Meteorology Flashcards
Constituents of gas in atmosphere
Nitrogen: 78%
Oxygen: 21%
Argon: <1%
CO2, ozone (O3) etc.
Percentage of water vapour in atmosphere
0.001% to 4%
Averages less than 1%
NOTE: This isn’t included in the usual breakdown of atmosphere which is based on DRY air
Temperature changes in stratosphere
Constant up to 20km
Increases up to -15C at 50km
Decreases from 51km again (stratopause)
Reason for increase in temperature with altitude in stratosphere
O2 breaks down into 2 O molecules by absorbing UV light (<240nm wavelength), which is exothermic.
Single O’s join with other O2 to form ozone (O3).
O3 absorbs UV light (<290nm wavelength) to breakdown into O2 and O, again exothermic.
Tropopause definition
Point where temperature stops falling with altitude increase (taken as rate of fall going below 0.61C per 1000ft - 2C per 1km)
What factor impacts tropopause height and temperature?
Temperature depends on height, the higher it is the more temperature is lost before reaching it.
Height is determined by surface temperature, high surface temp (equator) leads to higher altitude (thus high surface temp leads to low tropopause temp).
Typical tropopause heights and temperatures
53,000ft (16km) at equator, -75C.
36,000ft (11km) at mid-lat, -56C.
26,000ft (8km) at poles, -45C.
Tropopause heights summer & winter
- Latitudes 30, 50, 70
Summer Winter
30deg: 16km 16km
50deg: 12km 9km
70deg: 9km 8km
Mesosphere, thermosphere and exosphere altitudes
Mesosphere: 50km to 85km
Thermosphere: 85km to 600km
Exosphere: 600km to 10,000km
Mesosphere and Thermosphere features
Mesosphere: Meteors are burned up by the thickness of atmosphere here
Thermosphere: Absorption of high energy UV and X-rays leads to increasing temperature with altitude around -120C at base to 2000C at top.
ISA
- MSL temp, pressure, density
- lapse rates
- MSL temp 15C
- MSL pressure 1013.25 hPa
- MSL density 1.225kg / m3
- Lapse rate 1.98C / 1000ft to 36k
- Constant temp up to 20km
- Temp increase 1C / 1km to 32km
Pressure levels by altitude
30000’ 300
18000’ 500
10000’ 700
Altitude of:
- 50% of MSL pressure
- 50% of MSL density
- 50% of weight of atmosphere below
18,000ft: 50% pressure
20,000ft: Half weight of atmosphere
22,000ft: 50% density
Altitude of 25% MSL density
40,000ft
Barograph
Aneroid type pressure capsule connected to rotating drum which draws a line over time as pressure increases and reduces
Equation for (H), height change in feet per hPa
H = (96 x T) / P
T = Temperature in Kelvin
P = Pressure in hPa
Height change per hPa under ISA at:
MSL, 20000’, 40000’
MSL = 30 feet per hPa
20000’ = 50 feet per hPa
40000’ = 100 feet per hPa
Diurnal pressure variation
1hPa typically but up to 3hPa in tropics.
Needs to be considered as part of assessment of pressure changes over time.
QFE (Field Elevation)
Pressure setting to give zero elevation at HIGHEST point on airfield.
If touchdown point is significantly lower you may be given a touchdown QFE.
QFF
Similar to QNH (nautical height) but converts QFE to MSL using actual temperatures rather than ISA temperatures to establish true MSL pressure at a location which is relevant to meteorological charts.
Note - QNH gives correct airfield elevation when at the airfield.
QFF vs QNH when temp is higher than ISA and airfield above sea level
If warm and > sea level:
QNH > QFF
Flips if cold, both flip if below sea level.
QNE
NOT a pressure setting. This is the height on altimeter at touchdown point if standard pressure setting (1013) is set.
Used for high elevation airfields where QFE is too low to set in altimeter.
Risk of ground collision when heading to high or low pressure
Altimeter will over-read if heading into low pressure risking ground collision.
Think about what would happen if you adjusted to the correct pressure setting (turn down to the lower pressure setting, indicated altitude would fall).
Density altitude
- description
- calc
The altitude at which observed air density would be found in an ISA standard atmosphere.
Or, pressure altitude adjusted for ISA temperature difference (@120ft per C)
Effect of temperature of air column on pressure readings
In cold air pressure changes more quickly with altitude, i.e. the column of air between two pressure points is compressed. Thus higher risk of ground collision if heading into very low temperatures as altitude will over-read.
[Not an issue for traffic avoidance as all aircraft the same, but separation will reduce]
Impact of latitude on density at low and high altitude
At surface density is higher near the poles as surface temp is colder.
At high alt (50000’) however, the opposite is true, as higher equator temp leads to lower fall in pressure with height.
Specific Heat
The number of calories required to raise temperature of 1g of a material by 1C.
This is 1 for water (1 calorie raises 1g by 1C), but less for most materials (ice 0.5, rock 0.25).
Latent heat of water (ice to water, water to steam)
80 calories for fusion (melting)
540 calories for evaporation
Depression weather
- Cloud, prec, vis, temp, wind
Cloud - extensive (including vertically)
Prec. - Intermittent or continuous, light to heavy
Visibility - Good unless in rain
Temp. - Brings colder air in summer, warmer in winter
Winds - Strong
Anticyclone weather (summer)
- Cloud, prec, vis, temp, wind
Cloud - None
Prec. - None
Visibility - Moderate (haze)
Temp - Variable
Wind - Light
Anticyclone weather (winter)
- Cloud, prec, vis, temp, wind
Cloud - Low stratus
Prec. - Drizzle
Visibility - Poor (mist/fog likely)
Temp - Warm
Wind - Light
Cold anticyclones
Permanent overs the poles
Temporary in other places (e.g. Siberia) due to surface cooling (aka Cold Pool).
Cold land cools air which becomes dense, high pressured and creates an inversion. Above 500mb level get low pressure.
Cold pool/drop
- Cause
- Conditions
- Identification
Cold air BREAKING AWAY FROM A COLD FRONT or the polar region.
Convective weather due to instability, lots of rain and TS in afternoon due to insolation in the day.
Can be identified using isohypse charts at 500hPa level, surface charts will NOT show.
Polar air outbreaks
- description
- season
- locations
Outbreaks of cold polar air into tropical air. Typical in WINTER, in both USA and China (very cold continental air to the north).
Warm anticyclones
In subtropical regions (e.g. Azores) where Hadley cell air from equator descends along with air from Ferrel cell from temperate regions.
Creates a high pressure area on the surface AND above 500mb level. Recall warm air column has pressure levels further spaced out, so warm column over a high pressure area has to give high pressure at high altitude too.
Blocking anticyclones
Quasi-stationary
Typically warm anticyclone
50-70N
Persist in an area, blocking Eastwards flow of fonts.
Anticyclone vs cyclone (depression) size and speed
Anticyclones around 1500NM - slow, depressions around 300NM - quick (fast moving and lifecycle)
Col
Level pressure area between 2 highs and 2 lows
Look out for “flat pressure area”
Col weather
Generally settled, calm winds
Summer - Thunderstorms common due to calm winds allowing heating to start convection
Winter - Poor visibility, fog due to calm winds
Trough
- Description
- Weather
Trough is an extension of a low pressure area with sharp V shaped isobars causing windshear. Get unsettled weather, clouds and precipitation.
Subsidence
Vertical down draught of air
Celcius to farenheit
32 - 212
C = (5/9) * F - 32
Radiosonde
- description
- RoC
Radar reflector (GPS lately) attached to balloon which can be tracked to detect wind speed @ altitude.
1,200ft RoC up to 65,000 to 115,000ft height
Insolation
Heating of surface by solar radiation
Nature of solar radiation
Short wave radiation, passes through atmosphere and warms up surface
Nature of radiation from earth
Longer wave radiation than solar, can be reflected back by cloud
Clouds absorbing or reflecting radiation from sun and earth
Clouds REFLECT solar radiation into space.
Clouds ABSORB infra-red radiation from Earth, although they re-radiate some back again.
Conditions for maximal diurnal variation of surface temp
Calm winds (reduces mixing of lower and upper layers of air) and clear skies.
Times of minimum and maximum temperature with clear sky over continent
Min temp 30 mins after sunrise
Max temp about 2 to 3pm
Saturation Vapour Pressure (SVP)
This is the water vapour pressure at which air is saturated with water pressure and water will no longer evaporate.
It changes with temperature, the higher the temperature, the higher the energy level in water molecules and more water vapour pressure required to prevent them evaporating.
Warm or cold air holds more water vapour?
Warm air holds more.
Thus reach saturation as air rises, cools, and can support less water vapour
Absolute humidity
Grams of water vapour per metre 3 of air
Specific Humidity
Grams of water vapour per kg of air
Humidity mixing ratio (HMR)
Grams of water vapour per kg of DRY air.
This remains constant as the air rises up until saturation point.
Saturation mixing ratio (SMR)
Max amount of water vapour a unit mass of dry air can support at a given temperature
Relative Humidity (RH)
HMR / SMR
Humidity ratios as temperature changes
Measures of water vapour content (mixing ratio, absolute humidity, specific humidity) don’t change, but saturation levels (saturation mixing ratio) increase with temperature, so relative ratios (relative humidity) will decrease.
Saturation at below zero temperatures
(in terms of partial pressure and temperature)
Partial pressure of water for saturation for ice is less than partial pressure for saturation for water, therefore when saturation is reached below zero, water vapour sublimes to ice (hoar frost).
[To remember, increasing water vapour pressure reaches lower ice limit first and gets pushed into ice]
In terms of dew point/frost point, frost point > dew point.
[falling OAT reaches frost point before dew point]
Limitations on dew point
Dew point can’t be greater than the temperature of the air parcel
Psychrometer
Aka “dry bulb & wet bulb hygrometer”.
Wet bulb thermometer has wet muslin around it, if air is not saturated there will be evaporation which draws heat and therefore cools the thermometer. Difference between wet and dry thermometers indicates humidity.
Wet bulb temperature
This is NOT dew point, but in-between dew point and OAT.
Calculating cloud base
- Based on wet bulb lapse rate
Wet bulb falls at SALR (1.8C per 1000ft), so wet bulb rate (proxy for dew point) and unsaturated air “close in” on each other at (3-1.8 =) 1.2C per 1000ft.
Don’t need to know dew point lapse rate info, just that it does fall with altitude like the other temperatures.
Calculating cloud base (i.e. condensation level) based on surface temp & surface dew point
Cloud base in ft = (surface temp - surface dew point) x 400
Hair hygrometer
Tool that uses length of a human hair to measure humidity.
Diurnal variation of dew point
Dew point doesn’t vary through day/night
Calculation for relative humidity from temperature and dew point
RH = 100 - 5 x (temp - dew point)
Latent heat at different temperatures
More latent heat energy is released/absorbed when temperature is higher
Diabatic vs adiabatic
Diabatic involves movement of heat energy.
Adiabatic is due to changes in pressure
DALR
Dry Adiabatic Lapse Rate
1C per 100m
3C per 1000ft
SALR @ low level, mid latitudes
Saturated Adiabatic Lapse Rate
1.8C per 1000ft
SALR @ altitudes & latitudes
Cold air holds less water vapour so less latent energy, SALR tends towards DALR.
So high altitudes & polar low level can reach 3C per 1000ft.
Equatorial low level get down to 1C per 1000ft.
Effect of additional moisture on stability
Increase in moisture affects dew point, or alternatively increases mixing ratio. This reduces the altitude increase required before dew point (or saturation mixing ratio) are reached. Thus DALR changes to SALR at a lower altitude, lower lapse rate increases likelihood of layers where ELR > lapse rate, thus instability.
Stability of a layer of air
We consider the ELR of a layer of air, which can be any value, as compared to the DALR/SALR of a pocket that might enter that layer. Thus high lapse rate means unstable, as an unsaturated or saturated pocket of air entering it will be unstable.
Blue thermal
A rising column of air with no cloud. Requires absolute instability so that a rising pocket of very dry air is unstable at DALR (perhaps due to convection or orthographic lifting). Used by gliders.
Inversion in turbulent lee area
Moist air in stable atmosphere (so low ELR) with high winds to create the turbulent layer in lee of hills.
Initially temperature profile is ELR, but turbulence causes some air to rise and some to fall, which will happen at DALR. Over time temperature profile changes to match DALR within the layer (surface is warmer, top of layer is cooler) creating an inversion above the turbulent layer.
Calculation for temperature inversion at top of turbulence layer
((DALR - ELR) x Height of turbulent layer) / 2
[all in 1,000ft, i.e. DALR in per 1,000ft, height of turbulent layer in thousands of feet]
Vertical windshear
- description
- units
Change in windspeed with change in height.
Knots per 100 ft
Horizontal windshear
Change in windspeed with movement in horizontal plane
Knots per 1000 ft
Low level windshear
Up to 1600ft.
Severity ratings for windshear
Light: 0-4kt per 100ft
Moderate: 5-8kt per 100ft
Strong: 9-12kt per 100ft
Severe: >12kt per 100ft
Determinants of friction layer height
- Terrain roughness
- Wind speed
- Stability
Terrain roughness: more rough => more vertical deflection => higher layer
Wind speed: more deflection @ higher windspeed => higher layer
Stability: Stable air resists vertical movement => lower layer
Types of turbulence in friction layer (2)
Convection from thermal currents
Frictional/mechanical turbulence
Diurnal changes in friction layer and turbulence
Thermal effects in the daytime create instability and increase depth of friction layer.
At night only mechanical turbulence.
However surface cooling at night can lead to low level inversions, with light winds below and stronger winds above, causing windshear at the inversion.
Turbulent vs Laminar boundary layer
In a laminar boundary layer (1000 to 1500ft) each “slice” of the boundary layer slides over each other easily so windspeed reduces significantly to very low close to the ground. When the boundary layer is turbulent (2000ft thick), these layers mix and so the higher slices lose speed more quickly and lower layers retain more speed. Only very close to the ground does speed fall significantly (i.e. below the 10m anenometer).
Driver of boundary layer being turbulent or laminar
Boundary layer turbulence created by thermal disturbance and mechanical (e.g. hills) disturbance.
So laminar layers expected over smooth cold surfaces, turbulent over rough hot surfaces.
Time of day of greatest thermal turbulence
1500 - same as highest temp
More turbulence in stable or unstable atmosphere?
Link to thermal turbulence, less stability increases turbulence.
Definition of cloud
Hydrometeor consisting of minute particles of liquid water or ice (or both) suspended in the atmosphere, usually not touching the ground.
Conditions for mountain waves (standing waves, lee waves)
- Wind direction perpendicular to mountain range +/- 30 deg
- Wind speed at summit >15kt (30kt over large mountains)
- Wind speed INCREASING with altitude up to troposphere, but direction constant
- Stability around summit altitude (strongest if inversion)
Lenticular clouds
- description
- altitude
- cloud type
Formed above mountain tops and at crests of mountain waves in downwind direction.
Can be found up to and ABOVE tropopause.
Ragged edges => turbulence.
In the altocumulus family (altocumulus lenticularis)
Rotor (or roll) clouds
Formed under crests of strong waves downwind of the ridge.
Strongest rotor formed under first wave, level with or slightly above ridge crest.
Cap clouds
Form on the mountain ridge, may be blown down lee slope by strong winds.
Visibility of mountain wave clouds
Might be obscured by other clouds, or might not appear if air is dry, but the waves are still present.
Advice for mountain waves
Cross mountain ranges at 90 degrees, don’t fly parallel and downwind.
Don’t fly through rotor zone.
Allow height clearance = height of ridge.
Avoid low altitude flight on lee side and high altitude flight downwind.
Turbulence in troughs and ridges
Wind follows isobars so ridges and especially sharp edged troughs create a lot of horizontal windshear.
Mostly concerned with upper level troughs and ridges where wind strength is higher.
Frontal windshear/turbulence
Wind direction changes abruptly at a front, thus windshear/turbulence, particularly cold front as speed can change significantly. On approach to land this can be dangerous.
Turbulence that pilots should report
High level (> FL150) clear air turbulence (i.e. not associated with cumuliform cloud or thunderstorms).
Report time, location, level, intensity and aircraft type
Turbulence intensity (ICAO)
- IAS
- G force
- control
Light: <0.5g changes @ CoG (5-15ks IAS change)
Medium: 0.5 to 1g changes @ CoG (15-25kts IAS change), positive control at ALL times
Severe: >1g changes @ CoG (25+ IAS change), MOMENTARILY out of control
Jet stream turbulence location
Turbulence related to jet streams is around the boundaries due to windshear.
Strongest near to or just below jet axis on the cold air (low pressure) side.
Barometric error (altimetry)
Altimeter errors caused by wrong pressure setting on subscale
Temperature error (altimetry)
Altimeter errors caused by temperature being different to ISA.
Colder than ISA means true altitude is lower than indicated.
Calculation to adjust altimeter for temperature error
0.4% per degree C
e.g. 5,040ft mountain clearance 20C below ISA.
7040 * 0.4% * 20 = 563ft
[NOTE: Add 1,000ft (or 2,000ft in mountains) to an elevation figure, but NOT to a minimum safe altitude figure!]
Consequence of temperature error on decision height/altitudes
Need to be adjusted when temperature more than 15C below ISA
[increase by 0.4% per deg C]
Wind vector indicator meaning
Relevance of direction wind vector triangle/dashes point
Towards low pressure
Gust
Needs to be at least 10kts over stable wind speed
Squall
A sudden increase in wind speed (often along with change in direction), lasting a minute or more, can cover a large area.
Squall line
Narrow band of active thunderstorms
Indicated by sudden gusts and distinctive “ROLL CLOUD”
Gale
Sustained wind speed over 33kts or gusts over 42kts.
Equipment for measuring surface wind
10m above surface level
Wind vane detects direction
Anemometer (3 spinning half cups) measures speed
Pressure Gradient Force (PGF)
Force creating wind from high pressure to low pressure areas, with wind speed related to spacing between isobars.
This determines wind strength.
Coriolis Force (CF)
Causes wind to veer in NH, back in SH.
Force relates to windspeed (i.e. windspeed due to pressure gradient force) and latitude (minimal within 5deg of equator).
Formula for CF
CF = 2 x w x rho x V x sin(theta)
theta = angle of latitude
w = angular rotation of earth (15 deg per hour)
Formula for windspeed
- effect of air density and latitude on windspeed (geostrophic)
In geostrophic wind coriolis force balances PGF so:
PGF = 2 x w x rho x V x sin(theta)
=> V = PGF / (2 x w x rho x sin(theta))
Thus air density and latitude decrease windspeed, so fastest winds at high altitude and low latitude FOR THE SAME PRESSURE GRADIENT.
Creation of geostrophic wind
Initially wind moves in direction of PGF and coriolis forces starts acting on it 90 degrees to the right. As the net wind direction turns to the right the coriolis force keeps turning to be 90 degrees to the right of wind. This continues until the coriolis force is in opposite directions to PGF (so wind direction 90 degrees right of PGF), when the two forces balance and wind maintains its speed and direction.
Geostrophic wind definition
Created by combination of pressure gradient force and coriolis force.
- Above the friction layer
- Latitude > 15 degrees
- Pressure situation not changing rapidly
- Isobars straight and parallel
Gradient wind
- Cause in exam questions
Occurs when isobars are curved.
Combination of pressure gradient force, coriolis force and centrifugal force.
[If question refers to surface, likely FRICTION is the answer, otherwise CURVATURE OF ISOBARS]
Impact of centrifugal force on wind speed
Coriolis force is based on wind speed so adjusts to the combination of PGF and centrifugal force.
So around a low when centrifugal force opposes PGF, coriolis force is lower and windspeed is lower.
Around a high the centrifugal force acts with PGF, coriolis opposes and windspeed is higher.
[FOR QUESTIONS WHERE ISOBAR SPACING IS THE SAME - LOW SPEED ROUND LOW, HIGH ROUND HIGH]
Isolobaric effect
Wind blowing across isobars (high pressure to low pressure) when pressure is changing significantly. In other words, when pressure is changing a lot the geostrophic wind will be adjusted by an additional force from high to low.
Antitriptic wind
Wind that blows in low latitudes where CF is very small
Wind change as surface is approached
[Northern Hemisphere]
Day over land: backs 30 deg, 50% windspeed (compared to free stream flow/2000ft)
Night over land: back 40 deg, 30% speed
Over sea: back 10 degrees, 70% speed
[So windspeed at 2000ft in day is twice that over land]
Diurnal variation of surface wind
At night friction layer is stable, friction is high so surface effect causes wind to back and slow down relative to high level wind.
In the day thermal effects mix the layers more so that effect is reduced, wind speed increases and veers (relative to night time).
Diurnal variation of 1500’ wind
At night the friction layer can reduce to less than 1500’ so now 1500’ level is behaving like high level, increased windspeed and veering.
In day the friction level rises and ground effect impacts 1500’ level, so windspeed reduces and backs (relative to night time).
Sea breeze and land breeze
- wind speed
- distance from coast
- height
Sea breeze about 10-15 nm from coast, 10-15kts.
Land breeze about 5 nm from coast, 5 kts.
Recirculation in opposite direction at about 1000’.
Mountain/valley breezes
Thin air over mountains warms quickly in the day, rising and sucking up air from lower valleys. So mountain breeze is uphill in daytime, downhill at night.
Katabatic and anabatic winds
Katabatic/anabatic due to cooling/heating of ground/mountains thus falling/rising over mountains.
Kata - 10kts, ana - 5kts
[Remember KATANA!]
Fohn winds
Wind pushes air upwards against mountain, clouds forming on windward side resulting in warmer wind and higher cloud base on leeward side.
TURBULENCE expected on lee side.
Contour pressure chart
Chart produced for a given pressure level (e.g. 500 hPa), contour lines (isohypse) show relative altitudes of that pressure level.
Drivers of isohypse (and pressure levels) at high altitudes
1) Thermal differences - recall that a warmer column of air with a given pressure range will take up more space (i.e. altitude) than a colder column of air. So adjacent areas with different temperatures will create a pressure difference at altitude.
2) The pressure at the surface.
Which way do upper winds go relative to isohypses?
Bays Ballots law says in NH winds travel parallel to isohypses, with low numbers on the left (as with isobars).
Isotach
Line of equal windspeed, shown as dashed red line on contour pressure charts
Cause of winds in upper atmosphere
Thermal differences which cause pressure differences, ultimately coming from surface temperature effects.
Thus upper winds referred to as thermal winds.
General upper wind direction
Westerly (i.e. West to East) in both hemispheres.
From Buys Ballot’s, wind blows with low pressure (i.e. low temp) on left in NH, right in SH. Thus equator on the right for NH thermal winds, on the left for SH thermal winds.
Easterly upper winds
NOT jet streams
Tropical easterlies around ITCZ
Polar regions in summer (thermal effects limited so low level winds replicated).
[i.e. arctic in June, antarctic in Jan]
[Polar Easterlies are NOT jet streams]
Jet streams
- position relative to tropopause
- speed
- dimensions
Strongest upper thermal winds JUST BELOW tropopause, >60kts (up to 300kts).
Assumed 2000 miles long, 200 miles wide, 2 miles deep. [ratio 1000:100:1]
Location of jet stream around fronts
Jet stream will be in the warm part (although appears in cold part from surface level charts due to frontal slope).
Subtropical jet streams
Formed by subtropical anticyclones. Permanent but move with seasons as the anticyclones move.
NH: 25 to 40 deg in winter, 40 to 45 in summer
SH: 25 to 30 deg
Polar front jetstreams
Caused by pressure gradient between wTm and cPm on either side of polar front. The only frontal jet stream. Follows polar fronts around the globe but interrupted over Siberia in Winter.
Straight lines in SH but land mass disturbs the line in NH.
NH: 40 to 65 deg
SH: 50 to 55 deg
Tropical/equatorial easterly jet
In NH summer between 10 and 20 degrees, due to heated central Asian plateau.
Hotter over India than sea at equator so initially heads south, coriolis turns right so EASTERLY jet stream.
Runs from South China Sea across India, Ethiopia and sub Sahara.
Only 30 to 50kt.
Arctic Jet Stream
Boundary of Arctic and Polar air.
60 deg N around USA or 45 to 50 deg N elsewhere.
Transient feature during NH winter.
Westerly
Jet stream altitudes
Equatorial: 50,000ft, 150hPa
Sub-tropical: 40,000ft, 200hPa
Polar front: 30,000ft, 200hPa
Arctic: 20,000ft, 400hPa
Jet stream summary
- Altitudes
- Seasons
- Latitude
- Wind speed
Low level jet stream
Not technically a jet stream.
Caused by mass of cold air moving over ground, or intense uneven ground cooling at night.
Creates a strong INVERSION, temperature differential causes wind in the warm air above inversion up to 70kt.
Clear air turbulence
Caused at boundaries of jet streams.
Recall jet stream is on warm air side. Strongest turbulence is in the warm air, but on the cold side of the jet stream, just below the jet stream.
[Also mountain waves w/o cloud termed CAT]
Indicators of strength of CAT
Curving jet stream (change of direction => windshear)
Trough at surface level
Conjunction of two jet streams
Indicators of jet stream speed
Largest pressure gradients, so boundary between ridge and trough, or low and high pressure systems in proximity would give greatest speeds.
Detecting turbulence
Radar reflects liquids and solids, so turbulence related to cloud/rain phenomena can be interpreted. CAT is harder. LIDAR can detect it, but pilot reporting is helpful.
Jet streams stronger in summer or winter
Increase in strength in winter due to higher temperature differential.
Cloud associated with jet streams
Cirrus cloud can form on the equatorial side, due to corkscrewing of air around the jet stream, rising air on equatorial side (often moist, e.g. tropical front jet stream) leads to cirrus cloud.
Cloud ceiling
Height above aerodrome of lowest layer of cloud of more than 4 OKTAS
Measuring cloud base by day or night
By day release a balloon and time until it disappears
By night shine a searchlight at the cloud base and use alidade (trigonometry) to calculate height.
Ceilometer
Uses light or lasers and reflection to determine cloud base height.
Cloud height bands
Low: 6,500’
Alto: 6500’ to 23000’
Cirro: 16500’ to 45000’
[Alto & Cirro lower in polar regions, higher in tropical]
Cirrus cloud conditions
Ice crystals
No icing (as particles are ice already!)
Light turbulence in Cc, no turbulence in Ci/Cs
Fair visibility (>1000m)
Alto cloud conditions (As, Ac)
Water droplets + ice crystals
Light icing
Light to Moderate turbulence
Fair visibility (<1000m)
As: Light to Moderate rain
Stratus/Stratocumulus conditions
Water droplets
Light to Moderate icing
S: No turb, Sc: Light turb
Poor visibility (<30m)
Drizzle/light rain
Cumulus/Cumulonimbus/Nimbostratus conditions
Water droplets + ice crystals
Moderate to Severe icing
Moderate to Severe turbulence
Poor visibility
Heavy showers (rain Ns)
Cloud base low (low/medium for Ns)
Cumulus development stages
[Early stages start in am]
Cumulus Fractus
Cumulus Humilis (fair weather Cu)
Cumulus Mediocris (dark underneath)
Cumulus Congestus (towering cumulus)
Cn Calvus (rounded tops, no anvil)
Cn Capitallis (has an anvil)
Altocumulus castellanus (Acc)
Little towers of cumulus clouds forming on a flat cloud layer.
Requires considerable instability and can occur before thunderstorm.
Turbulence Cloud
Stratiform cloud formed in stable conditions, due to turbulence in the friction layer.
Turbulence at surface steepens the lapse rate and thus there will be an inversion above the turbulence cloud.
Mammatus
“Breast shaped” cloudbase of Cu/Cb indicating imminent precipitation
Convection Cloud
Cumulus type cloud formed by the earth warming the air, which then cools at DALR and forms cloud at dew point (cloud base) up to the point ELR takes over SALR when the cloud top is reached.
Only forms over land.
Nacreous cloud
Aka “mother of pearl” cloud.
Exist in lower stratosphere (20 to 30km) and have a bright shiny appearance due to reflected sunlight.
Noctilucent clouds
In mesosphere at 70 to 95km, reflect sunlight, thus the name meaning “night light clouds”. Made of ice crystals (potentially from rocket engines).
Bergeron Theory
(Norwegian or Ice Crystal theory)
Bergeron theory is that precipitation is caused by some water droplets turning to ice, growing in size through sublimation with water vapour and colliding with supercooled droplets. These droplets then become heavy and fall as rainfall or snow depending on temp.
Related to the partial pressure of water vapour over ice/water.
Coalescence Theory
Assumes a variety of droplet sizes, with larger ones falling faster and uniting with smaller ones, eventually overweight drops fall as DRIZZLE or LIGHT RAIN.
Likely or initial precipitation at high latitudes or in cloud where water droplets and ice crystals exist
If ice crystals exist in the cloud due to low temperature, partial pressure of water vapour over ice vs water will cause ice crystals to grow relative to water droplets (water droplets may even partially evaporate).
Thus initial precipitation is SNOW, which may melt by the time it reaches the ground, depending on temperatures.
Ice crystals are NOT hail!
Symbols for:
- Drizzle
- Rain
- Snow
- Hail
- Soft hail (graupel)
- Ice pellets
Diameters of precipitation types:
- Drizzle
- Rain
- Snow
- Hail
- Ice pellets
Drizzle: 0.2 - 0.5mm
Rain: 0.5 - 5.8mm
Snow: 1- 5mm (grains - pellets - flakes)
Hail: 5 - 50mm
Ice pellets: <5mm
What cloud do snow grains fall from?
Stratus or fog only, never fall in showers
Ice Crystals (precipitation)
Aka Diamond Dust.
Very small particles found in polar and alpine regions, forming at below -10C. Sparkle in the sunlight.
Hail
- how and where it is created
From Cb cloud only, made up of layers around a nucleus. Can fall (gathering moisture) and be lifted back up by an updraft, freezing again. Eventually too heavy, or thrown out of top of cloud.
High freezing altitude at equator means they usually melt there before reaching the ground.
Not common over sea due to lack of convective strength.
Ice pellets
Produced by frozen raindrops (as opposed to hail which has a frozen core).
Likely linked to FREEZING RAIN!
<5mm [Hail is >5mm]
Visibility in rain types
- drizzle, rain, snow
Drizzle: 0.5km to 3km
Rain: 3km to 5.5km (1km heavy)
Snow: below 1km (almost zero for drifting/blowing)
Visibility in sand/dust storm
Moderate: 200m - 600m (maybe <200m with sky not obscured)
Heavy: <200m
Duration of rain
Showers associated with convection cloud (cumulus).
Intermittent or continuous with layer cloud (continuous is no break for 60 mins+).
Surface temp for snow to reach surface
< 4 deg C
Definition of slight, moderate & heavy precipitation
Rain in mm/h, snow in cm/h:
Slight <0.5, Moderate 0.5-4, Heavy 4+
Showers <2mm/h, 2-10mm/h, 10+mm/h
Basic requirements for thunderstorms
- Lapse rate greater than SALR at least 10,000ft thick and above freezing level (i.e. CONDITIONAL instability)
- Sufficient water vapour
- Trigger action
Triggers for TS
Winter: Frontal uplift
Summer: Convection, orographic uplift, convergence (also advection)
Frontal type thunderstorms
Frontal type caused by fronts (cold front or occlusion in a depression or trough), usually in Winter, fast moving.
Air mass thunderstorm
Occur in daytime over land in summer, thermal convection from heated land.
Advection type thunderstorms
Medium level (base around 10,000ft). Can happen day or night, over land or sea, at any time of year.
Thunderstorm initial stage
- description
- time
- up draft speed
- size
15-20 mins. Small Cu combine and strong up currents (1000 to 2000fpm) draw air from sides and below.
Approx 5NM across.
Thunderstorm mature stage
15-20 mins. Precipitation starts. Rain/hail causes 2400fpm down currents carrying cold air down. Descending air warms at SALR so accelerates downwards creating gust fronts 13 to 17NM ahead and 6000ft in depth.
Up currents remain strong (up to 10000fpm), cloud top rising at 5000fpm.
Rising and falling raindrops cause static electricity leading to thunder.
Thunderstorm dissipating stage
1.5-2.5 hours. Sporadic showers, extreme turbulence.
Cloud extends to tropopause where it spreads out into cirrus and forms an anvil.
Movement of thunderstorms
In direction of 10000ft (700hPa) wind, though large and newly developed storms will differ
Lightning in Cb
Upper cloud +ve charge, lower -ve, differential causes lightning. Found +/- 5000ft from freezing level, where temp is around 10/-10C.
Supercell thunderstorm description
High winds at upper layers separate the updraughts from the downdraughts, thus preventing the usual dissipation method of single cell thunderstorms.
Occur around polar fronts typically.
Supercell thunderstorm (severe local storms) requirements
Great depth of instability
Strong vertical windshear
Stable layer between warm upper and cool lower air which is broken down by insolation.
Supercell mature stage
Very strong up and down draughts causing violent weather & tornados.
Can last several hours.
Supercell movement & location
Located over continental land masses generally (e.g. central USA).
Move 20 deg to the right of the 18000ft wind (500hPa).
Avoidance distance for thunderstorms on radar (by flight level)
0 - FL250: 10NM
FL250-300: 15NM
FL300+: 20NM
Vertical: 5000ft
[Visual avoidance - 10NM]
Thunderstorm risks
Turbulence
Hail (up to 45000ft)
Icing (airframe -45 to 0C, carb -10 to 30C)
Lightening (within 5000ft of freezing level, temp -10 to +10C)
Static (affects radio equipment)
Pressure variations
Microbursts
Water ingestion (in jet engine)
Tornadoes
Microburst
- downdraft speed
- windspeed & windshear
- size
- time
Down currents in cloud and also outwards due to ground impact.
c. 3000fpm downwards (up to 6000fpm) and 50kt horizontal (in 2 directions, so up to 100kt windshear)
Only 4km horizontal length and last < 5 mins.
Dry microburst
If air below the thundercloud is dry, the rain can evaporate before reaching the ground. This absorbs energy, reduces temperature of the air, increasing density and thus the intensity of the microburst effects (windspeed).
Detecting downbursts
- 2 methods
Detection @ aerodromes where it is a concern for large aircraft.
1) Low Level Windshear Alert System (LLWAS), set of anemometers around airfield @5 to 10NM distance
2) Low frequency doppler radar measures windspeed around aerodrome
Tornado
- description
- size
- time
Connected to thunderstorms, caused by opposing vertical airflow movements. Diameter generally less than 150m but can be up to 1.6km. Called funnel clouds if they don’t reach the ground (can be embedded within cloud).
Last a few mins up to 30 mins.
Dust devil
Small whirlwind on hot, sunny afternoons up to 2000ft
Action when flying through thunderstorm
Maintain heading, don’t turn, this will probably be quickest way out.
Don’t climb or descend to make up for moves in turbulence.
Avoid flying below or above cloud due to down draughts below and growth of the cloud above.
Limitation for reporting of fog or haze
Not reported if visibility >5000m
Radiation fog
- description
- time of day
Formed overnight or early morning, with light winds, clear skies, high relative humidity.
[Look out for reference to morning or afternoon, maybe advection fog if afternoon?]
Unusual instances of radiation fog
Southern Gulf in Winter - cool moist air from sea breeze is cooled overnight over cold earth, fog drifts back out to sea in land breeze.
Polar Maritime air winter (NE Europe)
Hill (Orographic) fog
Cloud (usually stratiform) with a base lower than the summit of the hills. Can be caused by air being forced up hills, or normal turbulence creating St or Sc cloud.
How orographic fog is lifted
A DOWNWIND blowing moist air back down the mountain, below the dew point level.
Or potentially solar radiation.
Advection fog
- description
- ideal conditions
Warm moist air (e.g. from water) moving over cold land.
Wind speed around 15kt (much greater than 15kt will lift the fog to form stratus cloud)
OAT close to dew point is important, but also higher temp means more potential moisture in air so 20/15 more likely than 10/5.
Notorious area of advection fog
Over sea around Newfoundland and the Kamchatka peninsula.
Several areas where warm and cold sea flows coincide.
Steaming fog (arctic smoke)
- description
- specific conditions
In high latitude areas, requires high level of stability.
Cold air (BELOW -10C) from above land moves over warmer sea.
Frontal fog
Occurs AT warm front (or occlusion), clears once it passes.
Warm front sliding over cold front, warm rain evaporates in drier cold air.
Also evaporation of standing water and mixture of saturated air with non-saturated air below.
Can form 200nm band ahead of warm front.
Freezing fog
With temperatures below 0C air won’t sublime due to lack of freezing nuclei, but will freeze on contact with an object.
Can also happen when fog forms over 0C and air then cools below 0C.
Ice fog
- description
- cause
Rare occurrence below -40C where warm moist air is introduced to cold saturated air (e.g. due to car engines). Condensation and immediate freezing leads to ice crystals in the atmosphere.
Prevailing visibility
- description (# candelas)
Based on black object on white background OR 1000 candela white lights on black background.
Refers to the visibility around at least half of the horizon circle from aerodrome (except areas <1500m or <5000m and <50% prevailing will be reported additionally [e.g. 1200NE - 1200m visibility in NE direction]).
Runway Visual Range (RVR)
- Description
- When is it reported?
Max distance that a pilot 15ft above runway in touchdown area can see marker boards (day) or runway lights (night) in direction of TO/L.
Reported when meteorological optical range (MOR) or RVR <1500m, or if shallow fog is reported/forecast.
UK RVR increments
25m from 0 to 400m
50m from 400m to 800m
100m above 800m
RVR measurement frequency
Every 30 minutes in regular usage, 15 mins before TO/L in irregular usage.
RVR segment reporting
Divided into touchdown, midpoint, stop end.
Always report touchdown. If other two are <400m, or <800m AND <touchdown then they are also reported.
If one of them reported only will say “midpoint” or “stop end” but not if all three are reported.
Is RVR or meteorological visibility better?
RVR is usually better as it is based on bright runway lights.
Transmissometer
Transmitter and receiver measure visibility over short distance. Can have 3 at middle and each end of runway.
Forward Scatter Visibility Meter
Transmitter and receiver positioned 20 to 50 degrees away from each other, thus senses scattered light reflected.
Can assess visibility and also the nature of the particles in the air for more accurate information.
Freezing & condensation nucleii
Required in the atmosphere for water to freeze and condense, regardless of temperature of the water.
Condensation nucleii are more common, so supercooled (below 0C) water is common.
Share of ice crystals and SCWD in temperature bands
0 to -15: Mostly SCWD (large & small)
-15 to -40: Mix of SCWD (small) & ice crystals
-40 below: Ice crystals only
Impact of ice/frost/snow layer on lift surfaces (%)
Thickness and roughness similar to sandpaper can reduce lift by 30% and increase drag by 40%.
Clear Ice
Freezing of supercooled water droplet impacting airframe releases latent heat, water drifts back causing thin clear layer of ice.
1/80th of a supercooled water droplet freezes for every degree below zero (80 calories latent heat), rest stays water.
Most dangerous form of icing as it builds up quickly, so just below 0C more dangerous than lower temps.
What conditions does clear ice form in?
Large supercooled water droplets.
Cu/Cb/Ns from 0 to -20C
Rime Ice
Small supercooled water droplets freeze instantly, creating white coloured rime ice on the leading edges.
Can happen all the way down to -45C, from -23C down only get rime ice.
What conditions does Rime ice form in?
Stratiform cloud from 0 to -30C
Visiblity of Rime and Clear ice
Rime ice more visible as air gets trapped between frozen droplets. No air trapped in clear ice so not visible (no refraction of light).
Mixed ice
Combination of clear ice and rime ice which can be experienced in -10 to -15C
Icing severity in cloud types
Worst icing requires large super-cooled water droplets just below 0C.
Cb, TS: severe/moderate
Cu, Sc, Ns: moderate
St, Ac, As: light
Ci, Cs: nil (trace in Cs)
[St could get moderate or even severe with orographic uplift]
Very worst icing area
Tropical Cb cloud at temperatures just below 0C.
Creates combination of large super-cooled water droplets held up by cumulus cloud, high level of moisture at low levels and the right temperature conditions at high altitude.
Rain Ice/Freezing rain
When rain becomes supercooled by falling through an inversion, so common in triangle of cold front underneath Ns clouds of the warm front.
Will freeze to form clear ice or rime ice when impacting airframe.
Can build up very quickly so climb or descend.
[Freezing drizzle also exists]
Rain ice (diagram)
Hoar Frost
White crystals forming in clear air if airframe is below 0C and ambient temperature lowered to saturation level.
Water vapour sublimates (requiring sublimation nucleii, usually inorganic like volcanic dust or soil particles).
Situations causing hoar frost (and how serious)
On the ground at night: Must be cleared due to skin friction, obscuring windscreens and affecting radio antennae.
In the air if descending from cold region to warm moist air (or up through inversion). Effects not severe, speed up or move to warmer air.
Active frost
- Description
- Conditions
Hoar frost that reforms (obviously more dangerous). Same conditions as hoar frost:
- Aircraft skin below 0C (perhaps due to cold soak fuel - very cold fuel close to aircraft skin)
- Air temp close to dew point (within 3C)
- Dew point < 0C
- Either cloudless sky & calm wind (get radiation cooling) or warm front (brings warm, moist air)
Ice Crystal Icing (ICI)
- description
- where it is found
- problems caused (2)
Very small ice crystals at high altitude.
Can’t be detected by radar as particles are too small.
Found downwind of TS and convective cloud at high altitude but BELOW the tropopause [OVER FL330]
Can cause damage to engines and problems with heated temperature probes (to do with partial melting).
Avoiding Ice Crystal Icing (ICI)
Steer clear of area above convective rain clouds by 50NM
Ice Water Content (IWC)
grams per m3 of ice in a cloud
(high at high levels in convective cloud)
Impact of surface/aerofoil shape on icing
Airflow adheres closer to a thinner profile so more icing will occur. Thus thin winged aircraft more affected and also thin surfaces (e.g. tailplane) might get more icing than the main wings.
Impact of aircraft speed on icing
Faster speed leads to striking more water droplets, however the kinetic effect of increased speed may offset this.
Skin Temp = OAT + (TAS/100)^2
Increase in temp to below 0C can increase icing!
Cloud base temperature impact on icing
Higher temperature leads to higher amount of water vapour content. So a cloud with a warm base has more potential for icing at the higher (colder) layers, than a cloud which is cold at its base.
Worst Cb layer for icing?
Middle
At the top precipitation will be ice, at the bottom temperature is too warm.
Supercooled water drops mostly in the 0 to -20C temperature band which is probably in the middle of the Cb.
Requirement to report icing
Pilot must report to ATC any experience of unforecast icing, or moderate & severe icing (meaning continuous de-icing or diversion required)
Airframe icing severity reports
This is different to the severity for forecasting purposes.
Trace: De-icing not necessary unless exposure for > 1 hour.
Light: De-icing occasional use removes accumulation, only a problem if over 1 hour exposure.
Moderate: Short encounters may become dangerous and de-icing is necessary. Change of heading or altitude may be desirable.
Severe: De-icing can’t control and diversion is necessary. Immediate change of heading/altitude essential.
Piston Engine Induced Icing types
Impact icing (from snow, snow+rain or supercooled water droplets). Only this one affects turbo/fuel injected planes.
Fuel icing
Carb icing (due to temp drop as fuel evaporates and expansion of air passing through venturi)
Most dangerous temp for carb icing
-10 to +25C
OR 0 to 15C
[Centred around 7.5]
Key factors for carb icing
Difference between temperature and dew point, or in other words - relative humidity (100% when temp = dew point).
Very worst conditions are temp = dew point = 10C.
Jet engine icing
Jet engines often have some degree of convergence (high air velocity, temp and pressure falls) which can cause intake icing. Operating manual will describe the risk and RPM, airspeed (etc.) to avoid (likely to be high RPM and low airspeed).
Impact of orographic uplift on icing
Air forced upwards loses heat rapidly at the DALR, thus freezing level falls to lower altitude and icing risk increases.
Updrafts support supercooled water droplets so icing more intense above mountains than flat land (orographic intensification).
Classification of air masses
e.g. mAc
1) m for maritime, c for continental
2) Equatorial, Tropical, Polar, Arctic
3) c for cold, w for warm
Note on stability in air masses
All air masses are stable at source (i.e. get their properties by hanging around for a while in a cold/warm dry/moist area), possibly under an inversion.
Air moving to colder land becomes more stable, moving to warmer land becomes less stable.
[Note air mass are uniform in horizontal plane, not vertical]
Main air mass sources for NW Europe
Constant high pressure areas of polar region (cold) and Azores (warm), plus the temporary high pressure over very cold Siberia in winter.
Give rise to 6 air masses:
Polar: mAc, mPc, mPw
Azores: mTw, cTw (more like North Africa)
Siberia: cPc
Polar maritime (mPc)
Starts as cold, dry, stable air.
Heading over warmer North Atlantic picks up moisture and is heated, becoming conditionally unstable.
Winds from NW/W.
If sufficient heating (e.g. summer) get convective cloud (Cu, Cb), rain showers, hail, TS.
Can also get clear skies (especially at night), good visibility, radiation fog.
Arctic maritime (mAc)
Similar to mPc but from North in winter, air is colder so relative heating from beneath is higher and becomes less stable over UK.
Cu, Cb and heavy snowfall.
Polar continental (cPc)
From Siberia in winter only, very cold and dry, becomes unstable over UK.
From over the continent very dry, no precipitation, creates inversion so poor vis.
If coming over Baltic/North sea more moist, get Cu and heavy snow showers.
Tropical Continental (cTw)
Mostly in summer, warm, dry, stable air.
No cloud or precipitation, haze due to stability.
Comes from SOUTHERN BALKAN REGION AND NEAR EAST.
Tropical Maritime (mTw)
Azores anticyclone - warm, stable, high absolute & relative humidity.
Stability and humidity cause low stratiform cloud, poor visibility.
In spring advection fog over sea.
In summer insolation breaks down the stability giving clear skies or a few Cu.
Returning Polar Maritime (mPw)
Polar air moves to the South in Atlantic and approaches UK from W or SW.
End up with a mix of mPc and mTw, could behave like either depending on conditions/season.
Coldest source of air
Continental Polar (i.e. Sibera).
Colder than arctic maritime largely as dry air is colder than moist air.
Polar Front
Boundary between polar and tropical air, 35-65deg latitude in NH, 50-55 in SH.
In winter mid-Florida to SW UK, in summer Newfoundland to NW UK.
Arctic Front
Boundary between arctic and polar air.
Mediterranean Front
Boundary between polar continental/maritime air from Europe and tropical continental from Africa.
Extends West to East over the Med.
Disappears in summer.
[Med is LOW pressure, sucks in the SGK winds from Africa]
Intertropical Convergence Zone (ITCZ)
The fourth of the main world weather fronts.
Separates air masses either side of heat equator, also know as thermal equator or equatorial trough.
Created by the trade winds coming from the sub-tropical high pressure zones towards the equator.
Polar Front Depressions
- creation
- pressure aloft
Main cause of UK bad weather, depressions formed in families along the polar front (mPc, mTw). Warm Tropical air pokes into cold air creating the warm sector. The warm air surrounded by cold will rise and make this a depression.
Move parallel to the warm sector isobars.
Low pressure at high altitude as well as at the ground (unlike warm depressions).
Time between polar front “waves” in Europe
A “wave” means the passage of a new front.
About 1-2 days between each front (bit more in winter, bit less in summer).
Death of polar front depressions
Their energy is fed from the warm air (mTw) to the south (poking into the cold mPc). As occlusion forms the warm sector is pushed out and the depression separates from the mTw air, losing the hot air that gives it energy. Eventually it dissipates.
Warm front
Low slope (1:150) rising over cold air, Ci and stratiform cloud types behind the front.
Total distance of 400/600nm, rain up to 200/300nm under the lower Ns clouds.
Moves at right angles to itself, at 2/3 of the geostrophic interval at the front.
Cold front
Steep slope (1:80) forces warm sector air up quickly, forming Cb cloud (& Ns) ahead of it - heavy showers. Low pressure at the front itself (so passing of the front sees pressure fall then rise within the cold air following).
Other cloud at warm & cold fronts
Stratus fractus due to high humidity
Warm sector conditions
mTw air in the polar depression.
Summer: good weather, fair weather Cu
Winter: stable conditions, stratiform cloud, drizzle, mist
Passing of warm front
- pressure
- wind direction
- cloud
- precipitation
- temp
- visibility
Pressure falls as the centre of the low is approaching you (travelling Eastwards to your north).
Sharp veer from S to SW.
Cloud increases as it approaches (Ci, Cs, As) then Ns and drizzle as it gets closer. Continuous rain as it passes.
Frontal fog as it passes.
Temp & dew point rise as it approaches.
Reducing visibility.
Passing of cold front
- pressure
- wind direction
- cloud
- precipitation
- temp
- visibility
Pressure low point around the front (closest to centre of the low) then increases after passing.
Sharp veer from SW to NW.
Cu/Cb/Ns cloud, heavy rain or snow showers, thunder/hail possible (air forced upwards steeply by cold front).
Temp & dew point fall.
Visibility good except in showers.
Distance covered by frontal area behind cold front
Warm air at altitude extends back 100-200 NM from surface cold front.
Precipitation extends 50-100 NM behind the cold front at surface.
[Questions about time taken for weather to clear on passing of cold front]
Conditions in cold air behind cold front
Cumulus clouds (cold air aloft gives steep pressure gradient therefore instability).
Showers
Good visibility
Faster or slower cold front more active
Slower cold fronts are more active (wider band of convective clouds and related activity).
Effect of mountains on passing warm & cold fronts.
Warm front can trap cold air on windward side of mountains, forcing warm air higher so longer term precipitation on windward side, less on leeward.
Cold front gets extra lift from mountains so again, lots of activity on windward side, less on leeward.
Movement of fronts
- speeds in kt
- in terms of geostrophic wind speed
Warm front around 15kt, cold front 20kt.
Speed <5kt means stationary front. Winds will blow along the front rather than over it (so 180kt windshear at the front).
Warm front 1/3 to 2/3 of geostrophic wind, cold front over 2/3.
(Quasi) Stationary front weather
Wide area of cloud (predominantly on cold side) with varying levels of precipitation (showers).
Polar Front upper winds
Jet streams parallel to fronts as driven by thermal differences.
One 400nm ahead of warm front, parallel to it, from NW.
One 200nm behind cold front, parallel to it, from SW.
In the warm sector will be parallel to isobars (i.e. similar to geostrophic).
Wind directions around polar front depression
Triple point
Or occlusion point, the point where the cold front catches up with the warm front.
Warm or cold occlusion
Depends on relative temperatures of the cold areas (ahead of warm front and behind cold front). If air behind warm sector is warmer than air ahead of warm sector, it’s a warm occlusion.
Identify warm vs cold occlusion on a chart
See if it follows the shape of the cold front or warm front that it connects to
Description of occlusion cloud
Whether warm or cold occlusion, warm sector gets pushed up off ground. Cumulus cloud ahead of cold front is forced upwards so get Ac or Cb (hail, TS).
Occurs towards the end of depression lifecycle so moves slowly, bad weather can hang around.
Occlusion types in Europe in winter/summer
Get cold occlusions in summer, warm occlusions in winter in Europe.
Due to land warming/cooling the air in front.
Lifecycle of polar front/mid-latitude depression
4-7 days
Orographic (lee) depression
Air flow redirected around sides of mountain range causes low pressure on lee side.
Creates convective conditions on the lee side.
Three possible weather conditions around orographic depressions
1) Dry stable air leads to Fohn effect, warm, clear & dry weather lee side.
2) Moist unstable air creates Cu and Cb with showers, thunderstorm and hail possible over mountain and the lee depression.
3) Cold front approaching the mountain range when it breaks over will be above warmer air in the depression, increasing the convective, unstable effect. Line squalls, heavy showers, TS, hail.
e.g. Northern Italy over Alps
Thermal depression
Surface air is heated causing convection, creating a low at the surface and cyclonic winds.
Get Cu, Cb (hail & TS), heavy showers, moderate/sev turb and good visibility outside showers.
Lots of types (monsoon, polar air, inland water, TRS, NOT tornados).
Monsoon low
A large thermal low developed over continents in summer which dominates weather patterns.
Polar Air Depressions (Polar low)
Formed when Arctic Maritime air is lifted on a large scale as it moves south over warmer seas. Gives Cu, Cb, heavy showers and secondary cold fronts. Like TRS, lose power over land.
NOT the same as polar front depressions.
ONLY formed over water
Inland Water lows
In winter, cold air (e.g. Siberian cPc) over warm lakes or inland seas (Caspian, Black, Mediterranean) can create convection and depressions.
Summer thermal lows over land
Surface heating can cause thermal lows over land in summer, causing TS or widespread rain if conditions are unstable.
Over land OR (rarely) water
Tropical Revolving Storms (TRS)
- Description
- windspeed
- Size
- Time
aka hurricanes. Thermal depressions over warm tropical oceans with sustained wind speeds over 33kt. Designated tropical cyclone if sustained wind speed over 63kt [NO LONGER A TRS!, max TRS windspeed 63kt!]
270 NM (500km) diameter
Heavy rainfall
Tornados may be faster, but only last for minutes, TRS last for a couple of weeks.
Formation of TRS
- general
- specific example in atlantic
Formed from complexes of thunderstorms.
Gain moisture from warm waters which condenses releasing latent heat, exacerbating the depression and the convergence, leading to more condensation & latent heat.
Atlantic - Easterly waves coinciding with SE trade winds
Airflow around TRS
Cyclone (anti-clockwise in NH) rotation at lower levels around the depression, changes to high pressure and opposite rotation at highs where the air is expelled (divergence).
Some air flows back into the eye and down from the top level.
Requirements for TRS
5 to 25 latitude (coriolis too low <5, sea too cold over 25)
Ocean temp over 26C
Sufficient depth of water to provide energy
Minimal shear otherwise storm topples
Instability in atmosphere
Origins of TRS
Mainly from the Intertropical Convergence Zone (ITCZ) or from equatorial easterly atmospheric waves (originating in North Africa).
Movement and life of TRS
- direction and speed of movement
- death
Driven West by the easterly trade wind belt at 10-20 kts.
Get turned away from equator by coriolis force, losing energy as they move over colder seas or over land.
TRS eye
- description
- size
10-20 NM wide, lowest pressure and calmest conditions.
Air forced up by the storm descends in the centre. Adiabatic heating causes clouds to evaporate. It rose at SALR creating clouds and descends at DALR in the column, so relatively warm.
Walls of the eye are the fastest winds.
TRS stages
1) Tropical Disturbance: Area of low pressure, rising air giving towering Cu, releasing latent heat. Winds light.
2) Tropical Depression: Stronger depression and cylonic circulation, driven by divergence at altitude. Cb forms. Winds up to 33kt.
3) Tropical Storm: Continues to strengthen, subsidence in eye develops, no cloud there. Wind 33kt to 63kt.
4) Tropical Cyclone: (or hurricane, typhoon dependent on location) Wind speeds >63kt
How to avoid a revolving storm
Heading towards a low if you experience rightward drift, so turn (either direction) until you get leftward drift - then you must be going away from the low.
Most dangerous quarter of TRS
Fastest wind speeds on the half furthest from equator as cylonic windspeed direction (anti-clockwise) same as TRS movement (Westwards).
So front-right quarter in NH is the most dangerous zone.
Seasonality of TRS
Driven by high ocean temperatures so occur when ITCZ is passing through an area.
Examples of global TRS
- frequencies and season
Typhoon: NW Pacific, most active region (16 a year) mostly at sea, mostly Jun-Nov (never Feb-Mar)
Hurricane: N Atlantic (6) and NE Pacific (9) (i.e. USA), Jun to Nov
Cyclones (NH): Indian Ocean (12) mainly autumn/spring
Cyclones (SH): SW Pacific (Australia) (Willy-willy) (9), SH summer
Forecasting TRS path
Cannot ACCURATELY forecast TRS path
TRS sizes (degrees lat)
Small: 2-3 degrees lat
Medium: 3-6 degrees lat
Large: 6-8 degrees lat
V Large: 8 degrees+
Secondary Depression
- description
- where is it likely to form
Secondary area of low pressure nearby a “primary” low. Can eventually form another primary low or simply act as a disturbance in normal flow.
Sometimes more active than the primary depression.
Likely to form on the COLD FRONT.
North American Tornados
- size
- time of day
- cause
- season
100m in diameter, between 1400 and 2200 (peak at 1700) in central USA.
High level cold air from West rises over Rockies above warm moist unstable air from Gulf of Mexico.
Localised reduction of pressure (20 to 200hPa) and high wind speeds in vortex (300kt) cause high level of damage.
Spring & summer.
West African Tornados
Line of thunderstorms moving from East to West between March and November.
Associated with passage of ITCZ.
Example of Easterly wave.
NOT similar to NA tornados.
Idealized Circulation
Consideration of weather and airflow patterns based on a simplified spherical earth entirely sea with only earths rotation to consider (no tilt).
Get trade winds at equator and air cells along latitude (Hadley, Ferrel, Polar). Air cells produce anticyclone at subtropical level where hadley and ferrel meet.
Trade winds
- Description
- Latitudes
Winds NEAR equator curve towards West (Easterly winds). So wind direction NE in NH and SE in SH.
The main low level winds between sub-tropical high pressure belt and the equatorial low pressure region.
Note: Primarily 10-20 degrees N or S
Koeppens - Geiger Climate Zones
- By degrees latitude
0-20: A) Tropical rain forest (wet), tropical monsoon (seasonal), tropical savannah (dry)
20-35: B) Sub-tropical and mid-latitude steppe and desert (dry)
40-70: C) Wide variety including Mediterranean, humid sub-tropical. Classified based on wet/dry and seasonality.
50-70: D) Sub-arctic climate (hot summer, cold winter)
70+: E) Snow climate (polar)
Earth tilt and impact on savannah regions weather
23.5 degrees tilt
The wet equatorial climate zone moves into the “summer” hemisphere, creating high rainfall in savannah regions in summer and dry trade winds in winter.
Impact of land mass on idealised circulation weather
Minimal in SH as less land, but big effect in NH due to Asia & North America.
In Winter the Rockies in NA and Himalayas in Asia block flow so gets very cold in NA and Northern Asia. No barrier below UK allows some warming from equator direction.
Less contrast in summer, warm air from gulf of mexico and warming of large land masses makes NA & Asia warm, UK more static.
Impact of land mass on idealised circulation weather
- Diurnal variation
Larger land masses have bigger impact from heating of the sun so more diurnal variation. Diurnal difference peaks at 15C in middle of Africa, NA, some other areas in middle of continents. Over seas goes below 5C.
Impact of land mass on idealised circulation weather
- Freezing altitudes
Generally gets higher over landmasses due to heating of the land.
Around 16000ft over equator, but up to 18000ft over parts of the land, so hail from thunderstorms melts before reaching land.
Less variation further north.
Global pressure spots - January
Equatorial low pressure zone to the south of geographic equator.
Sub-tropical high’s over oceanic areas either side of equator.
Travelling low pressure systems at latitude 40 to 60 N are interrupted by high pressure over very cold large land masses (Siberia & USA).
Global pressure spots
July
Equatorial low pressure zone to the north of geographic equator.
Sub-tropical highs below including Australia, and some ocean areas in NH.
Over land in NH (e.g. Asia) warming of land creates lows instead of sub-tropical high.
Sub-tropical high strong around Azores and pacific ocean.
Main global surface winds (4)
Westerlies at temperate latitudes (40-60), especially strong in SH where land doesn’t interfere.
Trade winds (tropical easterlies) at equator.
Monsoons, seasonal winds caused by cooling/heating of land masses in winter/summer.
At poles strong Easterlies (although westerly in summer over N Atlantic & Pacific).
Intertropical Convergence Zone (ITCZ) weather
[time of day of relevance?]
Generally unstable conditions, extensive Cu, Cb and TS (especially at MIDDAY).
In stable conditions instead get As and Ns and continuous rain.
Severe turbulence and icing.
Width of ITCZ
25nm to 300nm, no well defined frontal surface.
El Nino
Normal pattern is cold water blown West from Peru to far east, warming as it goes.
Every 3 to 7 years El Nino means the cold water goes deeper, so far east is colder and Pacific is warmer, disrupting weather patterns.
El Nina is when Pacific is colder than normal.
Monsoon definition
When trade winds blow towards continental lows, or from continental highs.
3 Monsoons
NE monsoon in Asia from Siberian winter high, cold dry air over continental Asia, heavy rain & TS where it crosses ocean.
NW monsoon is continuation of NE monsoon which backs as it crosses equator and heads to Australia.
SW monsoon caused by SE tradewinds crossing equator and veering to SW, taking heavy rain over south Asia (big impact on aviation).
West African Monsoon
This is the SW monsoon in summer when ITCZ is north of the equator. SE trade winds veer to SW over equator, very unstable and moist creating convection and heavy rain up to the ITCZ. After summer they get pushed south by ITCZ and replaced by harmattan, dry NE winds on other side of ITCZ.
Easterly Wave
- description
- where is activity relative to the trough?
Trough of low pressure in NH only, originating in Africa from 5 to 20 deg lat, moving towards Caribbean (over Atlantic).
Usually about 50 a year, similar to TRS but not as severe, can develop into TRS.
Caused by disturbance in the low pressure line (parallel isobars) along ITCZ.
Activity in REAR of trough.
Westerly Wave
Interconnected warm front and cold front bands (associated with polar fronts) moving west to east creating a wave pattern.
Chinook wind
Fohn wind over the Rockies to the east.
From southern Colorado up to Mackenzie basin, can create a rise of 20C in 15 minutes.
Blows for several days and can clear snow on the eastern side of rockies.
Mistral wind
Valley wind through Rhone valley (between Masif central and alps).
High pressure over central France, low pressure over Gulf of Genoa during winter.
Temps well below zero, turbulent flying conditions and 40 to 75kts.
Bora wind
Katabatic wind.
Cold, dry NE wind down mountains from central Europe over adriatic (high pressure central Europe, low over adriatic).
Strongest in winter (70kt, gusts up to 100kt)
Mediterranean winds
Blow out of winter high pressure region over Sahara and Northern Africa.
Hot & dusty southerly winds, usually spring time lasting about a day. Can reach southern Europe and pick up moisture (stratus, drizzle fog).
Scirocco - Algeria
Ghibli - Libya
Khamsin - Egypt
SGK - ALE [strong goat kick Ale]
Pampero wind
Burst of cold polar wind from W/SW/S on the pampas in south Brazil/Argentina/Uraguay.
Common in the southern hemisphere winter.
Harmattan wind
Winter wind in line with trade winds (NE towards ITCZ) from Sahara over West Africa.
Cool and dusty, reducing visibility below 1000m.
Dust can get to 10,000ft or more.
Doldroms
Occur where ITCZ coincides with the equator. Winds turned as the coriolis forces when crossing the equator, being heated from beneath, creating light and variable winds and lots of convective activity (Cb, TS).
Horse latitudes
30 degrees N/S
Slack winds, no rain, around subtropical highs
Mesoscale convective system
- description
- size
- shape
Thunderstorm regions found in ITCZ area.
Mesoscale convective complex is a convective system with 100,000sq km cloud tops below -32C and 50,000sq km with temp below -52C.
Eccentricity <0.7 so fairly round in shape.
Long lived and form overnight.
Cold Air Outbreaks in tropics (ITCZ)
Pampero in Argentina summer and Blizzards in North America.
Cold air (from antarctic or canada) pushes into warm, moist air at ITCZ. Big temperature differential form a low pressure area, with strong storm winds and significant snowfall.
NW Africa weather
Below 20 degrees N, get SW monsoon in summer and Harmattan in winter (NE trade winds).
Above 20 degrees N get winter wet season (polar lows, wind from canaries) and NE trade winds in summer.
NW Africa advection fog
In the area above ITCZ (changes from summer to winter) offshore winds towards Canaries can create advection fog, which can be blown back onto land by sea breeze.
Equatorial double rains
Mostly in East Africa (Nairobi).
ITCZ passing over the equator in Spring and Autumn. Especially North going rains in Spring, bring the “long rains” fed from Indian ocean.
South going rains from dryer areas but still bring 25% of annual rain.
North Atlantic weather - Winter
Cold air from NA high moves SE over sea and encounters warm gulfstream waters of NA east coast, creating the western end of the polar front.
Azores High ensures the depressions move NE towards UK (from SW Florida to SW UK).
Iceland statistical low caused by the trailing end of the depressions.
London vs New York winter
London further North but where London gets dry cold air from Siberia, New York gets cold continental flow going out over warmer sea, becoming unstable, then bringing snow back over land.
North Atlantic weather - Summer
Everything moves north, inc. Azores high and Iceland low. Temperature differences reduced so polar front, jet stream (etc) intensity reduced.
Polar front between Labrador/Newfoundland and Scotland/Norway.
Advection fog over cooler seas.
Hurricans and Easterly waves closer to equator.
NW Europe weather - Winter
Polar front depressions move from Atlantic to Russia, bounded between mountains of Norway and the alps.
Get some E/NE dry winds from Siberian high, mostly Westerly winds though.
Lots of cloud and precipitation.
NW Europe weather - Summer
Polar front further north and less strong.
Lots of local depressions due to insolation, which creates the dominant cloud being Cu/Cb type and heavy showery precipitation.
Mediterranean weather
Hot summers, warm wet winters
Much less than 700mm annual rain.
Arabian weather
Winter: Siberian high can impact the area, crossing med to pick up moisture and create instability (Cu, Cb, TS) in North.
Summer: Baluchistan low over rocky Baluchistan due to insolation, passing thermal equator so very hot
Crachin
In Hong Kong from Jan to Apr as ITCZ moves far to south, air comes from the warm Kurosiwo sea over seasonally cool HK waters.
This creates advection fog, low stratus drizzle and gloomy conditions in HK for this period.
Singapore to Tokyo via Hong Kong weather
Summer: Typhoons, SW monsoon
Winter: NE monsoon, Crachin Hong Kong
Both monsoons convective TS type weather
Australia weather
Winter (local): Polar front depressions over south, SE trade winds in north, subtropical jet stream above
Summer: Tropical cyclones North side (W and E), convective activity in N due to ITCZ
Nairobi region weather
ITCZ transits create long rains in spring and short rains in autumn.
ICAO weather domains (altitude)
Low level: SFC to FL100
Medium level: FL100 to FL250
High level: FL250 to FL630
Polar orbiting satellites
Inclined @ 99deg to Equator, take 1h42 to orbit earth at 820 to 870km height covering 1500nm wide band.
Any point on globe experiences a southbound pass in morning, northbound in afternoon.
Geostationary satellites
Stationary over equator at 36000km.
Less clear definition towards poles can be corrected with computer processing.
METAR
Meteorological Aerodrome Report
Half hourly report of current weather conditions at an aerodrome
Wind measurement basis:
- Local reports
- ATS units
- METARs
- SPECIs
Local & ATS: Average of last 2 mins
METAR & SPECI: Average of last 10 mins
METAR: “220V300”
Appears after wind information indicating variability of wind direction, if this is more than 60 degrees.
METAR visibility
- 2 special values
Four digits for metres of visibility.
“9999” means over 10km
“0000” means less than 50m
METAR RVR
- format
- point measured at
- value over what time
After visibility, state runway in use then RVR if either visibility or RVR are less than 1500m.
eg “R30/1100”
Note: Refers to RVR at touchdown point, not average of the 3 points.
Note: Lowest value in last 10 mins
METAR RVR
- codes
“R30/P1500”: RVR > 1500m (i.e. plus)
“R30/M0050”: RVR < 50m (i.e. minus)
If RVR increases by 100m in last 10 mins add “U” at end, if decreases add “D”, if no trend add “N”
Can have two figures separated by “V” if significant variation in last 10 mins.
METAR weather codes
“+”
“-“
“ “
“VC”
”+”: Heavy
“-“: Light
“ “: Moderate
“VC”: In the vicinity (within 8km)
METAR weather codes
“MI”
“BC”
“BL”
“SH”
“TS”
“FZ”
“PR”
“DR”
“MI”: Shallow (<2m above ground)
“BC”: Patches
“BL”: Blowing
“SH”: Showers
“TS”: Thunderstorms
“FZ”: Freezing (supercooled)
“PR”: Partial (covering of aerodrome)
“DR”: Drifting
METAR precipitation codes
“DZ”
“RA”
“SN”
“IC”
“PL”
“GR”
“GS”
“UP”
“PY”
“DZ”: Drizzle
“RA”: Rain
“SN”: Snow
“IC”: Ice Crystals
“PL”: Ice pellets
“GR”: Hail
“GS”: Small hail (<5mm)
“UP”: Unknown precipitation
“PY”: Spray
METAR obscuration codes
“BR”
“FG”
“FU”
“VA”
“DU”
“SA”
“HZ”
“BR”: Mist (vis 1000 to 5000m)
“FG”: Fog (vis <1000m)
“FU”: Smoke
“VA”: Volcanic Ash
“DU”: Dust
“SA”: Sand
“HZ”: Haze
What is obscuration?
Something blocking vision that is NOT precipitation (e.g. haze, mist, sand)
METAR other codes
“PO”
“SQ”
“FC”
“SS”
“PO”: Dust/sand whirls
“SQ”: Squall
“FC”: Funnel cloud (tornado)
“SS”: Sandstorm/duststorm
When is TS included in METAR?
Thunder heard in last 10 minutes
Cloud types specified in METAR
Cb: Cumulonimbus
TCu: Towering Cumulus
CAVOK requirements
- Visibility >10km
- Cloud base >MAX(5000ft, MSA)
- No Cb or TCu
- No significant weather in vicinity (e.g. RERA)
METAR temperature
- depiction
- rounding
Temperature and dew point as 2 digits with “/” in between. If negative use “M”.
e.g. “10/M02” -> 10C, dewpoint -2C
Temperatures rounded UP (+0.5 up to +1, -0.5 up to 0)
METAR pressure information
QNH reported with “Q” followed by four digits, rounded DOWN.
In USA inches of mercury reported with the letter “A” (e.g. 29.89 as A2989)
METAR “RE”
“RE”: Recent - in the last hour
e.g. “RETS” thunderstorm in last hour
METAR “WS”
“WS”: Windshear
Could specify a runway number or “ALL RWY”
METAR
“TREND”
“BECMG”
“TEMPO”
“NOSIG”
“TREND”: Valid for 2 hours FROM TIME OF OBSERVATION
“BECMG”: Changes becoming permanent
“TEMPO”: Less than one hour and less than half the time period
“NOSIG”: No changes expected in next 2 hours
METAR runway state
- Structure of code
Contaminated: “RXX/ABCCDD”
“RXX”: Runway no or 88 for all, or 99 message repetition
“A”: Type of deposit
“B”: Extent of contamination
“CC”: Depth of deposit
“DD”: Friction/braking coefficient
METAR runway state
- Type of deposit
0: Clear & dry
1: Damp
2: Wet
3: Rime/frost
4: Dry snow
5: Wet snow
6: Slush
7: Ice
8: Compacted/rolled snow
9: Frozen ruts/ridges
/: Not reported (e.g. being cleared)
METAR runway state
- Extent of contamination
1: 10% or less
2: 11%-25%
5: 26%-50%
9: 51%-100%
/: Not reported (e.g. being cleared)
METAR runway state
- Depth of deposit
00 to 90: mm of coverage
91: not used
92: 10cm
93…98: 5cm increments
99: Depth not reported, runway not operational
//: Not significant or measurable
METAR runway state
- Friction/braking coefficient
28: 0.28 friction coefficient
35: 0.35 friction coefficient
91: Braking poor
92: Braking medium/poor
93: Braking medium
94: Braking medium/good
95: Braking good
99: Figures unreliable
//: Not reported
METAR
“SNOCLO”
Closed due to contamination
METAR
“SPECI”
“An aviation selected special weather report”
Special report due to significant change in conditions (improvement or deterioration)
“SA” at beginning indicates METAR, “SP” indicates SPECI (not always there)
Will include all parameters updated to time of the SPECI, not just the change.
METAR & SPECI vs
Met Report & Special
In both cases SPECI/special means an updated version due to changing conditions.
METAR/SPECI is for “general public”, i.e. wider audience over VOLMET (for example)
Met Report/Special is for “local public”, i.e. ATIS for people at the aerodrome
TAF
“NSC”
No significant cloud
Means no cloud below 5000ft or sector altitude, no Cb or TCu, but CAVOK not appropriate.
TAF
“TX”
“TN”
“AMD”
“TX”: Maximum temperature
“TN”: Minimum temperature
“AMD”: Amendment
- FRQ
- OCNL
- ISOL
Which one is “well separated”?
FRQ: Frequent, spatial coverage greater than 75% of area
OCNL: Occasional - WELL SEPARATED, maximum spatial coverage of 50 to 75%
ISOL: Isolated, less than 50% of area
Form 215/415 abbreviations:
BLW
BTN
CIT
CLD
COR
COT
LAN
LCA
LSQ
BLW: Below
BTN: Between
CIT: Near or over large towns
CLD: Cloud
COR: Correction
COT: At the coast
LAN: Over land
LCA: Locally
LSQ: Line squall
Form 215/415 abbreviations:
LV
SEV
SFC
VAL
VRB
VSP
WRNG
WS
WSPD
LV: Light & Variable
SEV: Severe
SFC: Surface
VAL: Valleys
VRB: Variable
VSP: Vertical speed
WRNG: Warning
WS: Windshear
WSPD: Wind speed
Reasons for SIGMET
TS/TSGR (only OBSC, EMBD, FRQ or SQL)
Heavy hail
Tropical cyclone
Freezing rain
Severe turbulence
Severe icing
Severe mountain waves
Heavy sand/dust storm
Volcanic ash cloud
AIRMET
Lower level weather warnings (usually FL100, could by FL150+ in mountainous regions)
Reasons for AIRMET
SFC wind > 30kt
SFC VIS < 5000m
TS/TSGR (incl. ISOL, OCNL)
Mountain obscuration (MT OBSC)
Moderate Icing, turb or MTW
BKN/OVC cloud <1000ft
Cb or TCu (ISOL, OCNL or FRQ)
First 4 letters of SIGMET represent
Name of the air traffic services controlling unit
SIGMET abbreviations
NC
OTLK
STNR
TC
VA
WKN
NC - No Change
OTLK - Outlook
STNR - Stationary
TC - Tropical Cyclone
VA - Volcanic Ash
WKN - Weakening
Which agency prepares SIGMET and AIRMETs?
Meteorological WATCH office
AIRMET & SIGMET validity time
4 hours
Frequency for VOLMET
Locally on VHF
Globally on HF
VOLMET code
Uses plain language
VOLMET update frequency
30 minutes
VOLMET content
- 3 standard
- 2 potential
METARs, SPECIs & TRENDs
Potentially also TAFs and SIGMETs
ACARS
Ground air link with weather information, to pass general warnings, SIGMETs, ATIS for destination
When are ATIS broadcasts updated?
When aerodrome or weather info changes, or 30 minutes
Where else are ATIS messages broadcast other than a VHF channel?
VOR and potentially through ACARS
In-flight weather briefings (2)
Can arrange pre-flight a special in-flight enroute weather service with relevant Meteorological Authority.
Also get a diversion briefing from ATC unit if required to divert into area you don’t have weather info for, will include SIGMET and AIRMET warnings.
AIREP
Report from pilot on weather (PIREP in USA). Can be handed in at end of flight as written report.
AIREP SPECIAL (or ARS) reported immediately in case of:
Mod/Sev turb or icing
Sev MTW
TS (OBSC, EMBD, WDSPR or SQL)
Heavy dust/sand storm
Volcanic ash
Sections of a special AIRREP
1) Aircraft identification, position, time, level
2) n/a
3) Meteorological info
AMDAR
Aircraft Meteorological Data Relay
System that automatically transmits weather data from aircraft to WAFCs
GAMET
Plain language general aviation forecast for low level flights in an area.
Timing of MSL pressure charts
Data taken every 6 hours from midnight.
MSL charts published 4 hours later.
24 hour forecast charts published 5 hours after the observation (e.g. 5am for midnight data), covering the next 24 hours.
Weather chart symbols
- Fronts at surface and above surface
- Quasi-stationary fronts
- Convergence line
- Trough axis
- Ridge axis
Significant weather chart timing
Prepared 24 hours in advance of a time period 3 hours each side of the synoptic times (0000, 0600, 1200, 1800).
Upper wind and temperature charts
- How is temperature information displayed?
Number next to the wind symbol is temperature, assumed negative (has “+” or “PS” preceding if positive).
Interpreting airborne weather radar
Large areas of red might not be an issue, just warm moist areas of rain. Having the different colours close together however indicates sharp updrafts at the edge of a cell, more risk of turbulence.
Round shapes less concerning than fingers, hooks, U-shapes and scalloped edges.
Use of satellite imagery
To locate frontal systems, not areas of precipitation (radar better for this).
Gridded forecast
Produced by the World Area Forecast Centres (WAFC) [Washington/Kansas & London/Exeter] using Numerical Weather Prediction (NWP) models.
Produce 3d gridded forecasts including temp, humidity, wind, tropopause height, Cb, turb & icing - NOT jet streams.
Low Drifting Sand/snow/dust definition
Below 6ft/2m (visibility above eye level not affected)
AKA “Shallow”, “MI” in code (eg MIFG)
Sand diameter and wind speed required to impact visibility
0.08 to 0.3mm
20kt
Virga
- description
- what does it indicate?
Rain which evaporates before reaching ground. Typical of microbursts under TS.
INDICATION OF WINDSHEAR
White areas on IR satellite image
High cloud is bright white, lower cloud is darker, warm land is very dark.
If IR is bright white and visual image is not “whispy” that means cloud tops are high and not cirrus, so possibly TCU/Cb.
Cloud to cloud vs intra cloud lightning
Cloud to cloud is between 2 different clouds, intra-cloud is inside a single cloud
Frequency of windshear alert updates
Every minute until they drop below 15kt
Pressure level variation flying across mountain ridge in strong winds
World Area Forecast Centre responsibilities
Significant weather forecasts
Upper air forecasts
Sublimation
Deposition
Sublimation: Solid to gas
Deposition: Gas to solid
0 degree altitude (isotherm) altitude
- polar regions
- temperature regions
- tropics
Polar: Ground
Temperate: Ground in winter
10,000ft in summer
Tropics: 15,000ft in winter
17,000ft in summer
Stormscope
Device that identifies thunderstorms via electrical discharges (useful for embedded Cb)
Purpose of merging gridded forecasts with pilot reports
Improve situational awareness
Increase or decrease airspeed in turbulence
DECREASE - to avoid damage to airframe
But be careful of stall risk!
How does aircraft initiate lightning
Builds up charge by flying through electrically charged air, can initiate a lightning discharge itself.
What maintains the global potential (electricity) difference between ionosphere and ground?
Thunderstorms around the globe
St Elmos fire
Static discharge, doesn’t affect instruments or anything else, occurs on sharp edges (wipers, pitot tube)
Which cloud type has a halo?
Cs
Geographical location of fastest jet stream
Japan
How to get out of rain ice?
Do a 180
What percentage of all gas and all water is in the troposhere?
75% of gas
90% of water
Bays Ballots Law
In NH, low on the left (when going with the wind)
Vertical visibility
- steps and limit in metres & feet
100ft steps to 2000ft
30m steps to 600m
Where does doppler radar measure turbulence?
Thunderstorms, picks up precipitation.
