Meteorology Flashcards
What types of meteorological hazards are there?
- Visibility
- Fog
- Wind/Changes in wind
- Cloud height
- Turbulence (E.g. in clouds)
- Icing
- Lightning
- Weather fronts
What does WAFS stand for and what does it do?
- World Area Forecast System
- Supplies meteorological authorities and other users with forecasts of global meteorological parameters like upper wind and temperature information as well as forecasts of significant weather phenomena.
What does WAFC stand for?
There are 2 WAFCs within the WAFS. Where are these located?
- World Area Forecast Centre
- London and Washington
Why has an ICAO/International Standard atmosphere been defined?
To establish standards for:
- Calibration of instruments.
- Vertical separation of ACFT.
- To compare power parameters of different ACFT and engines.
Describe ISA at MSL
- Temperature: +15°C
- Pressure: 1013.25 hPA = 29.92 inch HG
- Barometric Lapse Rate: 30ft/1hPA
- Density: 1.225kg/m^3
- Relative humidity: 0%
What is the gas composition of ISA?
What is the maximum percentage composition that water can reach within the atmosphere?
- 78% Nitrogen (N2)
- 21% Oxygen (O2)
- 0.9% Argon (Ar)
- 0.03% Carbon Dioxide (CO2)
- No water by ISA Definition (In reality can be up to 4% all within the troposphere)
Describe the ISA Troposphere in terms of vertical limits and temperature gradient.
What is the name of the boundary between the troposphere and the stratosphere?
- Extends from MSL to approximately 6-8km (pole), 11km/36100ft (45° latitude) and 16-18km (Equator).
- Temperature Gradient: approximately -2(°C)/1000ft.
- Boundary between troposphere and stratosphere called the Tropopause.
Describe the ISA Lower Stratosphere in terms of vertical limits and temperature gradient.
- Extends from tropopause to around 20km/65600ft.
- Isothermal Layer - Temperature remains constant - around -56.5°C.
Describe the ISA Upper Stratosphere in terms of vertical limits and temperature gradient.
Where does ISA definition end?
- Extends from around 20km/65600ft to around 50km.
- Inversion - Temperature begins to increase.
- Temperature Gradient: around +0.3K(°C)/1000ft.
ISA definition ends at around 32km/105,000ft.
How is heat transferred from the surface to the overlying atmospheric layers?
Mainly by conduction and convection.
Where do almost all clouds and weather occur and why?
Within the troposphere due to water content.
Why do high temperatures occur in the upper stratosphere?
- Because of the presence of ozone between 20km and 50km which absorbs ultra violet radiation from the sun.
Note: Up to 50°C.
What is the atmospheric layer above the stratosphere called?
What are the vertical limits of this layer?
What happens to temperature with altitude within this later?
- Mesosphere.
- Extends from stratosphere to around 80km.
- Temperature decreases with altitude.
What is the atmospheric layer above the mesosphere called?
What are the vertical limits of this layer?
What happens to temperature with altitude within this later?
- Ionosphere
- Extends from mesosphere to around 400km
- Temperature increases again due to low air density - very little energy is required from the sun to heat the air.
What is the layer beyond the ionosphere called?
Exosphere
How is air pressure measured?
Which units are used?
Barometer (hPA, inch Hg)
How is air temperature measured?
Which units are used?
Thermometer (°C)
How is air density measured?
Which units are used?
It is calculated (kg/m^3)
How is dew point measured?
Which units are used?
It is calculated (°C)
How is relative humidity measured?
Which units are used?
Hygrometer (%)
How is cloud cover measured?
Which units are used?
Ceilometer (okta)
How is cloud base measured?
Which units are used?
Ceilometer (ft)
How is precipitation measured?
Which units are used?
Rain gauge (mm)
How is ground visibility measured?
Which units are used?
Estimated by observer (m)
How is RVR measured?
Which units are used?
Transmissometer (m)
How is wind direction measured?
Which units are used?
Wind vane/wind sock (°)
How is wind speed measured?
Which units are used?
Anemometer (kt)
What does RSMC stand for?
Regional Specialised Meteorological Centre
What does MWO stand for?
Meteorological Watch Office
What does VAAC stand for?
Volcanic Ash Advisory Centre
What TCAC stand for?
Tropical Cyclone Advisory Centre
What is SADIS and what is its purpose?
SATellite DIStribution
- High quality, cost effective and error free satellite data distribution system developed on behalf of ICAO.
- Provides vital flight info to aviation users around the worlds.
At what temperature does water have its highest density?
4°C
What is sublimation and deposition?
- Sublimation is when a solid turns into a gas without going through the liquid phase.
- Deposition is when a gas turns into a solid without going through the liquid phase.
What is the energy spent during a change in phase called?
Latent heat
Define humidity
Humidity is the content of water vapour in the atmosphere expressed as a %. It is measured using a hygrometer, psychrometer or dew point mirror.
What does it mean when air is unsaturated, saturated or supersaturated?
Unsaturated - If maximum content of water vapour has not been reached
Saturated - If water vapour content is at its maximum
Supersaturated - If more water vapour is added to saturated air it condenses spontaneously and forms droplets of fog or cloud
What is the dew point?
The temperature at which condensation occurs.
What is spread?
What would a high or low spread indicate?
The difference between the temperature and the dew point. A higher spread would signal a low probability of fog while a low spread would signal high probability of fog.
True or false - At higher temperatures, air can contain more water vapour than at lower temperatures
True
What are the 2 types of barometer used to measure pressure?
Mercury and aneroid barometer
How does an aneroid barometer work?
Operates on difference between air pressure and pressure of the atmosphere within the aneroid cell. Pressure changes cause the cell to deform.
What happens to pressure with altitude?
Decreases exponentially.
Note: It drops by around half every 18000ft.
How is the atmosphere heated? (7)
- Short wave radiation from the Sun absorbed as latent heat. Earth re-radiates long wave terrestrial radiation.
- Absorption of ultra-violet by the ozone.
- Scattering by air molecules
- Reflection by surface (ice especially!) and clouds
- Latent heat released by condensation.
- Conduction between air and surface.
- Long wave radiation absorbed by clouds and water vapour.
Define inversion
A layer in the atmosphere where temperature increases with height
What is density altitude?
The altitude in the ISA at which the air density would be equal to the actual air density at the place of observation
How much longer does ICAO say a RWY needs to be for every 1000ft elevation?
7%
What happens to the amount of oxygen in the atmosphere with increasing altitude?
It decreases.
Roughly how does an altimeter work?
The altimeter measures pressure using 3 coupled aneroid capsules. The pilot can set a reference pressure by altering the sub scale. The altimeter then indicates the vertical distance between pressure measured at the static ports and the set sub scale value under the assumption of ISA conditions.
The altimeter indication is usually not identical to the true vertical distance. What are the 4 possible reasons for this?
- Instrument error (hysteresis and elastic drift, friction and calibration - shall not exceed a prescribed value)
- Dynamic error (turbulent air and motion of ACFT)
- Incorrect sub scale setting
- Deviations from ISA such as air humidity, temperature and pressure.
When flying from a region of high to low temperature/pressure with a constant sub scale setting…
True level will decrease (From high to low, beware below)
What do the terms QFE, QNH and QFF mean?
QFE - Pressure measured at the surface/aerodrome.
QNH - Pressure reduced to MSL assuming ISA conditions.
QFF - Pressure reduced to MSL according to actual weather conditions (If Temperature is less than ISA then QFF will be greater than QNH)
What is QNE?
The altitude indicated at the landing RWY THR when the altimeter sub scale is set to 1013.2hPa. It is used if QNH is less than 950hPa or greater than 1050hPa
What is fog (FG) and what problems does it cause in aviation?
A thick cloud of tiny water droplets suspended in the atmosphere at or near the earth’s surface which reduces visibility to less than 1km.
- Airport capacity lowered due to higher separation
- Airport closure
- More ACFT going around
- ACFT diversions may occur
- Delays due to holding
- Taxi orientation problems
What are the conditions for radiation fog?
What is the thickness of radiation fog?
What time of day is radiation fog likely to occur?
In which season is radiation fog most likely to occur?
How is radiation fog dissipated?
Type of fog which influences aviation the most.
Can form over a solid surface after sunset when the ground cools, causing the air above it to reach saturation. It builds up throughout the night becoming its most dense just after sunset. The sun quickly dissipates the fog or a change in weather.
Other conditions are a clear sky/few clouds, winds of less than 10 knots and high humidity.
Thickness is less than 2m for shallow fog (MIFG) and MAX 100-200m for broken fog (BCFG).
More likely to occur during the winter (also long lasting in winter )
What are the conditions for advection fog?
What is the thickness of advection fog?
What time of day is advection fog likely to occur?
In which season is advection fog most likely to occur?
How is advection fog dissipated?
Forms when a warm, moist air mass is advected by the wind over a cold surface at least as cold as the dew point of the advected air.
Widespread and is 500-1000m thick.
Can occur at any time of day.
Occurs over the continent during autumn and the sea during spring.
Dissipated by a change in weather or strong winds.
What are the conditions for mixing fog?
What is the thickness of mixing fog?
What time of day is mixing fog likely to occur?
In which season is mixing fog most likely to occur?
How is mixing fog dissipated?
Forms when a warm humid air mass mixes with a cold humid air mass.
Can be very thick at fronts
Can occur at any time of day.
More likely to occur during winter.
Dissipated by a change in weather of if a front moves through.
What is steam fog?
A type of mixing fog which forms when cold air is overlying a warm surface.
What is upslope fog?
Clouds which form in mountainous areas may lie on the ground which can appear as fog to skiers.
Define visibility
The distance in which certain visual objects can be exactly identified. Dark objects against bright backgrounds during the day and bright objects clearly visible at night.
What is wind?
The vector of the horizontal motion of the air, characterised by the wind speed and direction. According to international agreement it is to be measured 10m above a ground which is free of obstacles.
What is the pressure gradient force?
The force exerted on the air when areas of high pressure air flow to areas of low pressure air. It acts perpendicular to isobars.
How does a sea breeze form and what are the hazards associated with one at coast aerodromes?
At the coast on a day with calm, fine weather, the sun heats the ground to a greater degree than the ocean.
The air above the ground subsequently heats up, causing it to rise and expand, creating a pressure difference along the coast line.
This subsequently causes the cooler, more dense air over the ocean to flow inland to replace the warm air, thus creating a sea breeze.
Hazards:
Wind shear, strong surface winds and possibly advection fog from the cold sea.
How does a land breeze form?
Opposite of a sea breeze. The circulation of air is reversed and the air will flow out to sea instead. Usually less intense than a sea breeze.
What is an adiabatic process?
A process where volume, pressure and temperature change with no transfer of energy across the boundary of the system.
What is the dry adiabatic lapse rate?
-3K per 1000ft (Upward movement)
Describes temperature changes during vertical movements without changes in phase (in unsaturated air)
What is the wet adiabatic lapse rate?
-2K per 1000ft (Upward movement)
Describes temperature changes during vertical movements with changes in phase (in saturated air)
What are the 2 main cloud types? Also state their origin, stability, vertical speed, particle size, horizontal extension, precipitation types, icing and turbulence levels.
Cumuliform: Origin - Thermal convection Stability - Unstable Vertical speed - 100-1000ft/min Particle size - Large Horizontal extension - Small Precipitation - Showers Icing - Clear ice Turbulence - Moderate-severe
Stratiform: Origin - Forced lifting Stability - Stable Vertical speed - 1-10 ft/min Particle size - Small Horizontal extension - Large Precipitation - Continuous Icing - Rime ice Turbulence - None
What cloud types can be found between the 20000ft and the tropopause?
Describe the appearance of each and state the likelihood of turbulence, icing and precipitation.
Cirro type clouds (Ice particles):
Cirrus (Ci) - Feather like structure. Nil turbulence, nil icing and nil precipitation.
Cirrocumulus (Cc) - Separate flakes or little balls. Rapid frequency turbulence and nil icing and nil precipitation.
Cirrostratus (Cs) - Thin white layer often with a halo. Nil turbulence, nil icing and nil precipitation.
What cloud types can be found between 8000ft and 20000ft?
Describe the appearance of each and state the likelihood of turbulence, icing and precipitation.
Alto type clouds (Water and ice particles):
Altostratus (As) - Layered uniform grey, sometimes with outline of sun or moon. Nil turbulence, rime ice possible and nil precipitation.
Altocumulus (Ac) - Separate flakes with vertical extent great than Cc. Turbulence, clear ice is possible and nil precipitation.
What cloud types can be found below 8000ft?
Describe the appearance of each and state the likelihood of turbulence, icing and precipitation.
Stratus (St) - Uniform grey with low base. Nil turbulence, rime ice possible and drizzle possible.
Cumulus (Cu) - Isolated clouds with vertical extension. Turbulence, clear ice possible, showers possible.
Stratocumulus (Sc) - Layered cloud with vertical extension, inversion above. Turbulence, mixed ice possible and nil precipitation.
What are the rain bearing cloud types which can extend across low, middle and high levels?
Describe the appearance of each and state the likelihood of turbulence, icing and precipitation.
Nimbostratus (Ns) - Dark grey with precipitation. Nil turbulence, rime ice possible, incessant rain.
Cumulonimbus (Cb) - And Towering Cumulus (TCu). Great vertical development, typical anvil. Severe turbulence, clear ice and heavy showers/rain/hail/snow
Define ceiling
The height of the base of the lowest layer of cloud below 20000ft covering more than half the sky.
What are the different levels of cloud cover?
Sky divided into 8ths called octa’s
0 Octa's - Sky clear 1-2 Octa's - FEW (Few clouds) 3-4 Octas - SCT (Scattered clouds) 5-7 Octas - BKN (Broken Clouds) 8 Octas - OVC (Overcast)
9 = Sky obscured
What is virga?
Precipitation which evaporates before reaching the ground.
Define precipitation and give examples
All forms of liquid or solid water falling from the atmosphere and reaching the ground.
Falling precipitation: Rain, snow, hail, etc
Deposed precipitation: Dew, hoarfrost and rime ice.
Stored precipitation: Snowcoverage
What types of precipitation can a stratus cloud produce?
Drizzle, freezing drizzle and snow grains
What types of precipitation can a nimbostratus cloud produce?
Snow, continuous rain, freezing rain and ice pellets
What types of precipitation can a cumulonimbus cloud produce?
Snow, hail and rain showers
Why does precipitation present a hazard to aviation?
It can:
Reduce visibility, affect engine and aerodynamic flight performance, increase braking distance, cause icing. Hail can damage an ACFT both on the ground and in flight (more dangerous)
What are the meteorological mechanisms for icing? (4)
Transport of warm droplets to above the freezing level:
- Clouds which start to form below freezing level. Further adiabatic lifting may cool the cloud below freezing point.
Freezing rain:
- Sometimes occurs in warm fronts in winter. Worst cause of structural icing - occurs extremely quickly.
Condensation below 0°C:
- Clouds which form above freezing level contain supercooled water and possibly ice crystals.
Cooling by emerging radiation:
- Top of a formerly warm cloud may be cooled at night by emerging IR radiation causing the freezing level to drop into the cloud.
What determines the type of icing?
The droplet size
What types of icing are there?
Rime ice:
Caused by small supercooled droplets like those found in St and Ns clouds which freeze instantly on collision with the ACFT retaining their shape. Inclusion of air leads to a rough and brittle structure. Changes the shape of the wing, therefore increasing drag an causing a loss of lift
Clear ice:
Caused by large supercooled droplets like those found in Cu and Cb clouds. Enough latent heat is released in the freezing process to delay it, allowing the droplets to flatten and even flow rearward on the wing. Builds up without air and is hard, clear and heavy. Difficult to remove. Increases ACFT weight, flight controls may block and causes a loss of lift and increased drag.
Mixed Ice:
A mixture of clear and rime ice. Due to presence of small and large droplets. Sc and Ns clouds on windward side of mountains)
What factors influence the intensity of icing?
Liquid water content:
- Main factor.
- Worst icing conditions are met between 0-6°C
- Intensity of icing is greater in cumuliform clouds than in stratiform although duration of cruise in stratus is usually much longer allowing for a lot of ice accumulation over time.
ACFT Type:
- Shape of wings, thin exposed parts.
- Onboard anti-icing and de-icing equipment.
TAS:
- Adiabatic compression at stagnation points leads to a temperature increase where icing is most probable.
- Light ACFT and helicopters are more vulnerable to icing than jet ACFT at cruising speed.
What actions should be taken for light icing during flight?
Remain on course an altitude and use anti-icing devices temporarily.
What actions should be taken for moderate icing during flight?
Use anti icing devices at all times, course and/or altitude change may be required.
What actions should be taken for severe icing during flight?
Change course and/or altitude immediately, anti icing devices are not sufficient to prevent/remove ice.
What are the hazards of ice accretion to ACFT?
- Increased weight
- Increased drag
- Loss of lift
- Loss of thrust
- Higher stalling speed
- Malfunctioning/blocked ACFT control surfaces and landing gear.
- Engine damage (Ingestion of large pieces of ice)
- Frozen windshield
Up to what OAT temperature and spread shall carburettor icing be anticipated?
Up to 20°C
Spread of 15-20K even outside of clouds.
What are the dangers of carburettor icing?
Insufficient fuel/air flow causing loss of power or engine failure.
What is an inversion?
A layer of the atmosphere where temperature increases with height.
What are the 3 types of inversion?
Surface inversion - Conditions for creation are the same as for radiation or advection fog.
Warm front inversion - Occurs in the transition zone between warm air mass (above) and cold air mass (below)
Subsidence inversion - develops in the friction layer. During the winter, high chance of low stratus, fog and drizzle. In summer low cumulus clouds.
Why are inversions dangerous when close to the surface?
- Wind shear
- Icing
- Reduced climb performance because of a sudden drop in density
These negative effects may accumulate.
With regards to inversions, what would trigger an aerodrome warning
A strong inversion with a temperature rise >=10K below 650ft
Where does the tropopause have its greatest height?
At the equator (16-18km)
What is the predominant wind direction in the atmosphere?
Westerly
Which weather phenomena is related to SW trade winds
Monsoon
What is an air mass?
A large volume of air of a single origin, characterised by horizontally uniform temperature, humidity and purity (pollution level)
Which air masses affect the UK?
Polar maritime, returning polar maritime, arctic maritime, tropical maritime, polar continental and tropical continental
What are the 3 cells of general circulation in the atmosphere?
Hadley, ferrel and polar cells
What type of weather is common at the Inter Tropical Convergence Zone (ITCZ)?
Lightning and heavy rain
What are the characteristics of polar maritime air?
Most common air mass in the UK Temperature: Cold Weather: SHRA, SHSN, GR, TS Clouds: Cu, Cb Wind: Strong, gusty NW wind Visibility: Good Hazards: TS, GR, TURB
What are the characteristics of returning polar maritime air?
Temperature: Mild Weather: Largely dry, SHRA Clouds: Low level Wind: Westerly wind Visibility: Good Hazards: Nil
What are the characteristics of arctic maritime air?
Uncommon in summer Temperature: Very cold in Scotland. Not as cold as polar maritime over the rest of the UK. Weather: SHGR, SHSN Clouds: None, possibly Cu Wind: N wind Visibility: Good-very good Hazards: +SH, +TS, low temperatures
What are the characteristics of tropical maritime air?
Temperature: Mild in winter, warm and humid in summer
Weather: DZ, FG (SW of coast and hills), Fine weather in lee of hills
Clouds: Low clouds
Wind: SW, regular, no turbulence
Visibility: Moderate-poor
Hazards: Nil
What are the characteristics of polar continental air?
Only in winter (Nov-Apr). Characteristics depend on length of sea track. (long sea via North sea and short sea via channel)
Temperature: Long sea track: cold, short sea track: Coldest temp with severe frost
Weather: Long sea track: SHRA, SHSN, short sea track: Nil
Clouds: None, possibly Cu
Wind: NE
Visibility: Good-very good
Hazards: Nil
What are the characteristics of tropical continental air?
Temperature: Cool in winter, hot/highest in summer Weather: Dry Clouds: None Wind: SE Visibility: Moderate-poor Hazards: Nil
What is a weather front?
Boundaries or transition zones between air masses where most intensive weather phenomena take place.
What is a warm front?
Describe a warm front in terms of its:
- Rough dimensions
- Cloud types
- Migration speed
- Weather
- Wind
- Pressure
- Temperature
- Cloud base/ceiling
- Visibility
- Flight hazards
Air mass boundary where warm air mass moves towards cold air mass and glides above it.
- Rough dimensions: ~1000km long and ~10km high with ~300km area of intensive weather.
- Cloud types: Ci, Cs, As, Ns (with embedded Cb during the summer)
- Migration speed: ~20kts
- Weather: Continuous RA, SN, FRZA, PL
- Wind: Turns to the right (From S and W)
- Pressure: Decreases as warm air replace cool air
- Temperature: Increases as warm air replaces cool air
- Cloud base/ceiling: Decreasing until ground
- Visibility: Decreases, possibly mixing fox
- Flight hazards: Light to moderate icing, no VMC
What is a passive cold front?
Describe a passive cold front in terms of its:
- Rough dimensions
- Cloud types
- Migration speed
- Weather
- Wind
- Pressure
- Temperature
- Cloud base/ceiling
- Visibility
- Flight hazards
Air mass boundary where cold air moves towards warm air mass and wedges underneath the warm air forcing it to rise. Wind speed decreases with height.
- Rough dimensions: ~200-500km long and ~10km high with ~100km area of intensive weather.
- Cloud types: Ns with embedded Cb, As, Cs, Ci, (Cu)
- Migration speed: ~30kts
- Weather: SHRA, SHSN, TS
- Wind: Gusty, turns to the right (From W to N)
- Pressure: Increases as cold air replaces warm air
- Temperature: Decreases as cold air replaces warm air
- Cloud base/ceiling: Rapidly decreasing/low
- Visibility: Bad during rain
- Flight hazards: Icing, embedded Cb, TS, Gusty winds
What is an active cold front?
Describe an active cold front in terms of its:
- Rough dimensions
- Cloud types
- Migration speed
- Weather
- Wind
- Pressure
- Temperature
- Cloud base/ceiling
- Visibility
- Flight hazards
Air mass boundary where cold air moves towards warm air mass and wedges underneath the warm air forcing it to rise. Wind speed increases with height which causes deep instability in the upper layers of air.
- Cloud types: Cu then TCu then Cb
- Migration speed: ~50kts
- Weather: +SHRA, +SHSN, GR, GS, +TS
- Wind: Strong and gusty, turns to the right (From W to N)
- Pressure: Rapidly increasing
- Temperature: Notable decrease (~10K in summer)
- Cloud base/ceiling: Rapidly decreasing
- Visibility: Bad during rain
- Flight hazards: Heavy TURB, gusts up to 50kts, severe icing, wind shear, Cb, TS
What is SQL and when do they form?
Squall line - A SIGMET item - A line of Cb’s which ahead of the surface of an active cold front.
Usually during the summer, surface warming by the sun adds to the instability of an active cold front which can develop quickly into an SQL.
Hot and humid, very bad visibility.
