Met Lesson 2 Flashcards
ISA Environmental Lapse Rate
+ 15°C, with a lapse rate of -1.98°C/1000ft
Dry Adiabatic Lapse Rate
3°C/1000ft
Never changes
Saturated Adiabatic Lapse Rate
1.5°C/1000ft
Never changes
Dew point temperature has been reached
Latent heat is released during condensation
Calculating Environmental Lapse Rate
ELR = (Change in temperature)/(Change in height)
Unstable Atmospheres
A lapse rate > 3°C/1000ft
Produce towering cumulus if the air is humid and the air parcel reaches dew point
Stable Atmospheres
A lapse rate < 1.5°C/1000ft
Conditionally Stable
Air so dry it cannot form clouds
Eg. A parcel of 13°C air in a 14°C atmosphere will not rise
Conditionally Stable or Unstable
Air that resists an upsetting tendency until it becomes saturated and then, due to the smaller lapse rate, continues to rise by itself
A lapse rate in between 3°C and 1.5°C/1000ft can either be stable or unstable
Stable when dry
Unstable when saturated
The Adiabatic Process
Heated air expands and becomes lighter due to a lower air density than the surrounding/ambient air
The rising air cools and the overall temperature inside the parcel reduces
Air Expansion
Expansion cools a gas
Rising air cools adiabatically due to expansion
Air Compression
Compression heats a gas
Subsiding air will warm adiabatically
Actual Environmental Lapse Rate (ELR)
The vertical temperature profile for the atmosphere over a given point at a specific time during the day
Varies from day to day
Stability
The ability of the air to resist any upsetting tendency
Atmospheric stability depends upon the ELR
Absolute Stability
A parcel of air that is forced to rise, will sink back when the lifting force is removed
Absolute Instability
The inability of air to resist an upsetting tendency even after the removal of the upsetting force
Associated with steep ELRs
High Level Clouds
Cirrus (Ci) Cirrocumulus (Cc) Cirrostratus (Cs) Can create a halo effect Light to moderate turbulence
Middle Level Cloud
Altocumulus (Ac)
Altostratus (As)
If thick, intermittent or continuous rain or snow is common
Can produce virga
Risk of icing: moderate rime and clear ice in the lower levels
Low Level Cloud
Cumulus (Cu)
Stratus (St)
Stratocumulus (Sc)
Nimbostratus (Ns)
Stratus
Cloud ceiling very low Cloud base often ragged/diffused Poor visibility VFR flying difficult or impossible High ground, hills and mountains may be obscured
Nimbostratus
Heavy continuous rain
definite risk of icing: moderate rime ice, clear ice more probable in the lower levels
Cumulonimbus (CB)
Develops from normal cumulus due to any lifting mechanism with a deep unstable atmosphere
Heavy showers, lightning, squalls at the surface, severe to extreme turbulence
Risk of icing: dangerous clear ice likely
Reporting Cloud Cover
FEW: 1 - 2 Oktas
SCT: 3 - 4 Oktas
BKN: 5 - 7 Oktas
OVC: 8 Oktas
Cloud Ceiling
Height AGL of BKN or OVC cloud
Cloud Base
The bottom of any amount of cloud
Types of Precipitation
Drizzle Rain Showers Hail Snow Virga
Intensity of Precipitation
Light (-)
Moderate
Heavy (+)
Continuity of Precipitation
Showers (convective cloud - TCu or CB): Short duration
Intermittent: Short breaks
Continuous (Ns): Periods longer than an hour without breaks
Virga
Falling moisture that evaporates before reaching the ground
Cirrus Cloud
Forms ice crystals and therefore has no precipitation
Cirrostratus
Prevents sun rays from increasing the daytime temperature significantly
Wind
The horizontal movement of air
Pressure Gradient
The PGF is the ‘initiating force’ by initiating the movement from a high to a low
Isobar Spacing
Indicates wind strength
If the isobars are closer the wind is faster and stronger due to the larger pressure differential
Gradient Wind
Blows parallel to the isobars
Gradient wind = PGF + coriolis force
Coriolis Force
Is the deviating or backing force
The air appears to be turning to the left in the Southern Hemisphere due to the Earths rotation eastwards
Weakest at the equator and strongest at the poles
Increased speed = stronger coriolis force
Surface Wind
Is gradient wind affected by the friction of the Earths surface
Up to 3,000ft AGL
Gradient Wind Verus Surface Wind
Surface wind veers more as the coriolis force reduces due to the reducing wind speed
Veers 30 degrees over land compared to the gradient wind
Veers 10 degrees over the sea compared to the gradient wind
Highs
Anti-cyclones
Rotates anti-clockwise in the Southern Hemisphere
Ridge
Isobars stretching out from a high
Weather Associated with a High
Subsiding air is stable and clouds tend to disperse
Subsidence inversions
- may trap impurities and lead to poor visibility
In summer: fine but hazy
Clear nights: radiation fog due to maximum terrestrial radiation
Weather Associated with a High Over Land Versus Sea
Clear and dry when over land
Over the coast higher humidity may produce stratiform clouds with rain
Low
Depression
Clockwise rotation
Air flows into a low pressure system
Trough
Isobars extending out from a low and forming a ‘valleyt’
Weather Associated with a Low
Rising air will cool
Cloud will tend to form
Large Cu, Cb, Ns cloud depending upon stability
Rain and heavy showers
Turbulence
Good visibility (convection disperses the impurities)
Buy Ballot’s Law
If facing your back into the wind the low pressure system will be to your right
Col Area
Area of almost constant pressure between two highs and two lows
Isobars bending away from the centre
Weather in a Col
Wind light and variable
Possible fog in winter
High temps in summer may lead to thunderstorms
Veering
Increasing number or clockwise
Backing
Decreasing number or anti-clockwise
Surface Friction
Due to uneven and different types of terrain
Winds affected by Surface Friction
Speed increases with height (less friction)
Up to about 3000ft AGL
Wind direction veers with decreasing height
Strength of the Wind Affected by Surface Friction
Over land surface wind is one third of the original gradient wind speed
Over water surface wind is two thirds of the original gradient wind speed
Right Drift
Occurs when you are right of track
Left Drift
Occurs when you are left of track
Lowest Wind Speed
Around dawn
At night air is cool and friction is max resulting in calm winds
Maximum Wind Speed
Around 3pm
Instability of the air with convection currents in the afternoon results in a stronger surface wind
Diurnal Variations in Wind Speed and Direction: Night to Day
Surface wind increases
The direction backs (anti-clockwise)
Diurnal Variations in Wind Speed and Direction: Day to Night
Surface wind decreases
The direction veers (clockwise)
Squalls (SQ)
Ahead of convective clouds and CB
Outflow of cold air
Down-draughts
Gust fronts
Line Squalls (LSQ)
A band of intense thunderstorms
Gust (G)
A sudden increase in wind speed of more than 10kts and lasting for only a few seconds
Seabreeze
Daytime
Occurs about 1000ft AGL
The sun heats the ground faster than the water
The hot air rises over the land
Cooler air moves in from the sea to replace it
Strongest in the mid-afternoon
Land Breeze
Night
The sea heats up the air
Cooler air from the land replaces the rising air over the water (lower pressure)
Strongest just after sunrise
Thermals
Temperature versus dew point split could indicate possible thermals
Cause the undershoot/overshoot affect
Pilot Actions Whilst Flying Through Thermals
Control airspeed
Divert if necessary
Delay for cooler temperatures
Increase the approach speed within the recommended safety margin
Control/capture IAS while maintaining the approach path
Maintain best ROC airspeed
Dust Devils
Short lived and localised
Only a few metres in diameter
The air and surface is dry
Large temp/dew point split causes the ground to be heated maximally
Visible only due to dust, sand and leaves
Dust Storms
Moderate to strong winds
Instability
<1000m visibility
Katabatic Wind
Night or in the early morning
Land loses heat by terrestrial radiation
Gravity pulls the cooler air down the slope
Anabatic Wind
Daytime, strongest at 3pm
The sun heats the mountain slopes
The hot air rises
Cooler air flows up the slope to replace it
Weaker than katabatic wind as it is fighting gravity
Mountain Wave Requirements
A mountain greater than or equal to 1000ft
A wind perpendicular to the mountain greater than or equal to 20 - 25kts
A stable layer above the mountain (unstable below)
Windward
Upwind side of the mountain
Leeward
Downwind side of the mountain
Mountain Wave Turbulence
Significant turbulence in the lee side
Down draughts and rotor zones below the crest
The Föehn Effect
Moist air is forced up against a mountain and creates orographic lifting
The air cools to dew point and cloud forms
The rain falls on the upwind side and the moisture content reduces
The air then descends and warms adiabatically on the lee side with a higher cloud base
The Föehn Wind
The föehn effect causes a warm, dry wind on the lee side of the mountain
Requirements for a Low Level Jet
Strong surface (radiation) inversion
High pressure system approaching from the west
North-south mountain range
Subsidence inversion on top preventing the air from flowing over the mountain
The Low Level Jet
A strong southerly wind along the mountain range
Located over a plain and to the west of a mountain range
Usually below 3000ft AGL
Characteristics of a Low Level Jet
Strongest in the early mornings as it is colder
Most prevalent in winter with long cold nights
Produces a wind speed of up to 70kts
Usually southerly winds
Strong windshear and turbulence
Disperses when the sun heats the surface inversion
Altocumulus lenticularis
A stationary cloud formed at or above the crest on the downwind side of a mountain when mountain waves are present if there is sufficient moisture in the air
Kelvin-Helmholtz Wave Cloud
Clouds get their appearance because the top layer of air moves faster than the lower layer
The top layer gets scooped into a wave-like structure
Indication of severe vertical windshear
Dangers of Mountain Waves
Updraughts (hypoxia/altitude bust)
Downdraughts (exceed a/c climb capability)
Rotor zones (exceed aileron roll rate)
Abnormal Throttle Position
Occurs with windshear
Aircraft decreasing in altitude
Pitch the nose up (raise attitude)
Possible stall and decreased speed
Increased power
Leads to decreasing altitude in a full power aircraft seeting
Same can occur when increasing in altitude
Pilot Actions When Facing Mountain Waves
Fly 1000ft above the crest of the mountain
If heading upwind towards the mountain on the lee-side, cross the ranges at an angle of 30-45 degrees to allow for a faster escape
