Meteo Flashcards
What are the three stages of thunderstorms development?
INITIAL STAGE: updrafts move air aloft, allowing condensation to take place throughout the ascent of
the convective currents. (20min)
MATURE STAGE : water drops start to fall through the cloud. The downdraft is often in the middle of
the cloud, surrounded on all sides by strong continuing updrafts.
Downdrafts can reach 3000ft/min
Updrafts can reach 6000 ft/min.
The mature phase of a CB is the most
hazardous stage : heavy rain, hail, severe turbulence,
severe icing, windsher and microbusts, lightning. (20min)
DISSIPATING STAGE : End of the thunderstorm, with continuous rain, showers, sometimes as virga due to a temperature inversion beneath the cloud base which can still cause a marked windshear.
At higher levels it may take on the familiar anvil shape under the tropopause. Strong windshear is generally associated with the anvil. (2 hrs)
How far laterally should an aircraft avoid a thunderstorm?
15 Nm
What is Jetstream?
Narrow bands of high speed winds at very high altitude. (>60kts).
- 1000 Nm long
- 150 Nm wide
- 10 000ft deep
Generated by a difference in temperature over large areas at upper heights.
Jetstream are always in warm air below the tropopause with Clear Air Turbulence facing to the
cold air. With the wind in our back, cold air is on our left.
In Europe: the Polar front Jetstream located between
40°N and 65°N and close to FL300.
The SubTropical Jetstream between 25° and 45°N at FL390.
In winter the Jetstream tend to have a more southerly position.
What is windshear?
Sudden change in speed and/or direction of the wind including vertical currents.
Vertical WS: (change of height)
Horizontal WS: (change of direction).
What conditions are required for ice to form?
Change of state of water : Liquide => solid
Can be formed by two states:
By water vapour, sublimation, causing hoar frost
By water droplets, freezing rain, supercooled
water droplets.
The conditions required are a water drops present in clouds, high moisture, and temperature
between +10°C and -40°C.
What is rime ice?
When supercooled water droplets are small, the droplet freeze on impact and a white
opaque deposit is visible. (small scwd: upper levels -20 to -40°C CU/CB/NS and ST/SC/AS/AC 0°
to -40°C).
What is clear ice?
Occurs if large supercooled water droplets strikes an aircraft, it will start to freeze and release latent heat.
This will delay the freezing process and the ice will flow back over the surface forming clear ice.
Lscwd: temp 0°>-20°C CU/CB/NS)
What is radiation fog?
Fog is present when the RH is saturated (100%). Visibility reduces below 1000m.
Cloudless night (earth surface to lose heat), moist air, light wind (5kts). Radiation fog occurs inland, especially in valleys and low flying areas.
What is advection fog?
Fog is present when the RH is saturated (100%). Visibility reduces below 1000m.
Horizontal flow of air: Occur suddenly, day or night, land or sea, if a warm, moist air moves over a colder surface, which is cooled from below. Light to moderate winds (10-15kts).
Warm front?
Boundary between two air masses (warm air behind cold air).
Warmer air, less dense rises up and replaces at altitude the colder air mass.
The slope of a warmfront is about 1/150
The top of the warm front = Cirrus and can be up to 600nm ahead of the front. Cirrus, CS, AS, NS, Stratus.
Approaching Warm front:
Lowering cloud base is experienced, possibility of virga giving way to nimbo stratus clouds with increasing rainfall.
Poor visibility, atmospheric pressure usually fall.
As the warm front passes: Rise in temperature (warm sector) Low level cloud base, wind veers in NH Pressure stop falling, visibility is improving. Speed : 2/3 of the geostrophic wind
Cold front?
Boundary between two air masses (cold air behind warm air)
Colder air , denser replaces the warmer preceding air mass.
The slope is relatively steep 1:50.
Approaching cold front:
Unstable clouds like cumulus, even CB associated with
thunderstorms (band of 30-50 miles).
Pressure will start to rise after the cold front.
Temperature low, visibility good except in showers.
Speed : geostrophic wind (denser air, less tendency to resist)
What is microburst?
Severe downdraft, often a the base of a cumulonimbus and associated with thunderstorms.
During the mature phase of the CB, typically only 5km across and concentrated in the
middle of the TS.
The vertical speed can reach up to 3000ft/min and 100kts.
