AV20: Thunderstorms - DC Flashcards
What are the four conditions required for the development of a TS?
High water vapour content
Strong lifting agent
Condensation nuclei
unstable air to high altitude
Define Cell
Part of a thunderstorm complex that is in a particular stage of development
Define Cluster
Group of cells that form a thunderstorm complex
What are the 3 stages in the life of a thunderstorm?
Cumulus Stage
Mature Stage
Dissipation Stage
What are the characteristics of the cumulus stage of a TS? (4things)
mainly ascending currents (updrafts)
No precip
Mainly water droplets
Drop in pressure at the surface
What are the characteristics of the mature stage of a TS? (8 things)
- Both strong updrafts and downdrafts
- Start of Precip (+SHRA, SHSN, SHPL, SHGR)
- Very high tops of cloud
- Severe turbulence (gusts, squalls, wind direction changes)
- All types of lightning
- Possible microbursts, or +FC
- Variations in surface pressure and temperature
- Composed of RA, SN, GR, and IC
What are the characteristics of the dissipation stage of a TS? (7 things)
- Gradual end to precip
- Anvil top
- mainly downdrafts
- little vertical movement in upper portion
- increased pressure at surface
- drop in surface temperature
- contains water drops, SN and IC.
What are the two categories of thunderstorms?
Air Mass TS, and Frontal TS
What are the 3 causes of air mass TS?
Daytime heating (convection), Advection heating (cold air over warm surface), orographic lift
What are the characteristics of a convection TS?
Caused by unequal heating of the ground,
Happen in afternoon or early evening,
Scattered over a large area,
Dissipate in late evening / overnight.
What are the characteristics of advection TS - over land?
Caused by advection of cold air over a warm surface,
Generally in a northerly flow,
Maximum activity happens in the afternoon,
Scattered over a large area,
dissipate in late evening / overnight.
What are the characteristics of advection TS - over water?
Caused by advection of cold air over a warm surface,
Maximum activity in the morning,
Tend to be close together,
dissipates with southerly flow.
What are the characteristics of orographic TS?
Caused by air lifted by sloping ground,
In a stationary line (over the lifting point),
Can happen day and/or night,
Dissipates with change in wind direction.
What are the characteristics of a Warm Front Thunderstorm?
Humid Unstable air is lifted by frontal surface,
Embedded in stratiform clouds,
day and night,
dissipates with movement of the front.
What are the characteristics of a Cold Front Thunderstorm?
Humid / Unstable air is lifted by the frontal surface,
in a line parallel to the front,
day and night,
dissipates with movement of the front
Define Microburst
Intense, small scale downdrafts which spread outwards from the downflow centre when they reach the ground
What 2 forms of precipitation are indicators of a POSSIBLE microburst?
Showers & Virga
What are the formation requirements for microbursts?
Convective Activity,
Downdrafts - created by cooling of the air by evaporation and/or friction on the air caused by falling precip.
What happens when the downdraft of a microburst reaches the ground?
spreads out in all directions in the shape of a “vortex ring”.
What happens at the edge of a vortex ring?
airflow curls upwards on itself, which is associated with dangerous low level wind shear for aircraft
What is the typical diameter of a downdraft of a microburst?
up to 6000 feet (1.8Km)
What is the typical diameter of a vortex ring?
up to 5 miles
What is the typical velocity of the downdraft of a microburst?
Up to 6000 feet per min
What is the typical velocity of the gust speeds outwards from the center of a microburst?
up to 80 Knots
What is the typical lifespan of a microburst?
10-20 minutes
What are the indication signs of a microburst?
Dust ring at the surface (dry microburst),
heavy rain shaft (possibly),
VIRGA (sometimes),
Opposing wind directions over a short distance.
What are the 9 hazards associated with thunderstorms (for aviation)
Turbulence, Hail, Lightning, Icing, Squall lines, Low Ceilings and Visibility, \+SHRA, \+FC, Microbursts
Where is turbulence most pronounced in a TS?
In the mature stage in the mid and high levels. Between opposing air currents, especially in and near zones of precip.
AND
Between the air and ground (Sfc to 1000’)
At what altitude does the maximum occurrence of hail happen?
10000 to 15000 feet
The stronger the vertical currents, the ________ will be the hail.
larger
Define Lightning
Visible electric discharge produced by a thunderstorm, always present in or in the vicinity of CB clouds
Between what temperatures is lightning most prevalent in CB cloud?
+8 to -8 degrees C (+/- 5000 feet from freezing level) in the mature stage
How does the stability of the air affect lightning?
The more unstable the air, the more lightning activity
For lightning to occur, CB cloud tops must reach a temperature of ___________
-20 degrees Celsius
What are the 3 types of lightning?
In cloud (LTGIC)
Cloud to Cloud (LTGCC)
Cloud to Ground (LTGCG)
What effects does icing have on aircraft?
modifies aerodynamics,
causes engine to stall by blocking air intake,
causes instrument errors by blocking sensors (pitot tube / static port),
reduces visibility.
When is icing most common and most severe in a TS?
SEV CLR ICG in the upper half of TS in mature stage, especially when temperatures are between 0 and -25 C, but could be down to -40.
What is a squall line?
A non-frontal band of highly active thunderstorms that generally form ahead of active cold fronts in moist and unstable air, with the gust front acting as a lifting agent.
Heavy precipitation saturating the ground or air could cause __________ forming.
Fog or SF
+SHRA are associated with which stage of the TS?
Mature
What are some effects +SHRA can have on aviation? (4 things)
- Contaminate wing (causing early stall)
- Reduced visibility on approach
- refraction on windshield (also limiting visibility)
- Hydroplaning on runway (destroyed braking action and possible loss of control)
Tornadoes are generally associated with _______________ and ________________
Cold Fronts and Squall Lines
What type of cloud is a good indicator of possible tornadoes?
Mammatus
