Chapter 14 - Thunderstorms Flashcards
What kind of cloud give rise to thunderstorms?
Well developed cumulonimbus CB, which give lightning and heavy precipitation.
What is needed for a thunderstorm to develop?
1) Unstable air through significant height
2) Moisture
3) A trigger for uplift
4) No wind shear
What risks to flying do thunderstorms bring?
Icing
Turbulence
Hail
Lightning
Pressure variations
Microbursts
Water ingestion to engines
Tornados
Where do thunderstorms occur?
At cold fronts
The ITCZ
From strong thermals
Squal locations
Locations with orographic lift
What level is lightning most common?
+/- 5000ft of freezing level (area between 10 and -10 degC).
What are the characteristics of the initial stage of a thunderstorm?
cauliflower/capillatus/incus cumulus
Rapid growth, 5000fpm
All updrafts inside cloud (with down outside)
Reaches the tropopause
Moves in the direction of 700hPa wind
No precipitation
Last 15-20mins
What are the characteristics of the mature stage of a thunderstorm?
Defined by onset of precipitation
Updrafts reaching up to 10000fpm
Down drafts caused by precipitation, 2000fpm
Heavy rain
Friction causes static electricty build up and lightning
Anvil/incus starts to develop
Lasts around 30-45mins
Can get micro/macrobursts
Tornados
What are the characteristics of the disspating stage of a thunderstorm?
Down drafts are stronger
Water supply running out
Distinct anvil top
Down drafts across the whole cloud
Cloud spread to stratucumulus/stratus
Moderate showers
Can feed adjacent systems
Lasts around 90-150mins
What is a Tropical rotating storm?
A combination of multiple multicell, supercell groups of thunderstorms that develop into one big rotating storm.
What is the minimum wind speed to a TRS?
64kts
Why do thunderstorms have localised low pressures?
Due to internal convection in the system.
Where has the highest probability of a thunderstorm?
ITCZ
What are squalls and where can they develop?
A line of thunderstorms, that can be several hundres miles long. They can usually develop just ahead of a cold front.
What is a downburst?
A very strong downdraft caused by the combination of precipitation’s friction with air and evaporation cooling the air making it fall even faster.
What is a microburst?
A type of downbust with only up to 5km diameter and last no longer than 5 minutes, but with downdrafts of aroud 60kts.
What is a macroburst?
A type of downburst with a diameter of more than 5km, lasting longer than 5 minutes.
What kind of surface winds can be expected with downbursts?
Up to 90kts
Can a downburst be dry?
Yes, if virga occurs then there will be no rain at the surface.
What is worse, a wet downbust or a dry downburst?
A wet downburst due to the increased downdrafts caused by precipitation friction and there is more ability for evaporation to occur.
Where can roll/rotor cloud develop?
At the front of a mature stage thunderstorm, between the updrafts and downdrafts underneath the cloud.
Where can shelf cloud develop?
At a gust front, for example at the front, at the base of a thunderstorm.
What are the characreristics of a tornado?
Usually 100-150m across
Wind velocities of up to 2000kts
Typically move across the surface at around 20-40kts
What powers a thunderstorm?
The latent heat released from condensing water vapour.
