Chapter 14 - Thunderstorms

Question 1

What kind of cloud give rise to thunderstorms?

Answer 1

Well developed cumulonimbus CB, which give lightning and heavy precipitation.

Question 2

What is needed for a thunderstorm to develop?

Answer 2

1) Unstable air through significant height
2) Moisture
3) A trigger for uplift
4) No wind shear

Question 3

What risks to flying do thunderstorms bring?

Answer 3

Icing

Turbulence

Hail

Lightning

Pressure variations

Microbursts

Water ingestion to engines

Tornados

Question 4

Where do thunderstorms occur?

Answer 4

At cold fronts

The ITCZ

From strong thermals

Squal locations

Locations with orographic lift

Question 5

What level is lightning most common?

Answer 5

+/- 5000ft of freezing level (area between 10 and -10 degC).

Question 6

What are the characteristics of the initial stage of a thunderstorm?

Answer 6

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

Question 7

What are the characteristics of the mature stage of a thunderstorm?

Answer 7

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

Question 8

What are the characteristics of the disspating stage of a thunderstorm?

Answer 8

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

Question 9

What is a Tropical rotating storm?

Answer 9

A combination of multiple multicell, supercell groups of thunderstorms that develop into one big rotating storm.

Question 10

What is the minimum wind speed to a TRS?

Answer 10

64kts

Question 11

Why do thunderstorms have localised low pressures?

Answer 11

Due to internal convection in the system.

Question 12

Where has the highest probability of a thunderstorm?

Answer 12

ITCZ

Question 13

What are squalls and where can they develop?

Answer 13

A line of thunderstorms, that can be several hundres miles long. They can usually develop just ahead of a cold front.

Question 14

What is a downburst?

Answer 14

A very strong downdraft caused by the combination of precipitation’s friction with air and evaporation cooling the air making it fall even faster.

Question 15

What is a microburst?

Answer 15

A type of downbust with only up to 5km diameter and last no longer than 5 minutes, but with downdrafts of aroud 60kts.

Question 16

What is a macroburst?

Answer 16

A type of downburst with a diameter of more than 5km, lasting longer than 5 minutes.

Question 17

What kind of surface winds can be expected with downbursts?

Answer 17

Up to 90kts

Question 18

Can a downburst be dry?

Answer 18

Yes, if virga occurs then there will be no rain at the surface.

Question 19

What is worse, a wet downbust or a dry downburst?

Answer 19

A wet downburst due to the increased downdrafts caused by precipitation friction and there is more ability for evaporation to occur.

Question 20

Where can roll/rotor cloud develop?

Answer 20

At the front of a mature stage thunderstorm, between the updrafts and downdrafts underneath the cloud.

Question 21

Where can shelf cloud develop?

Answer 21

At a gust front, for example at the front, at the base of a thunderstorm.

Question 22

What are the characreristics of a tornado?

Answer 22

Usually 100-150m across

Wind velocities of up to 2000kts

Typically move across the surface at around 20-40kts

Question 23

What powers a thunderstorm?

Answer 23

The latent heat released from condensing water vapour.