Thunderstorms

Question 1

Three conditions must be met for the development of thunderstorms, along with the five trigger possible actions:

Answer 1

1. There must be sufficient moisture present in the atmosphere.
2. Instability must persist to a height of at least 10 000ft
3. There must be a lifting process, called a trigger action, which will give rise to a sample of air becoming saturated. These trigger actions are:

A. Convective currents
B. Fronts
C. Convergency
D. Orographic uplift
E. Nocturnal effect

Question 2

The type of thunderstorm which will usually develop in the afternoon -

Answer 2

Convective thunderstorm

Question 3

The three thunderstorm stages:

Answer 3

1. Developing stage
2. Mature stage
3. Dissipating or anvil stage

Question 4

Explain the main prominent characteristics of the developing stage:

Answer 4

Several small cumulus clouds start to merge and form into one large cumulus cell. Some of these cells have stronger convergent currents which entrain(suck) in the other less dominant cells until the cumulonimbus cloud is formed. The developing stage is marked by strong up-draughts throughout the cell reaching velocities up to 3 000ft/min, but 6 000ft/min has been reported. Rain does not usually fall at this stage due to suspension by the strong up-draughts.

Question 5

Explain the main prominent characteristics of the mature stage(includes the gust front):

Answer 5

Rain indicates the start of this stage, and it is the most intense out of all. Lighting and thunder is frequent at this stage along with hail and severe turbulence. Large amounts of precipitation dispensed from the top of the cloud will cause down-draughts of around 2400ft/min and retards the up-draughts which will match this velocity. This down-draught spreads out on reaching the ground and causes severe squall, the leading edge of this gust is called the “first gust”. The shear produced between the up and down draughts of a mature cell causes the formation of roll cloud, called the “storm collar”, this will set up severe turbulence ahead of the leading edge of the cloud.

A phenomenon often associated with thunderstorms is the gust front. This comprises an area of cold air flowing outwards from a down-draught usually in all directions, but it does tend to be ahead of the storm’s line of movement. These gusts can be quite powerful often extending horizontally to 20 - 30km from the storm and up to 6 000ft vertically. This does not appear on weather radar.

Question 6

Explain the dissipating or anvil stage:

Answer 6

Eventually the down-draughts overpower the up-draughts spreading out across the cloud leaving only a gentle down-draught near the lower levels of the cloud leaving very little rain. Some up-draughts are still present and they are responsible for the anvil shape at the top. The cessation of the up-draughts cuts of the moisture supply to the cloud causing it to dissipate.

Question 7

Surface weather changes associated with thunderstorms:

1. Wind -
2. Surface pressure -
3. Visibility -

Answer 7

1. An observer ahead of the storm will experience a gentle surface wind towards the cell (remember the developing stage)

Prior to the passage a strong gust of about 15kts, this will the “first gust”

2. Pressures will fall during the advance of the storm. In the rain area, it rises sharply and then returns to normal with the passing of the storm. With a frontal storm, it will rise steadily.
3. Visibility is good during the developing stage but reduces at the onset of the mature stage

Question 8

Perhaps the greatest hazard that may be faces in a thunderstorm(but is infrequent)?

Answer 8

Hail.

Question 9

The six hazards within a thunderstorm:

Answer 9

1. Turbulence and draughts
2. Hail
3. Ice accretion
4. Lightning
5. Static electricity
6. Squall lines

Question 10

Lightning striker occur most frequently where the temperature is between -

Answer 10

-10 and +10

Question 11

What will be the effect of static electricity to an aircraft?

Answer 11

Static on the radios and inaccurate ADF needle readings.

Question 12

Explain a squall line:

Answer 12

A line of fast moving active thunderstorms which forms in the warm air ahead of a cold front. It causes strong and destructive winds along with hail.

Question 13

When flight through a thunderstorm is imminent and unavoidable, the worst turbulence may often be avoided by penetrating the storm at or below -

Answer 13

10 000ft

Question 14

A good general rule is to avoid a storm by _ for every _____ of vertical development.

Answer 14

1nm for 2 000ft

Question 15

The type of thunderstorms with are difficult to avoid due to it being obscured by other cloud?

Answer 15

Convergent and frontal storms

Question 16

The approach and landing should be delayed for this amount of time with the approach of such a storm in close proximity to the airfield -

Answer 16

20 - 30 minutes

Question 17

During flight, how to identify the worst part of the storm(day and night)?

Answer 17

During the day it will be darker parts, and during night the parts with lightning.

Question 18

The tops of a thunderstorm is usually?(tropical and temperate latitudes)?

Answer 18

20 000ft in the temperate latitudes and up to 50 000ft in the tropical regions.

Question 19

Penetration of thunderstorms for jet liners are usually at or above -

Answer 19

33 000ft

Question 20

A method to calculate the cloud base of a Cumulo type cloud?

Answer 20

Find the difference between the surface temperature and dew point, then multiply it by 410ft.