Lightning Part 2

Question 1

Where is positive cloud-to-ground flash more common?

Answer 1

Stratiform clouds

Question 2

Where is negative cloud-to-ground flash more common?

Answer 2

Areas of convection

Question 3

When is a positive flash most likely to occur?

Answer 3

In the mature stages of the storm and the anvil spreads out

Question 4

When is a negative flash most likely to occur?

Answer 4

In the early stages of the storm

Question 5

Where are positive flashes more common?

Answer 5

With an increase in latitude and increase in height of local terrain. Also in winter storms

Question 6

Why are positive flashes more common in winter storms?

Answer 6

Because the lower freezing level places the positive charge center closer to the ground thus increasing the likelihood of a flash

Question 7

Does a stroke between clouds and ground travel upwards or downwards?

Answer 7

Both, depending on where it begins and whether you are considering the motion of electrons or of the illumination

Question 8

What are some facts of a typical cloud-to-ground stroke?

Answer 8

• Strikes low objects, not tall buildings or towers.
• Lowers 10 to 25 Coulombs of negative charge from the N-region to the ground
• Lasts a few tenths of a second
• Electrons move downward and channel branches downward
• Flickering suggest multiple strokes along a single channel

Question 9

What is a Dielectric Breakdown?

Answer 9

When the voltage differential across an insulator is sufficiently large the electrons flow from the negative region

Question 10

How does a dielectric breakdown cause lightning?

Answer 10

It frees electrons attached to the cloud and precipitation from the negative region which causes them to produce a downward-moving electron “cascade”. The cascade neutralizes the low positive charge in the cloud base, then proceeds in 50m steps along a low luminosity channel towards the ground.

Question 11

How long does a stepped leader last?

Answer 11

~5ms

Question 12

How long does a step last in a stepped leader and how much time passes between steps?

Answer 12

A step lasts ~1μs and there are ~50μs between steps

Question 13

How many steps of a stepped leader does it take to reach the ground?

Answer 13

~100 steps

Question 14

How fast is a return stroke?

Answer 14

~10^8m/s

Question 15

How could a return stroke cause a forest fire?

Answer 15

A continuing current of a return stroke can raise the temperature of wood to its combustion point.

Question 16

How many multiple return strokes can occur?

Answer 16

up to about 50

Question 17

How fast does thunder travel?

Answer 17

~3000m/s but slows as it expands and becomes a sound wave traveling at ~330m/s

Question 18

How can you calculate how far away a lightning channel is from the thunder?

Answer 18

Take the time (s) between a flash and the sound of thunder divided by 3.

Question 19

What are the stages of a forest fire?

Answer 19

1) Ignition - lightning strikes
2) Survival - smoldering ground
3) Arrival - fire starts