8: Thunderstorm Structure

Question 1

Thunderstorm Structure

Answer 1

• Lightning is associated with convective activity
• Cumulonimbus clouds are the largest for of convective cloud and produce lightning
• Cumolonimbus clouds with lightning activity are referred to as thunderclouds

Question 2

Branches of thunderclouds

Answer 2

• Updraft: warm moisture rises
• Downdraft: cold air descends

Question 3

Thunderstorm Structure

Answer 3

image 14

• Model described as a positive electric dipole with a positively charged region (P)above a negatively charged region (N)
• A weaker, positively charged region at the base of the cloud (p) gives it more of the tripole structure, but because its weak, it can be called double-dipole
• N and P regions have the same charge (+/- 40 Coulombs), creating a positive dipole due to weaker p (+10 Coulombs)

Question 4

thunderstorm charge distribution

Answer 4

• At the surface we generally have negative particles (also some positive)
• charges in the atmosphere are attracted to surfaces with negative/positive charges

Question 5

Charge Generation theories and main points

Answer 5

1. Convective Theory: free ions in the atmosphere are captured by cloud droplets and are moved by the convective currents in the cloud to produce the charged regions
2. Gravitation Theory: negatively charged particles are heavier and are separated from lighter positively charged particles by gravitational settling

Question 6

Convective Charging Theory

Answer 6

• Positive charges are drawn from the middle portion of the cloud and as it reaches the top of the cloud, it is distributed laterally throughout the cloud
• Screening later: made up of negatively charged carriers around the perimiter of the cloud
• Screening layer negative charges are attracted to the positive charges inside the cloud
• image 15

Question 7

Gravitational Theory

Answer 7

• for this theory to work there must be some charge exchange process between particles of different sizes
• Charge can be exchanged betwen particles in various states by inductive and non-inductive processes

Question 8

Inductive Theory

Answer 8

• assumes that charge is exchanged between colliding particles polarized by the fair weather electric field
• large particle has positive charge at bottom and negative at top, when it collides with the small particle, the large particle takes the negative charge of the small particle and the big particle falls down because it is heavier from getting the neagtive charge

Question 9

What Type of Collisions Can There Be?

Answer 9

• Two water drops colliding often stick together but if it’s a glancing collision, there isn’t any charge separation
• Ice-ice particles colliding aren’t in contact long enough to separate charge
• Water-ice and ice-riming collision can transfer charge but its not believed that this process can induce a thunderstorm’s electric field

Question 10

Non inductive theory

Answer 10

• The effectiveness of the ice-ice process lies in the thermo-electric properties of ice. The mobility of the hydronium ions in ice is greater than the hydroxide ions and the number of these ions increases with temperature. H2O is a mix of H30+ and OH- ions
• Charges from H3O+ and OH- exchange

Question 11

Which Charge Theory is Right?

Answer 11

• No unique mechanism to generate the required charge
• Most likely explanation is most likely a combination

Question 12

What are the types of lighting

Answer 12

1. Intracloud (IC) flashes: redistributing the charge within the cloud, accounting for over half the lightning flashes in northern latitudes
2. Cloud to cloud and cloud to air are less common. Little impact on people
3. Cloud to ground (CG) flashes: very common and affect people
4. Ground to cloud flashes. Originate from the ground, observed from large buildings
5. Ball lightning: luminous sphere that lasts a few seconds

Question 13

Stages of the lightning flash (just names)

Answer 13

1. Stepped leader
2. Return stroke
3. Dart Leader

Question 14

What is the Stepped Leader

Answer 14

• Small packet of negative charge that descends from the cloud to the ground along the path of least resistance
• the leader leaves a trail of ionized gas. It moves in steps
• After a step, the leader pauses for 50 microseconds then takes the nexg step, the leader charge packet sometimes breaks up to follow a different path, giving it a forked appearance
• As the leader approaches the ground, electrons on the surface retreat from the leader, creating a region of positive charge
• Positive corona discharge/St. Elmo’s Fire: are released form tall objects on the surface and reach out to the approaching leader
• image 16

Question 15

What is the Return Stroke

Answer 15

• When the downward moving leader connects with a surface corona discharge, a continious path between the cloud and the ground is established a return stroke is triggered
• The return stroke rapidly moves as a wave upwards into the cloud , following the ionized trail and stripping electrons from its path (makes negative charges to positive)
• image 17 and 19

Question 16

What is a Dart Leader

Answer 16

• After the return stroke, the lightning flash may end or if enough charge in the cloud is collected, a dart leader may come down from the cloud that follows the already established path to the surface.
• Dart leader triggeres a second stroke

Question 17

Does a stroke between cloud and ground travel upward or downwards?

Answer 17

• Both depending on where it begins
• Human eye not fast enough to determine direction of a lightning flash
• Branching of thel ightning channel is the key to its direction

Question 18

A Typical Cloud to Ground Stroke

Answer 18

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

Question 19

The Stepped Leader Mechanism

Answer 19

• Lighting stroke begins as a local discharge (dialectric breakdown) between the positive and negative regions
• Dialectic breakdown is when the voltage differential across an insulator is large enough so that electrons flow from the negative region
• This free electrons attached to cloud and precipitation particles. They produce a downward moving electron cascade
• The cascade neutralizes the low positive charge in the cloud base
• image 18

Question 20

Stepped Leader Facts

Answer 20

• Step length: 50m, means 100 steps to reach grounf
• Total duration: 5x10^-3s
• Average current is 1000A
• Each step lasts 1x10^-6s

Question 21

Return Stroke Fact

Answer 21

• Return stroke is visible
• Drains 5C of negative charge from the stepped leader channel
• Velocity is 10^8m/s
• Leader channel becomes positively charged
• Current = 5c/50x10^-6 = 10^5 A

Question 22

Multiple Strokes

Answer 22

• The dart leader taps negative charge higher in the N region
• Dart leader moves without steps since there is sufficoent ionization remaining so showsl ittle branching

Question 23

How is Thunder Caused

Answer 23

• Rapid heating increases the air pressure in the lightning channel to 100 atm
• At the surface, the prssure is 1atm
• This causes rapid expansion in a shock wave that travels 3000 m/s. The shock wave slows down as it expands and the pressure decreases until it becomes a sound wave with speed ~330m/s
• The time in seconds between a flash and the sound of thunder is divided by 3, gives the nearest distance to the lightning channel (in km)

Question 24

Different Strokes

Answer 24

• 5% of lightning strokes lower positive charge to the ground
• Positive strokes have a higher current, fewer multiple strokes, and a longer continuing current –> concern for forest fires
• Strokes from tall structures branch upward (ground to cloud lightnting)

Question 25

On a clear day there is an increase of electric potential with increasing height above the ground. At 50m above the ground, the electric protential is what in volts?

Answer 25

Electric potential = 100V/m x distance (50m) = 5000V

Question 26

A thunderstorm lasts for 2 hours and gives rise to 8,000 lightning strikes, each of which transfers 10 Coulombs of charge to the ground. The average current between the storm and the ground is ... Amperes?

Answer 26

Total charge = 8000 x 10 = 80000 Coulombs (convert to seconds) * 2 hours = 2 x 60 x 60 = 7,200 seconds * I = 80000 Coulombs/(2 x 60 x 60 seconds) = 11.1111 Amperes

Question 27

If you see a lightning stroke and then, 30 seconds later, hear the thunder, how far is the lightning?

Answer 27

Distance (km) = 30s/3 = 10 km

Question 28

Stages of a Lightning Caused Fire

Answer 28

* Ignition * Survival * Arrival