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EAS 202 > 8: Thunderstorm Structure > Flashcards

8: Thunderstorm Structure Flashcards

1
Q

Thunderstorm Structure

A
  • 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
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2
Q

Branches of thunderclouds

A
  • Updraft: warm moisture rises
  • Downdraft: cold air descends
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3
Q

Thunderstorm Structure

A

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)
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4
Q

thunderstorm charge distribution

A
  • At the surface we generally have negative particles (also some positive)
  • charges in the atmosphere are attracted to surfaces with negative/positive charges
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5
Q

Charge Generation theories and main points

A
  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
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6
Q

Convective Charging Theory

A
  • 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
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7
Q

Gravitational Theory

A
  • 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
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8
Q

Inductive Theory

A
  • 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
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9
Q

What Type of Collisions Can There Be?

A
  • 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
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10
Q

Non inductive theory

A
  • 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
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11
Q

Which Charge Theory is Right?

A
  • No unique mechanism to generate the required charge
  • Most likely explanation is most likely a combination
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12
Q

What are the types of lighting

A
  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
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13
Q

Stages of the lightning flash (just names)

A
  1. Stepped leader
  2. Return stroke
  3. Dart Leader
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14
Q

What is the Stepped Leader

A
  • 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
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15
Q

What is the Return Stroke

A
  • 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
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16
Q

What is a Dart Leader

A
  • 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
17
Q

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

A
  • 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
18
Q

A Typical Cloud to Ground Stroke

A
  • 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
19
Q

The Stepped Leader Mechanism

A
  • 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
20
Q

Stepped Leader Facts

A
  • Step length: 50m, means 100 steps to reach grounf
  • Total duration: 5x10^-3s
  • Average current is 1000A
  • Each step lasts 1x10^-6s
21
Q

Return Stroke Fact

A
  • 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
22
Q

Multiple Strokes

A
  • 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
23
Q

How is Thunder Caused

A
  • 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)
24
Q

Different Strokes

A
  • 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)
25
Q

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?

A

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

26
Q

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?

A

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

27
Q

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

A

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

28
Q

Stages of a Lightning Caused Fire

A
  • Ignition
  • Survival
  • Arrival

EAS 202 (14 decks)

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