NATS 101

Lecture 28Lightning

Review: Thunderstorms

• A cumulonimbus with lightning and thunder!• Deep layer of conditionally unstable air is

necessary to produce a thunderstorm. • Several types of thunderstorms.

Single Cell, Multicell, Squall Line, Mesoscale Convective Complexes, Supercells

• Pose major hazards to public and economy.

Lightning, Hail, Microburst Winds, Flash Flooding, Tornadoes

Lightning Basics

• What is lightning?

An electric discharge, or spark, that occurs in thunderstorms (usually)

80% occurs within clouds

20% occurs between cloud and ground

• Lightning is ubiquitous, with more than 6,000 ground strikes per minute from 40,000 thunderstorms per day worldwide

Lightning Videos

• Examples were shown of

In-Cloud (IC) Lightning

Cloud-to-Cloud (CC) Lightning

Forked LightningIC Lightning Video from MetEd/UCAR

• MCC Lightning from Space ShuttleLightning from Space Video from NASA

Lightning Pictures

• Examples were shown of

Cloud-to-Ground (CG) Lightning

In-Cloud (IC) Lightning

Cloud-to-Cloud (CC) Lightning

Forked LightningChuck Doswell's Lightning Pictures-Very Nice!

• Excellent photography tips can be found at Chuck Doswell’s web site. He’s good!

Charge Separation

Lightning requires the separation of different charges into different regions of a cloud.

How does charge separation in clouds occur?We don’t know for certain, but we observe this:

Lightning only occurs in cold clouds with supercooled droplets and temps below 5oF.

Thus, the ice crystal processes responsible for precipitation in cold clouds likely plays an critical role in charge separation.

Charge Separation: One Theory

Hailstones are covered by a layer of liquid water.

The thin layer of liquid is positively charged.

When hailstones and ice crystals collide, some of liquid molecules stick to the ice crystals.

Along with the mass transfer, positive ions transfer from the hailstones to the ice crystals.

The heavier, negative hail falls to cloud bottom.

The lighter, positive ice crystals drift to cloud top.

Produces negative lower, positive upper cloud.

Charge Separation

• Top of cloud top has a positive charge.

• Lower and middle of cloud has a negative charge.

• Charge separation in cloud maintains the earth’s fair weather electric field denoted by the arrow E

• E points toward positive polarity

Williams, The Weather Book

PolarizationPolarization

––

++

E

• An electric potential exists between the ionosphere (positive) and surface (negative)

• Potential varies between 200,000 - 500,000 Volts

• Average current is 2x10-12 Amps/m2

• Power is 10-6 W/m2

Fair Weather Electric Field

www://thunder.msfc.nasa.gov/primer

Lightning StrokeCloud-Ground Sequence

1) Downward stepped leader. Stepped leader is invisible.

2) Upward return stroke.

3) Downward dart leaders.

4) Upward return strokes.

Dart leaders-return strokes: up to 25 cycles, 3-4 usually.

Ground strikes are usually negative, that is electrons flow from cloud to ground.

Williams, The Weather Book

www://thunder.msfc.nasa.gov/

Types of Discharges

Lightning Safety

Williams, The Weather Book

Thunder

What Causes Thunder?• Lightning rapidly heats air

to more than 30,000oC.• The intense heating causes

the air to expand rapidly. • The expanding air cools,

then contracts rapidly. • The expansion-contraction

generates sound waves.

Williams, The Weather Book

How Far Away Is It?

• We see lightning instantly.• But sound travels 1,000 ft

every second. If you hear thunder 10 seconds after seeing lightning, the bolt is 2 miles (~10,000 ft) away.

• We hear thunder from closest part of flash first, farthest part last. This causes the rumble sound.

Williams, The Weather Book

Why Thunder Rumbles?

• Assume that you are one mile away from a a one mile long bolt.

• You hear thunder from the lower part of flash in 5 seconds, from the upper part of flash 7 seconds.1 mile

5 seconds

1 m

ile

1.4 miles

7 seconds

National Lightning Detection Network

thunder.msfc.nasa.gov/primer

www.nssl.noaa.gov/western/kaney

Tucson

Phoenix

FlagstaffGallup

Tucson

Phoenix

www.nssl.noaa.gov/western/kaney

Global Lightning Distribution from Satellite, Take 2

www.msfc.nasa.gov

Let’s Play “Who Gets Toasted”

• What is the probability in Tucson of a Cloud-to-Ground lightning stroke hitting within a certain Radius R of you in an “average” year?

• Guesses? No peeking!

Courtesy Prof. E.P. Krider

Dept. of Atmospheric Sciences

CG Lightning over Tucson(2000-2002)

• 65,000 flashes in 80 km 80 km over 3 years

~3.3/km2 per year• Much higher

during monsoon ~12/km2 per year

Courtesy Prof. E.P. Krider Dept. of Atmospheric Sciences

Summary: Key Points

• Lightning - electric discharge in thunderstorms 80% within clouds, 20% cloud to ground

• Lightning is ubiquitous, with more than 6,000 cloud-to-ground strikes per minute from more than 40,000 thunderstorms per day worldwide.

• Lightning requires the separation of different charges into different regions of cloud.

• Charge separation maintains the earth’s fair weather electric field.

Assignment for Next Lecture

• Topic - Tornadoes • Reading - Ahrens, p277-290• Problems - 10.25, 10.26, 10.29