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MET 125 Physical Meteorology

Atmospheric Electricity, Part 1Lightning

Henry Bartholomew (M.S.)

San Jose State University

What is electricity?

It is energy carried by charged particles

Flow of charge is called current

Thunder

Sound heard as a result of lightning

Lightning is an electrical discharge

Peak temperature of lightning bolt is around 30,000 K (about 55,000°F)!

Due to this intense heating of the lightning “channel,” air spreads out, and sound travels faster than it would in cooler surrounding air

Outward moving pulse causes shock wave

Sound of Thunder

When lightning is nearby, thunder often sounds like clap

Farther away, it may sound more like a rumble– Can be caused by sound originating from

different locations of stroke, and highlighted when sound wave reflects off obstacles, such as buildings and mountains

Determining the distance from lightning

You can determine your distance from lightning by counting the number of seconds between when you see the flash and hear the thunder

The speed of sound is approximately 1 mile per 5 seconds

Distance = Time*Speed

Thus, multiply time (in seconds) by speed of sound (1 mile/5 seconds) to get distance from lightning (in miles)

Thunderstorm

http://www.youtube.com/watch?v=2Ey4KSnoReo

Three Types of Lightning Strikes

Cloud-to-Ground (CG)

Cloud-to-Cloud (CC)

Intra-Cloud (IC)

Lightning Schematic

Cloud-to-Ground Lightning

Electrical Discharge between negatively charged base of cloud and positively charged ground or object (usually)

Responsible for about 20-25% of global lightning strikes

Cloud-to-Cloud Lightning

Electrical Discharge between two oppositely charged regions of different clouds

Responsible for the majority of lightning strikes

Cloud-to-Cloud Lightning

Intra-Cloud Lightning

Electrical Discharge between two oppositely charged regions within the SAME cloud

Makes up a small percentage of lightning strikes

Intra-Cloud Lightning

Charge Separation in Clouds

Still a subject of debate as to WHY charge separation develops within a cloud

There are two plausible mechanisms

Mechanism 1

After evaporation from surface, rising moisture can condense and form clusters of droplets, which collide with falling supercooled droplets and ice crystals

The collisions may tear off electrons from rising droplets, which then collect near base of cloud

Mechanism 2

As clusters of rising droplets travel higher, temperature drops, and freezing can occur

Inner portion of cluster may freeze first and become negatively charged, while outer portion may stay as liquid and develop positive charge

Air currents can tear away positively charged surroundings and carry them to top of cloud

Steps of Lightning

1. Stepped Leader extends down from cloud

2. Upward Leader extends upward from objects on ground to meet Stepped Leader

3. Return Stroke occurs

4. Additional Strokes take place

Stepped and Upward Leader

A stepped leader is a negatively charged channel of air that extends downward from cloud

Positively charged channel (upward leader, or streamer) then extends upward from object or ground

These leaders occur too fast and too close to time of main discharge to appear visible to naked eye

When these two channels meet, return stroke occurs

Return Stroke

Electrical discharge that carriers positive charge UP into cloud

Additional Strokes

A typical CG lightning strike is made of 3 or more strokes (can be as many as 30)

After initial “return stroke,” additional strokes can occur using discharge channel taken by previous stroke

Strokes are separated by 30-40 milliseconds, and thus appear to us as one lightning strike

Lightning Flash Sequence

Steps of Lightning: Animation

Lightning Sequence Video

http://www.youtube.com/watch?v=6MUYsIjTKvk

Lightning in Slow Motion

http://www.youtube.com/watch?v=W9xzU0xjlhE

More Types of Lightning

Positive Lightning

Heat Lightning

Red Sprites and Blue Jets

Positive Lightning

This is a variation of CG lightning

Electrical discharge between positively charged upper part of cloud and negatively charged ground

Positive Lightning

Positive Lightning

Heat Lightning

On clear night, lightning can be seen from hundreds of miles away

Too far away to hear thunder

Called “Heat Lightning” because of its occurrence during hot summer nights over Eastern USA

Red Sprites and Blue Jets

Electrical discharges that occur above thunderstorms

Faint and fast moving

Red Sprites occur in mesosphere, and are result of positive CG lightning

Blue Jets occur in stratosphere

Red Sprites and Blue Jets

Lightning Frequency

Observations

Greater frequency of lightning flashes over land than ocean

The region with the highest amount of flashes per time is equatorial Africa

Can notice the effect of ocean currents!