MET 125 Physical Meteorology

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Presentation transcript:

MET 125 Physical Meteorology Atmospheric Electricity, Part 1 Lightning 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!