Wednesday, June 24, 2015

Where There is Thunder, There is Lightning

This week is Lightning Safety Awareness week in many parts of the country. If your local National Weather Service office is participating then you may have seen their links to some lightning information on their web page. I'll be sharing some information about this topic in my posts over the next several days.

Lightning is one of the oldest recorded natural phenomena. Despite our long study of lightning (remember Ben Franklin?) it remains on the frontier of atmospheric science.

Lightning occurs throughout the country and in all seasons. The area of highest incidence extends from the central and southern Plains through the Midwest and in the Southeast. Florida is the lightning capital of the U.S. with an average of 27 to more than 33 flashes per square mile per year.

We credit Ben Franklin with discovering that lightning was electrical in nature and was in fact static electricity. Ben was one incredibly lucky man with that experiment.

As Ben discovered, lightning is a sudden electrostatic discharge from a thunderstorm. These giant sparks can extend from the cloud to the ground or objects on the ground, between clouds, within the cloud, or even between the cloud and air.

In many respects lightning is similar to the static electricity spark you may see or feel during the winter when the air is very dry and you touch a metallic object. When you walk across a carpet, for example, electrons move from the atoms in the carpet to you.  You are, in effect, negatively charged. When you touch a metallic object like a door knob, the electrons move from you to the knob. The zap you feel and may hear are the electrons moving from you to the door knob through an electric spark.

Similar processes occur in a thunderstorm. As the thunderstorm develops the updrafts and downdrafts within the storm result in collisions between the precipitation particles within the cloud. Near the top of the storm these are usually small ice crystals. The ice crystals become positively charged and are carried higher into the storm because they are lighter. As a result the top of the storm becomes positively charged, while the middle and lower layers become negatively charged. Small ice crystals and small hail occur in the middle of the storm, while in the lower layer raindrops and melting hail occur. The collisions between these particles cause some to lose electrons and become negatively charged. The negative charge in the middle and lower layers of the thunderstorm cloud induces a positive charge in the ground underneath the storm, and the positively charged anvil induces the ground under the anvil to become negatively charged.

How electrical charges are distributed in a thunderstorm. Credit: NOAA

In the early stages of thunderstorm development the air acts as an insulating layer between the cloud and its surroundings. As the electrical charges build up within the thunderstorm, the difference between, for example, the negatively charged middle portion of the cloud and the ground become large enough to overcome the insulating effects of the air, and a lightning discharge occurs. When this discharge occurs between the middle of the cloud and upper portion of the storm, “in-cloud” lightning occurs.

In-cloud lightning. 

When the discharge occurs between the negatively charged region of one storm and the positively charged region of another, it is called cloud-to-cloud lightning.

Cloud-to-cloud lightning over Washington D.C.
Credit: Kevin Ambrose, Capital Weather Gang

 Lightning can also occur between the cloud and the surrounding air.

Cloud-to-air lightning.
Source: YouTube

Cloud-to-ground lightning occurs when the discharge happens between the cloud and the ground.

Cloud-to-ground lightning.
Credit: UK Met Office

Cloud-to-ground lightning strikes account for about 25 percent of the lightning flashes worldwide. They are some of the most spectacular and also the most dangerous because they hit the ground or objects on the ground. The lightning discharge lasts only a few microseconds, but the process of its formation is complex.

Credit: NOAA
A lightning strike begins when an ionized channel of air, called a step leader, develops from the thunderstorm to the ground. As the step leader zigzags toward the ground, the electrical field increases as the quantity of positive charge residing on the Earth's surface becomes even greater. The electric field is strongest on grounded objects whose tops are closest to the base of the storm such as trees and tall buildings (that’s why you stay away from tall objects during a thunderstorm). This charge begins to migrate upward through buildings, trees and people into the air. When this upward rising positive charge – an upward leader or streamer – meets with the leader in the air above the surface, a conductive path is completed. Electrons surge along this path creating the visible lightning bolt. The rapid flow of electrons heats the surrounding air causing it to explosively expand, sending out a shock wave we hear as thunder.

Lightning is the third highest cause of weather-related deaths after flooding and extreme heat, causing an average of 51 fatalities a year with hundreds more injured. The National Lightning Safety Institute estimates that costs and losses due to lightning in the U.S. could be as high as $8 - $10 billion per year.

In my next post we'll look at how people are struck by lightning, the effects of being struck, and some statistics  on lightning fatalities that may surprise you.

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