What is an electric field?

How do field mills work?

Question: How does lightning form?
Electrically charged particles result from the friction created by rapidly rising and decending air within a convective cloud. The weight of the negatively charged particles tends to migrate to the bottom of the cloud while lighter positively charged particles tend to migrate into higher regions. The natural forces of nature attempt to keep things in balance, equal amounts of negative and positive charge thus, this separation is not normal. The air between these separated areas acts as an insulator until the charge is too great and breaks down. The resulting spark is what we have come to call lightning as the positive and negative charged areas equalize.

Several types of lightning are common:
* In-cloud lightning extends from one charged region of a cloud to another
* Cloud-to-cloud lightning extends between two clouds
* Cloud-to-air lightning extends from a cloud to the air, not touching the ground
* Cloud-to-ground lightning stretches from a cloud to the ground

Question: What are "positive" and "negative" strikes?
The polarity of a lightning strike depends on what type of charge is lowered to ground. About 95% of all cloud-to-ground flashes worldwide are negative. Although less frequent, positive strokes generally contain more current.

Question: How would knowing where lightning strikes are occurring be beneficial?
Over 20 million flashes strike the ground in the United States every year affecting virtually every facet of our lives, from personal safety, to knocking out our local power, to impacting daily travel and communications. Measures can be taken to protect ourselves and the infrastructure around us with a little planning.

Question: Is it possible to predict lightning?
To address this question adequately we need to break it down. If the question asked is to determine if it is possible to "predict" whether a particular storm is likely to generate lightning activity the answer is yes. Electric Field Mills (sensors) have been used for over 25 years to evaluate the potential for lightning by monitoring the electrostatic fields present in the atmosphere. Research has shown that when these fields exceed 2000 volts/meter the potential is very high for lightning to develop.

If the question asked is to "predict" where and when an actual lightning strike will occur the answer is no. If the question asked is to "predict" the direction and path that lightning is likely to travel the answer is yes. To properly address this aspect, one needs to collect and analyze a number of factors that make up a thunderstorm and whether these factors will continue to support the continued growth and movement of a storm. Typically such a system would need to ingest lightning strike data, precipitation data, temp, winds speeds and direction data, etc into a sophisticated algorithm which outputs the storms time of arrival and departure.

Question: What causes a "bolt from the blue"?
Lightning can travel 10 or more miles horizontally from a cloud before striking the ground giving the impression that there is no thunderstorm in the immediate area (overhead).

Question: How many people are struck by lightning?
A visit to the National Weather Service Natural Hazard Statistics will show that lightning kills 75 to 100 people each year in the U.S. alone. It is estimated that there are 10 times as many lightning-related injuries as deaths, totaling several hundred to a thousand injuries each year.

Question: How hot is lightning?
Within the lightning bolt the air is superheated to temperatures exceeding 50,000°F, which is substantially hotter than the surface of sun.