What Are Thunderstorms? What Causes Them?
Thunderstorms affect relatively small areas when compared with hurricanes and winter storms. The typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. Despite their small size, ALL thunderstorms are dangerous! Of the estimated 100,000 thunderstorms that occur each year in the United States, about 10 percent are classified as severe.
The National Weather Service considers a thunderstorm severe if it produces hail at least 3/4-inch in diameter, winds of 58 mph or higher, or a tornado. Every Thunderstorm Needs:
MOISTURE - to form clouds and rain.
UNSTABLE AIR - warm air that can rise rapidly.
LIFT - cold or warm fronts, sea breezes, mountains, or the sun’s heat are capable of lifting air to help form thunderstorms.
There are probably 1500 to 2000 thunderstorms active around the world at any one time. It has been estimated from satellite observations that lightning flashes approximately 50 -100 times per second on a global basis. Lightning therefore flashes as often as three billion times each year across the whole planet.
Virtually anything you do outside during spring and summer involves a lightning risk. Swimming, boating, hiking, golfing, soccer...if you are out there, you are a target. Most lightning deaths in the U.S. (in descending order) occur (1) in open fields or ball fields, (2) under trees, (3) while boating and fishing, (4) near tractors and heavy equipment, (5) on golf courses, (6) and on telephones (but not cellular or cordless ones).
It is a myth that if it is not raining there is no danger of being struck by lightning. Bolts can and often do strike as much as ten miles outside of the rain area of the parent storm. Recent research on lightning deaths finds that most fatalities occur in the period when the storm appears to be ending. During the height of most thunderstorms, people are inside seeking protection from the rain. For the ten or more minute period after the rain ends, and even after the sun comes out, lightning is still a threat. People apparently have enough sense to come in out of the rain, but not always to get out of the way of lightning. If you can hear thunder, you are close enough to the storm to be struck by lightning.
Most lightning strikes average 2 to 3 miles long and carry a current of 10000 Amps at 100 million volts.
STRAIGHT LINE WINDS
Straight-line winds are responsible for most thunderstormwind damage. Winds can exceed 100 mph! One type of straight-line wind, the downburst, is a small area of rapidly descending air beneath a thunderstorm.
What YOU Can Do Before Severe Weather Strikes
Be Prepared...It's Up to YOU!
Each year, many people are killed or seriously injured by severe weather despite advance warning. Some did not hear the warning, while others heard the warning but did not believe it would happen to them. The following preparedness information, combined with timely severe weather watches and warnings, could save your life. Once you receive a warning or observe threatening skies, you must make the decision to seek shelter before the storm arrives. It could be the most important decision you will ever make.
What YOU Can Do When Threatening Weather Approaches
Lightning Safety Rules
If Caught Outdoors and No Shelter Is Nearby
What to Listen For...
Severe Thunderstorm Watch: Tells you when and where severe thunderstorms are likely to occur. Watch the sky and stay tuned to know when warnings are issued.
Severe Thunderstorm Warning: Issued when severe weather has been reported by spotters or indicated by radar. Warnings indicate imminent danger to life and property to those in the path of the storm. You should seek safe shelter immediately.
Stay Informed. . . Use NOAA Weather Radio
The National Weather Service continuously broadcasts warnings and forecasts that can be received by NOAA Weather Radios, which are sold in many stores. The average range is 40 miles, depending on topography.
Purchase a radio that has a battery back-up and a Specific Area Message Encoder feature, which automatically alerts you when a watch or warning is issued for your county.