Homepage > ... > Divisions, Bureaus & Units > Emergency Management > ThunderstormsE-mail storyPrint friendly format


All thunderstorms - also known as electrical storms or thundershowers - produce lightning and thunder which usually follows or is followed by a brief period of heavy rainfall. They can occur at any time of the day or night and during any season of the year; however, they are typically seen as cooler weather follows after a long dry period of weather.

These storms are created when warm rising moist air converges with cooler air at high altitude forming clouds known as "cumulonimbus" (Latin for "rain heaps"). As the water vapor in the moist air rises into the cooler air, the vapor becomes a combination electrically charged ice and water droplets that build up static electricity. When too much static electricity is built up in the cloud, it discharges as a bolt of lightning. The thunder immediately following is the sound barrier being broken as the lightning courses through the air. 


Thunderstorms typically fall into four different categories:

Single Cell ThunderstormSingle Cell: The most observed type of thunderstorm, a Single Cell is an individual thunder-storm lasting between 20-30 minutes. These are fast passing thunderstorms that are accompanied by or immediately following a brief period of heavy rainfall.

Multi-cell Cluster: These are the second most typical type of thunder-storms observed in Southern California. The multi-cell cluster is a group of thunderstorms all in various stages of development.
These thunderstorms can last several hours for periods of 20-30 minutes at a time as each cell moves through its cycle. These storms can produce hail and flash flooding.

Squall Line - Squall Lines (or "derecho" Spanish for straight) is a long line of severe thunderstorms best known to produce extremely stong downdraft winds.

They are also called "bow" and/or "comma" echos due to the nature of their shape, or "cold fronts" because they usually form ahead or along a long band of cold moist air. They are capable of producing extremely heavy rain and hail, frequent lightning, very strong winds, tornadoes, and over bodies of water they can produce waterspouts.

Supercell - These are thunderstorms that mainly exist in the mid-west. A Supercell is a highly organized dangerous type of thunderstorm which poses the highest threat to life and property. These storms can produce winds up to 175 miles per hour which create a rotating updraft of moist air (mesocyclone) responsible for producing extreme weather such as tornadoes on land, and waterspouts and hurricanes over bodies of water. A Supercell thunderstorm can last for extremely long periods of time and is always followed by torrential rainfall or hail that can measure larger than 2 inches in diameter

An average of 300 people are injured and 80 people are killed in the United States each year by lightning.  Lightning often strikes as far as 10 miles away from any rainfall. Most lightning injuries occur when people are caught outdoors in the summer months during the afternoon and evening.

Thunderstorms can also produce strong winds, hail, flash flooding, and/or tornados. On average, 140 people are killed by flash floods annually.

Remember these tips if you are caught in a thunderstorm:
  1. If you can hear thunder, you are close enough to the storm to be struck by lightning.
    • If you are outside, get inside a home, building, or hard top automobile.
    • If you cannot get inside a safe place, avoid areas that will attract lightning such as tall isolated trees, hilltops, open fields, the beach, or on open water.
    • Also avoid anything metal such as farm equipment, motorcycles, bicycles, golf carts, and golf clubs.
    • Seek shelter in a low area under a thick growth of small trees, or a low place such as a ravine or valley.
      If you are on a boat or in the water, get to land and find shelter immediately.
    • Distance yourself several yards from other people rather than huddling together.
  2. Avoid showering or bathing during a thunderstorm.
  3. Use a corded telephone only for emergencies.
    • Cordless and cellular phones are safe to use.
  4. Turn off electrical items such as computers. Power surges from lightning can cause serious damage.
  5. If you feel your hair stand on end, it may be an indication that lightning is about to strike.
  6. Make yourself the smallest target possible and minimize your contact with the ground.
    • Squat low to the ground on the balls of your feet and place your hands over your ears with your head between your knees.
    • Do not lie flat on the ground.