All About Thunderstorms

Thunderstorms and springtime go hand in hand and the reasons are directly related to the change of airmasses as the atmosphere transitions from cold to warm.

Longer days and higher sun angles create warmer days and warmer air masses. These contrast with lingering cold air masses still being pulled out of Canada into the continental United States.

Contrasts between warming land surfaces and still cold upper levels create much stronger instability that can produce stronger storms with high winds, hail, and in extreme cases, tornadoes. 1

What is a thunderstorm and what do they produce?

• A thunderstorm occurs when a cloud producing a shower develops to a sufficient height and begins to produce lightning, and thus thunder.
• 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.
• Nearly 1,800 thunderstorms are occurring at any moment around the world. That`s 16 million a year!
• Despite their small size, all thunderstorms are dangerous.
• Every thunderstorm produces lightning, which kills more people each year than tornadoes.
• Heavy rain from thunderstorms can lead to flash flooding.
• Strong winds, hail, and tornadoes are also dangers associated with some thunderstorms.

How do thunderstorms form?

Every thunderstorm needs moisture to form the clouds and thus the rain. They also need instability-- this comes in the form of warm air near the surface rising rapidly, or instability due to the forcing of upwards motion. Instability can be caused by fronts, sea breezes and mountains.

Life Cycle of a Thunderstorm:

• Developing Stage: Towering cumulus cloud indicates rising air. Usually little if any rain during this stage. Lasts about 10 minutes. Occasional lightning during this stage.
• Mature Stage: Most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. The storm occasionally has a black or dark green appearance. Lasts an average of 10 to 20 minutes but stronger storms may last much longer.
• Dissipating Stage: Rainfall decreases in intensity. A few thunderstorms produce a burst of strong winds during this stage. Lightning remains a danger during this stage.

Of the estimated 100,000 thunderstorms that occur each year in the United States, only about 10 percent are classified as severe. The National Weather Service (NWS) considers a thunderstorm severe if it produces hail at least 3/4-inch in diameter, wind gusts of 58 mph or higher, or spawns tornadoes.