Project Atmosphere American Meteorological Society Weather series: Thunderstorms

Key Concepts Formation of a thunderstorm Stages of thunderstorm Damages of thunderstorm Safety during a thunderstorm

What Is a Thunderstorm? A thunderstorm is a rain shower during which you hear thunder. Since thunder comes from lightning, all thunderstorms have lightning. A thunderstorm is classified as "severe" when it contains one or more of the following: hail three-quarter inch or greater, winds in excess of 57.5 mph, tornado.

How Does a Thunderstorm Form? Three basic ingredients are required for a thunderstorm to form: moisture, rising unstable air (air that keeps rising when given a nudge), and a lifting mechanism to provide the "nudge."

What Fronts Cause Thunderstorms? Most Likely a Cold front A cold front occurs when a mass of cold air plows into a mass of warm air. Because the cold air is denser it forces the warm air upward. As the air rises it expands and cools. This causes the moisture in the warm air to condense and release large amounts of energy that can power a thunderstorm and possibly a tornado.

The Thunderstorm Life Cycle Thunderstorms have a life cycle of three stages: The developing stage, the mature stage, and the dissipating stage.

Developing Stage Marked by a cumulus cloud that is being pushed upward by a rising column of air (updraft). The cumulus cloud soon looks like a tower (called towering cumulus) as the updraft continues to develop. There is little to no rain during this stage but occasional lightning. The developing stage lasts about 10 minutes.

Mature Stage The updraft continues to feed the storm, but precipitation begins to fall out of the storm, and a downdraft begins When the downdraft and rain-cooled air spreads out along the ground it forms a gust front, or a line of gusty winds. The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. The storm occasionally has a black or dark green appearance.

Severe Thunderstorm In order for a thunderstorm to be considered severe 1 of the following must occur: Hail ¾ inch or bigger Winds greater than 57.5 mi/hr Tornado

Dissipating Stage Eventually, a large amount of precipitation is produced and the updraft is overcome by the downdraft beginning the dissipating stage. At the ground, the gust front moves out a long distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. Rainfall decreases in intensity, but lightning remains a danger.

Thunderstorm Hazards Hail Downbursts Lightning Flash floods Tornadoes (this will be covered at another station)

Hail Falls from cumulonimbus clouds. Largest recorded was 7 inches in diameter (soccer size). Most damage occurs to crops. Can dent automobiles and smash windows. Not produced by all thunderstorms.

Downbursts An exceptionally strong downdraft that diverges horizontally as a surge of potentially destructive winds. Most hazardous to aircraft.

Lightning Safety Go inside a structure, a vehicle, a cave… If you are outside and your hair stands on end, crouch down, but do not lie down. Stay away from metal of all kinds. If you can hear thunder, you are close enough to get struck by lightning. It doesn’t have to be raining at your site.

Flash Flood Safety Floodwaters only 18” deep can carry away most cars. Watch where you camp. Never cross a stream deeper than your knees. Abandon your car if it stalls, and seek higher ground.

Watches and Warnings Severe Thunderstorm Watch: conditions are conducive to the development of severe thunderstorms in and close to the watch area. Warning: a severe thunderstorm has actually been observed by spotters or indicated on radar, and is occurring or imminent in the warning area.