Dr. James Stoos yahoo–Spring is a time of transition from the cold, snowy winter to the hot days of summer. In most parts of the United States, this time of year corresponds to the "rainy season" during which thunderstorms frequently occur. We have all heard that the month of March comes "in like a lion and out like a lamb," but this is really due to the changeable nature of the weather at this time of year. It is just as likely to be the other way around, with storms occurring at any part of this transition season. Some facts about thunderstorms:
What causes thunderstorms?
Thunderstorms are the result of powerful convection -- the rapid upward movement of warm, moist air through dryer, cooler air. As this air moves higher it cools and the moisture condenses forming towering cumulonimbus clouds. Water droplets and ice form and begin falling through the clouds. As they fall, they collide with each other and grow larger. The falling droplets create a downdraft of air that generates the strong winds associated with thunderstorms. The strongest and largest type of thunderstorm is called a supercell.
Where do thunderstorms occur most frequently?
Thunderstorm intensity is related to the contrast in moisture and temperature of adjacent air masses, and this is strongest in mid-latitudes. It is further facilitated by flat open terrain such as that in the Great Plains of the US. This region is known as "Tornado Alley" due to its large number of severe storms, which often spawn tornadoes. As seen in this graphic showing the number of hail days per year, there appears to be a bulls-eye right over the state of Oklahoma. It is no coincidence that this is the location of the National Severe Storms Laboratory!
Are storms happening more often due to climate change?
Many scientists think so. Although any single storm cannot be attributed to climate change, an increasing trend can be. This seeming paradox requires understanding the difference between weather and climate.
How should we prepare for severe storms?
Damage from thunderstorms is mainly from downburst winds, large hailstones, and flash flooding caused by heavy precipitation. Therefore, it is wise to stay indoors and out of low-lying areas. Watch out for downed power lines and trees. Some thunderstorms spawn tornadoes which of course cause more extreme damage from the higher winds in the resulting vortex.
What is a severe thunderstorm watch?
The National Weather Service will issue a severe thunderstorm watch when "conditions are favorable for the development of severe thunderstorms in and close to the watch area".
What is a severe thunderstorm warning?
The National Weather Service will issue a severe thunderstorm warning when "either a severe thunderstorm is indicated by... radar or a spotter ... and/or winds equal or exceed 58 miles an hour; therefore, people in the affected area should seek safe shelter immediately."

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