In conditions of great instability, the cloud will develop into a full-fledged cumulonimbus, or thunderstorm, cloud.Ī number of factors may trigger the uplift of warm air. If the atmospheric instability is great enough, this air will rise to great heights, and the cloud will develop vertically (upwardly) into a towering cumulus cloud. On reaching the dew point, the temperature at which the air is saturated, the moisture within the air condenses, forming a cloud. Thunderstorms occur when warm, moist air rises quickly through an unstable atmosphere. When the air parcel reaches a height at which the atmosphere is stable, meaning that the surrounding air is warmer than the air parcel, it will rise no further. As long as the atmosphere remains unstable, an air parcel will continue to rise. "Unstable" means that the surrounding air is colder than a rising air parcel.
The other is that the atmosphere must be unstable. The first is that the surface air must be warm and humid. Two atmospheric conditions are required for the development of a thunderstorm. The storm clouds move away, the sun comes out, and the air is cooler and less humid. Then it strikes nearby and is followed by a thunderclap so loud it jolts you to your feet. Lightning brightens the sky here and there. Soon the rain intensifies to a downpour and the wind turns cold and blows wildly. Next the wind picks up and the rain begins to fall in large drops. Then comes a calm period in which the air feels still, hot, and very muggy. As these clouds approach, the sky darkens, almost blocking out the sunlight. On the horizon there are enormous thunderstorm clouds, with their whitish tops and dark undersides. The morning's haze, when the sky appeared milky, has given way to a line of tall cotton-ball-like cumulus clouds. Imagine standing on your front porch on a hot, humid afternoon. Watching a thunderstorm approach can be quite an exhilarating experience. Earth is struck by lightning from these storms one hundred times every second. About forty thousand thunderstorms occur worldwide each day, and fourteen million thunderstorms happen each year. A typical thunderstorm unleashes 125 million gallons (568 liters) of water and enough electricity to provide power to the entire United States for twenty minutes.Īt any given time, there are about two thousand thunderstorms underway around the world. Thunderstorms are produced by cumulonimbus clouds, which are tall, dark, and ominous-and they are giant storehouses of energy.
Flash floods, which are sudden, intense, localized floods caused by heavy rainfall, kill an average of 140 people yearly. It is the second biggest weather killer in the country, topped only by flash floods. Furthermore, in the United States lightning causes between 75 and 100 deaths and about 550 injuries annually. Lightning, which occurs with all thunderstorms, sets off about ten thousand forest fires each year in the United States alone, causing several hundred million dollars in property damage. While straight-line winds called derechos occur less frequently and result in fewer fatalities, a single derecho can cause millions of dollars in damage. Tornadoes also cause about eighty deaths and fifteen hundred injuries annually. Hailstorms, for instance, storms which bring frozen precipitation called hailstones that range in size from peas to softballs, are responsible for nearly $1 billion a year in crop damage. Severe thunderstorms and their related phenomena produce significant human injuries and fatalities, as well as property damage, each year. Only about 1 percent of all thunderstorms are the source of tornadoes, whose violently rotating winds reach the ground and can cause great damage. Only 10 percent of thunderstorms are considered "severe," meaning they produce some combination of high winds, hail, flash floods, and tornadoes. Lightning is a short-lived, bright flash of light that is produced by a 100 million-volt electrical discharge in the atmosphere. During that time, it produces strong winds, heavy rain, and lightning. An average thunderstorm is only 15 miles (24 kilometers) in diameter and lasts about thirty minutes. Thunderstorms are relatively small, but intense, storm systems.
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