What causes thunderstorms?

Dateline article| David Murphy|

by David Murphy

Thunderstorms are caused by the clash of two different air masses and the violent column of rising air that results from this collision. Usually, a thunderstorm forms within a warm, moist mass of air that's being assaulted by a cooler, drier air mass.

The process is most often started by a cold front. Cold fronts steer cooler air toward warmer air. The cooler air wedges beneath the warm air and pushes it up from the surface. This causes the warm air to cool and its moisture to condense from a gas into a liquid (forming clouds and rain).

But by itself, the average cold front is usually not strong enough to cause a thunderstorm. Other forcing mechanisms are needed to get the air rising even faster. Examples include Shearing Winds, or winds that change direction or speed as you move higher from the surface. Another is Vorticity, or an area of curving or spinning air above the surface. A third is a Jet Streak, or a fast moving ribbon of air within the jet stream, even higher above the surface.

In each case, these additional forcing mechanisms encourage the air to rise more rapidly, either by acting as a vacuum that sucks the air up from the surface, or by causing the air to rotate counter-clockwise which makes it easier to lift. Eventually, a column of rising, rotating air called an updraft is formed. The rising, turning wind inside an updraft grows so turbulent and violent, the electrical charges inside the cloud become separated, a key component of any thunderstorm. Lightning and thunder result as these separated charges are drawn back together.

By the way, another forcing mechanism is a layer of dry air in the developing storm's middle section. This doesn't encourage spinning, but it does evaporate some of that moisture rising from the surface. Since evaporation is a cooling process, the middle of the storm becomes cooler---which encourages even more of that warm air below to quickly rise! And here's another one: in South Jersey and Delaware, an afternoon Sea Breeze rolling-in off the ocean can spark a thunderstorm, as it draws cooler air toward warmer inland air. A case in point occurred in the summer of 2005 in parts of Burlington, Camden and Gloucester Counties. The resulting thunderstorms did not affect a wide area and didn't last long, but where they hit, they produced drenching downpours, gusty winds and a bit of property damage.

Thunderstorms can also be triggered by boundaries of cool air left over from old thunderstorms. These outflow boundaries can help spark a new thunderstorm as much as a day later.

To summarize, a thunderstorm is caused by warm, moist air being forced to rise by cooler air and any of several other forcing mechanisms that make the air rise even faster. In the end, the air rises so quickly and violently, that lightning, thunder, hail and high winds can also result. For details on thunderstorm development, click on the topic: How do thunderstorms form?

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