Weather 101: The Madness to March Winds
On average, March is the windiest month in the D.C. area, and this March certainly has not disappointed. Already this month, wind gusts at Reagan National Airport have reached 74 mph on March 5, 60 mph on March 8 and 45 mph on March 12.
It's no coincidence that our strongest winds tend to come during the month we transition from winter to spring, when the contrast sharpens between cold air to the north and warm air to the south. Differences in temperature are what create differences in pressure, and differences in pressure cause wind. But often the strongest winds come from a different source -- thunderstorms.
One way to look at it is large-scale wind versus localized wind ...
Pressure, in its simplest definition, is the weight of the air above a given location. High pressure makes air sink, while low pressure allows air to rise. When sinking air under high pressure reaches the ground, it has nowhere to go but to spread out horizontally toward areas of low pressure, where it replaces the air that's rising. This air moving across the Earth's surface -- turned to its right by the Coriolis effect -- is what we know and feel as wind. The bigger the difference between areas of high and low pressure, and the shorter the distance over which the pressure changes, the faster the wind blows.
March winds tend to be on the strong side because, as spring tries to push winter out, differences in temperature tend to be more extreme over a shorter distance, which leads to more dramatic horizontal changes in pressure and thus stronger winds over widespread areas. In the mid-Atlantic and Northeast, gusty winds like those that caused power outages on March 8 can occur when stormy areas of low pressure strengthen as they move away, to the east or northeast, while high pressure approaches from the west or northwest.
Meanwhile, the 74 mph gust that occurred just after midnight on March 5, and many other strong gusts across the area that same night, was born of a different cloth. These were more localized winds caused by severe thunderstorms that rolled through the region. As it turns out, an environment with strong large-scale winds tends to favor severe thunderstorms -- which is why transition seasons have the most severe thunderstorms -- and thus strong localized winds .
Thunderstorms can produce strong wind gusts when some of the rain evaporates as it falls through air that is not completely saturated in the lower half of a thunderstorm. The heat used to convert the water from liquid to gas comes from the water itself and from the surrounding air, thus cooling the air. The cooled air sinks, because it is more dense than surrounding warmer air, and this sinking air is called a "downdraft." A downdraft that becomes localized, hits the ground and spreads out horizontally is called a "downburst." Downbursts can result in localized winds upward of 150 mph, sometimes producing damage on par with that from tornadoes.
No one wants to be in the path of winds that strong. But a little breeze would sure be welcome relief from the heat and humidity in August, our least windy month of the year.
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