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Posted at 11:00 AM ET, 03/14/2008

Weather 101: The Madness to March Winds

By Dan Stillman

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 ...

Large-Scale Wind


Model forecast map of surface pressure (blue lines) for this coming Sunday shows low pressure (red spotlight) and associated precipitation heading out to sea, and high pressure (blue spotlight) moving toward the D.C. area (white asterisk). Each blue line represents a change in pressure of 4 millibars. The closer together they are, such as over portions of the mid-Atlantic and Northeast, the stronger the wind will be. Sunday's winds are forecast to be near 20 mph. Credit: Unisys.

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.

Localized Wind


Credt: National Weather Service.

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.

By Dan Stillman  | March 14, 2008; 11:00 AM ET
Categories:  Education  
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Comments

Wow! Dan! As someone who is fascinated by weather but understands very little about it, I'm giving a definite thumbs up to this post. Thank you for devoting a whole post to explaining recent weather phenomena and answering some questions that I was definitely asking as I watched the tree in my back yard drop branches like leaves last week.

And thanks for using layman's terms!

Posted by: Laura in NWDC | March 14, 2008 11:43 AM | Report abuse

The Iceberg Regatta tomorrow is the traditional Potomac River sailing season kick-off for my sailing club. I've shipped onto a club boat....what kind of winds do we face? I heard NW 10-15kt somewhere - and from Camden's post earlier, it sounds like we have a chance of staying mostly dry tomorrow. We depart Washington Sailing Marina at 9am - can CWG give me some idea of the wind outlook? Thanks!

Posted by: ~sg | March 14, 2008 12:38 PM | Report abuse

Posted by: FuelForFire | March 14, 2008 12:49 PM | Report abuse

All this wind talk is getting me pumped up about hurricane season. I dont wish for destruction, but I do enjoy following the storms.

What a bummer this winter. All I saw was a couple flurries flying around. Down in Virginia our snow drought this winter was worse then DC. Oh well...Next year, I hope we are luckier.

Posted by: skinsfn | March 14, 2008 1:15 PM | Report abuse

Nice post Dan! Still learning all the time here!

Posted by: Mike from the Blue Ridge | March 14, 2008 1:28 PM | Report abuse

Love it! I'm sure one Weather 101 a day is too much to ask, but I'll take as many as I can get...

Posted by: mookie | March 14, 2008 2:21 PM | Report abuse

sg- a northerly wind at about 10 knots or so sounds right and yes, a mostly dry day time.

Posted by: Jason, Capital Weather Gang | March 14, 2008 3:06 PM | Report abuse

We're usually a little too far north for a lot of thunderstorms in March. The typical severe-weather season starts in February on the Gulf Coast, gradually works its way north with the advancing sun angle, and reaches the northern tier of states along the Canadian border by June or July. Our chief month for tornadoes seems to be April.....with May and June for non-tornadic severe thundrstorms.

This March has been a little more active for thunderstorms than usual in this area, though, because of the passage of several strong low-pressure centers just to our north that allowed us to be in the turbulent warm sector.

Posted by: Mike | March 14, 2008 3:12 PM | Report abuse

A good diagram, Dan, but there is another big factor in thunderstorms that produces strong winds.....not just a dry layer at 10,000-15,000 feet.

Thunderstorms usually reach their most severe intensities with what are called "tilted-updraft" storms. A normal, summer, air-mass thunderstorm occurs in a weak upper-level flow, thus allowing the storm's downdraft to gradually choke off the updraft flow of incoming warm, moist air, and the storm's intensity abates and dies.

This does not happen when winds aloft are strong....such in the vicinity of a jet stream. That is why one of the key ingrediates forecasters look for in predicting severe thunderstorms is the presence, aloft, of cold air and strong winds....what is known as "wind shear". In a tilted-updraft storm, the downdraft does not fall back down on the updraft and choke it off....it is tilted by the jet stream winds at an angle, and descends with considerable force out ahead of the updraft. This can allow for storms of incredible violence....triple-digit wind gusts, large hailstones, and, if the wind direction varies with height, spinning and tornadoes.

Storms known as super-cells, that also produce tornadoes and extremely strong winds/hail, can also form from these conditions, but they are different, structually, from tilted-updraft storms. Supercells are huge, individual storms that sweep in warm, moist air from all sides in a spinning motion. The entire storm becomes one large, spinning dynamo.

Posted by: Mike | March 14, 2008 3:25 PM | Report abuse

Thanks Mike -- Nice description. I also didn't mention that thunderstorm downdrafts can transport strong winds from the upper levels down to the ground. But alas, there's only so much detail and information that I wanted to get into here.

Posted by: Dan, Capital Weather Gang | March 14, 2008 3:29 PM | Report abuse

Thanks, Jason! I'm looking forward to a cold but dry sail.

Posted by: ~sg | March 14, 2008 4:19 PM | Report abuse

So that's where the winds come from! Thanks very much for the explanation, Dan, and I second the motion for more Weather 101's.

Posted by: tinkerbelle | March 14, 2008 9:12 PM | Report abuse

The comments to this entry are closed.

 
 
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