Why Does Colder Air Move in Faster and Sometimes Carry in Dust?

Weather Talk
Thursday, May 1, 2014 - 11:19am

Some Cold Fronts do seem to blow in quickly, bringing in dust along with the cooler air

Another cold front blew in from the northeast yesterday, carrying in dust from the drought stricken Texas Panhandle, the eastern plains of New Mexico and some white gypsum sand dust from White Sands, New Mexico. Before I studied Meteorology, I wondered why cold fronts speed in so fast producing gusty winds and a turbulent atmosphere.
When I arrived in El Paso to work at News Channel 9, 19 years ago, I also was amazed about how much dust was picked up by the cold fronts moving through the Borderland! In today’s “Weather Talk” I thought I would write about why cold fronts move so fast, the reasons why they kick up and carry in so much dust and how long these dust storms can last.

What is a cold front?

A cold front is the leading edge of a colder air mass which is plowing or pushing into a warmer air mass.

Why do most cold fronts move so fast?

The primary factor why cold fronts usually move faster than warm fronts is that the cold air behind the cold front is heavier and denser than the warm air behind the warm front. The heavier, denser, cold air can push the warmer lighter air ahead of the cold front out of the way much easier than the warm air can push the cold air ahead of the warm front.
In fact, warm air never really "pushes" the cold air out of the way. Instead the warm front's movement depends on how fast the cold air is retreating. When warm, lighter air battles with cold, heavier air, the warm air almost always rises up and over the cold air at the surface.

Why is cold air heavier than warm air?

Cold air is denser than warm air. The molecules are packed closer together in cold air. The amount of water vapor in the air also affects the density of the air. The more water vapor that is in the air, the less dense the air becomes.

Because cold fronts move along the ground where they encounter friction, they move slower at ground level than they do further up in the atmosphere. For this reason, cold fronts tend to be more sloped than warm fronts.

Typically cold fronts move faster than warm fronts. The combination of higher speed and slope push warm air masses upward very quickly. This quick upward air movement causes the warm air being displaced to cool quickly, becoming turbulent. This turbulence often can be the cause of extremely violent or severe weather. Because cold fronts move quickly, the weather associated with them typically also moves quickly and passes over a particular location in a short period of time. The turbulent weather generally stays right in line with the front.

A cold front is where a cold air mass is pushing into a warmer air mass. Cold fronts can produce dramatic changes in the weather. They move fast, up to twice as fast as a warm front. Cold air is dense so it is able to quickly plow a warm air mass ahead of it. Most of the time, when the cold front is passing, winds become gusty; there is a sudden drop in temperature, and heavy rain, sometimes with hail, thunder, and lightning. Lifted warm air ahead of the front produces cumulus or cumulonimbus clouds and thunderstorms.

Atmospheric pressure changes from falling to rising at the front. After a cold front moves through your area you may notice that the temperature is cooler, the rain has stopped, and the cumulus clouds are replaced by Stratus and stratocumulus clouds or clear skies.

Why do Cold fronts kick up and carry more dust?

Dust storms are created when these cold fronts sweep onto much warmer or hot, dry land. With the drought conditions in many parts of our country, we have seen more large dust storms over the past few years. As I stated before cold fronts are slanted this works kind of like a bulldozer blade to lift up the warm air and dirt and debris as well. When cold fronts undercut warm air, the pressure gradient increases, and winds shift and become very strong and gusty, as high as 50 to 100 miles per hour (80 to 160 kilometers per hour). These shifting winds also create turbulence and the higher surface temperature creates convective currents at the leading edge of the storm. These forces essentially act to lift particles higher and keep them aloft for longer periods of time. Haboobs, strong wind produced desert dust storms, last on average for 2 to 3 hours. Cold front produced dust storms can carry dust for very long distances and the dust can remain airborne for days! Dust storms in the Out Back of Australia can last up to one to two days!

That is nothing compared to a winter “Shamal” (an Arabic word for “north”). Dust blows off the coast of Iran and over the Strait of Hormuz and the Gulf of Oman. In winter time dust storms are associated chiefly with the passage of westerly depressions in this region. A shamal is sustained winds of 25 knots about 29 mph or stronger and the wind direction is almost always northwesterly. The Shamal produces the most widespread dust storms for this region. The winter Shamal is generally lasts 24-36 hours, but 1 to 3 times a winter it can last as long as 3-5 days! This impacts mainly the Persian Gulf seas where visibility drops below 3 to 5 Nautical Miles.

Cold fronts this winter have brought record cold around most of the rest of our country. Here in the Borderland we really had a rather mild winter. Now whether it is a strong Pacific cold front from the west –northwest or a cold front that backs in from north east-east they can move in quite quickly sometimes and carry in a large amount of blowing dust, especially from February through the beginning of May, during our windy season. We welcome the colder or cooler now because we know the upper 90º to 100º plus degree heat is right around the corner!

To find out more about our Borderland Dust storms read my "Weather Talk"; Are you ready for “Another One Bites the Dust?” http://www.ktsm.com/weather/weather-talk/are-you-ready-%E2%80%9Canother-...

Chuck DeBroder, Chief Meteorologist
KTSM, NewsChannel 9, NBC, El Paso, TX
www.twitter.com/ Chuck DeBroder NC9 @wxchuckNC9

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