Cumberland Times-News

Bob Doyle - Astronomy

July 13, 2014

Fronts, highs, lows determine weather

— Weather news on television and internet focus on violent weather, extreme temperatures and flooding.

But the real determinants of our everyday weather are the movements of fronts, highs and lows (both refer to air pressure). The fronts are the battle lines between air masses of different temperatures.

You can see the fronts, highs and lows on the colored map of the continental U.S. that appears on the back page of the front section of each Cumberland Times-News.

The symbol for a warm front is a line with bumps that point in the direction of the warm front advance. (I remember this by thinking of fever blisters.)

For the cold front, its line has little triangles pointing in the direction of the cold front’s movement. (I imagine the little triangles as icicles.) An occluded front has been lifted above the surface with little movement. Its line is marked by alternating semicircles and triangles.

As a cold front advances, the denser cold air pushes the warm air upward; this may cause thunderstorms and showers.

I think of a cold front as a chisel that gets underneath the warmer air. Then after a cold front passage, the sky often is very clear, lending itself to night sky observing of stars and planets.

For a warm front, the warm air slides over the colder air. So well before the warm front passes, there is a gradual change in the clouds, going from the high cirrus clouds, then the middle clouds. When the warm front passes over, there are low clouds which often bring rain.

I imagine the warm front as less abrupt, trying to tip toe over the colder air. In an occluded front, a cold front overtakes a warm front and the warm front loses its contact with the ground.

An occluded front over the Tri-State area may result in some rain.

The highs are region where the air is descending, resulting in higher than normal air pressure. Highs mean calm skies and gentle breezes with the winds moving in a clockwise direction around the high in the northern hemisphere.

So if a high sits over a parched region for weeks on end, there’s little chance of rain. Lows are regions where the air is rising, lowering the air pressure.

Winds around a low move in a counterclockwise direction in the northern hemisphere. Lows are connected with rainy weather. So if a low settles over our area, you can be sure of rainy day after rainy day. Between a high and a low on the weather map, the winds tend to blow from high to low.     

How are lows and high pressure regions formed? A low pressure system starts with a cold air mass meeting a warm air mass. The warm air rises above the cold air, resulting in lower pressure. As the warm mass is lifted upward, its moisture forms clouds and rain. It is at this time that the fronts begin to rotate.

In the mid latitudes (where our Tri-State area is), there is a belt of high pressure regions that circle the globe. During the cold months, this belt is broken up by invasions of cold polar air from the north. During the warm months, the high pressure belt is fragmented by warm, tropical air masses from the South.

There is a general drift of fronts from West to East across the United States. So barring interfering fronts moving North or South, a front passing over the Midwest will be over our area in a few days.

My reference for the above article is: Weather: A Visual Guide by Buckley, Hopkins and Whitaker, published in 2008 by Firefly Books.

SKY SIGHTS AHEAD: This evening the yellow planet Mars is above the white-blue star Spica, separated by 1.3 degrees (a little over 2 moon widths). Look in the southwest after 10 p.m. The brighter Mars shines steadily while Spica will flicker. Mars will appear a little bit brighter than Spica.

This week, the moon will be rising about 40 minutes later each evening. By the coming weekend, the moon won’t rise until very late in the evening.

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

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Bob Doyle - Astronomy
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