My last two columns have dealt with the basics of our atmosphere, the direction of prevailing winds, the major air masses in and around North America and the different kinds of fronts (sort of a battleground of air masses).

To start a discussion of bad weather, consider the passage of fronts, which can result either in a prolonged bout of rainy, drizzly weather (passage of a warm front) or a sudden thunderstorm, followed by clear skies (passage of a cold front).

A warm front moving over this area comes from the mT (maritime tropical) air mass. Originating over the tropical Atlantic Ocean, this air mass will be warm and moist. Warm air (being lighter than colder air) will ride over the colder, surface air.

This gentle incursion will first be noted as wispy, cirrus clouds. Then more layered cirrostratus clouds will appear (halos may be seen around the moon). Next come altostratus clouds (alto = middle height) that are lumpy (like badly made mashed potatoes) or cirrocumulus (mackerel) clouds. Lastly, nimbostratus clouds bring in the rain. This whole sequence of cloud types, from high to low clouds may take several days.

A cold front moving over this area may come from either the Canadian cP (continental polar) air mass or the Atlantic mP (maritime polar) air mass. Unlike the warm front, the cold front is like a chisel, driving underneath the warmer air. As the warm air is rudely shoved upward, great towering clouds can develop; squalls and violent thunderstorms can come up suddenly.

If a cold front and a warm front, traveling in opposite directions, collide, we can have a stationary front. This can cause long periods of rain, low winds and dull cloud ceilings.

A good weather map used by meteorologists closely resembles a topographical map used by hunters and surveyors.

On a topographical map, the hills and mountains are surrounded by lines of constant elevation that are spread apart where the terrain has a low slope and close together where the terrain is steep. On a weather map, the mountains and valleys are the high air pressure regions and the low air pressure regions.

The lines on a weather map are lines of constant air pressure or isobars. A region of high pressure is called an anticyclone. Here the winds swirl clockwise around the center of the high. A region of low pressure is called a cyclone.

Naturally, the winds around a low move (in the opposite direction as a high) counter clockwise. Where the isobars are close together, the winds blow strongly; where the isobars are far apart, the winds are low.

The granddaddys of the cyclones are the hurricanes that form over the moist, warm waters of the equatorial Atlantic Ocean. (Typhoons are the Pacific version of the hurricanes.)

A minimum sea water temperature of 81 degrees F is required for a hurricane to form. Atlantic hurricane season is from June 1 to Nov. 15 while Pacific typhoon season is from May 15 to Nov. 30. Once a hurricane passes over land, it loses most of its energy and is downgraded to a tropical storm (less than 74 miles/hour).

Tornadoes are much smaller and are often connected with the passage of a cold front. In the eye of a hurricane, the air is descending (high pressure) and the winds are negligible.

In the funnel of a tornado, the air is ascending (very low pressure) and here the winds can be much faster than any hurricane. Most tornadoes form over the contiguous United States where moist hot air comes in contact with cold, dry air.

Tornadoes typically last a few hours while a hurricane formed over the ocean can last a week.

The evening moon grows in lighted width this week as it moves from Taurus (tonight) through Gemini and into Cancer.

The moon will be full this Friday night, when it appears near the planet Mars. Mars has a yellowish tint; this Wednesday, Mars will be brightest and closest to the Earth (62 million miles) for 2010.

To find Mars, take Orion’s belt and go down and left to the brilliant star Sirius (sounds like serious). From Sirius, go leftward to the bright star Procyon (pro-SEE-on). Then yet farther to the left is steadily shining Mars.

Now showing at the Frostburg State Planetarium is “Our Glorious Atmosphere,” with free public showings at 4 p.m. and 7 p.m. today and next Sunday. Following the planetarium program, you can also visit the Science Discovery Center where there is a great collection of preserved animals from five continents.

If you would like a copy of our Planetarium bookmark with our 2010 schedule and a small map (showing Planetarium, Science Center and parking), leave your name and mailing address at (301) 687-7799.

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