Weather

What is an Air Mass?

A large body of air with similar temperature and moisture throughout is called an air mass. Air masses take on the temperature and humidity of the land below. For example, and air mass over snow covered areas is cold and dry. Four main types of air masses influence our weather.

Continental polar	this air mass forms over northern Cananda and Alaska. These dry air masses move south to east. They cause winter cold waves in the central and eastern United States.
Maritime polar	These masses form over the northern Pacific Ocean. These bring winter rain to the West Coast. As the mass moves East it rises over the Rockies loses its moisture and becomes heavy snow.
Maritime Tropical	These air masses form in the south and come from the tropical waters of the Atlantic, Pacific and Gulf of Mexico. These masses bring hot, humid weather in the summer and warm spells in the winter.
Continental tropical	These air masses form in the south eastern United States and Mexico. These are hot, dry air masses form during summer and may cause drought. Sometimes they move into Oklahoma and Kansas bring dry heat instead of humidity.

Warm air masses and cold air masses do not mix very well because they have different densities. So, instead of mixing a boundary forms between the two masses. This is called a front. Fronts, like air masses usually move from west to east. These fronts form when one type of air mass enters an area occupied by a different kind of air mass

A cold front forms when cold air rushes in to replace the warm air which will rise. The result is the formation of large cumulonimbus clouds appear. These clouds often bring thunderstorms and rain showers

A warm front is formed when a warm air mass pushes out a cold air mass. The warm air glides up and over the cold air. As the warm front approaches Cirrus clouds will form in the sky. After the cirrus clouds, stratus and nimbostratus clouds appear. The barometer will fall and a steady rain or snow begin.

Sometimes two different types of air mass will stay over a region for several days. The weather is much like that of a warm front--a long, steady rain or snow.

An occluded front forms when a cold front overtakes a slower moving warm front. It is more complicated than other fronts because the two air masses interact instead of forming a boundary. Steady rain and snow occur at an occluded front.

A dry line is a boundary between a dry air mass and a moist air mass. Dry lines are most commonly formed at the eastern edge of the Rocky Mountains. Thunderstorms and tornados can form along the dry line.

Basically, the interaction of all these air masses creates are weather. The type of air masses that interact decide what type of weather this will be. For a review of air masses click this sentence.

These large high and low pressure air masses move from West to East across the United States. They create the changing weather that is common is many parts of the country.

Nature's Slide Show! CloudCam.com

Scientists use a barometer to measure air pressure. A mercury barometer is a very accurate way to measure air pressure.

How it works...

A long glass tube that is open at one end is filled with mercury. The tube is place, open end down, in a dish. Some of the mercury flows out of the tube into the dish, leaving a space with no air at the top of the tube. Air pressure pushes down of the surface of the mercury in the dish. This pressure holds the column of mercury at a certain height inside the tube. If air pressure decreases, the column of mercury falls. If air pressure increases the column rises. Standard barometric pressure at sea level is 76cm.

What's it mean?

When barometric pressure falls we can often expect rain and stormy conditions to form. A rising barometer means fair weather is ahead.

Investigate

The type of barometer in most schools and homes is called an aneroid barometer. Find out how it works and how it is different than a mercury barometer.