Air Masses and Fronts

An air mass is a huge body of air that has similar 1. temperature 2. humidity (amount of water vapor in the air) 3. air pressure

Types of Air Masses Scientists classify air masses according to two characteristics: temperature and humidity. Whether an air mass is warm or cold depends on the temperature of the region over which the air mass forms. Four major types of air masses influence the weather in North America (and most of the world): Continental tropical Continental polar Maritime tropical Maritime polar

The types of air masses are based on four root words: Tropical-- A warm air mass that forms in the tropics and has low air pressure. Polar-- A cold air mass that forms in the northern Latitudes and has high air pressure. Maritime—a humid air mass that forms over oceans. Continental—a dry air masses forms over land, usually in the middle of continents. Don’t the names and characteristics make sense?

Put together, they make the 4 basic types of air masses—with first names that tell use wet or dry and last names that tell us cold or warm! Maritime tropical: as wet/humid and warm Continental tropical: as dry and warm Maritime polar: as wet/humid and cold Continental polar : as dry and cold

Types of Air Masses

How Air Masses Move The prevailing Westerlies (major wind belt in the US) generally push air masses from west to east.

Fronts: Huge air masses move around and bump into each other but they don’t mix very well because of differences in density and temperature. The area or edge where the air masses meet and do not mix becomes a front (battle area).

When air masses meet at a front, the collision often causes storms and changeable weather. There are four types of fronts: Cold fronts Warm fronts Stationary fronts Occluded fronts The kind of front that develops depends on the characteristics of the air masses and how they are moving!

Cold Fronts Cold air is dense and tends to sink. Warm air is less dense and tends to rise. When a rapidly moving cold air mass runs into a slowly moving warm air mass, the denser cold air slides under the lighter warm air. The warm air is pushed upward. The front that forms is called a cold front. Cold fronts move quickly, so they can cause quick weather changes, including violent thunderstorms. Afterwards, look for clear skies & cooler temperatures!

Cold Fronts

A cold front symbol—The direction that the teeth point indicate the direction the front is moving.

Warm Fronts   At a warm front, a moving warm air mass collides with a slowly moving cold air mass. Because cold air is more dense than warm air, the warm air moves over the cold air. Clouds, storms, and rain also accompany warm fronts. The more humid the warm air, the greater the chance of rain. If not so wet, expect clouds. Afterwards, expect warm and humid weather with a few clouds.

Warm Front

Warm Front symbol—The directions that the bumps face is the direction the front is moving.

Stationary Fronts   Sometimes cold and warm air masses meet, but neither one has enough force to move the other. The two air masses face each other in a “standoff.” In this case, the front is called a stationary front. Look for days of drizzle.

Stationary Front Symbol

Occluded Fronts   A warm air mass is caught between two cooler air masses. The denser cool air masses move underneath the less dense warm air mass and push it upward. The warm air mass is cut off, or occluded, from the ground.

Occluded front symbol – The directions that the bumps face is the direction the front is moving.

Cold Fronts

Warm Front

Stationary Front

Occluded Front