Air masses and fronts

Fronts

The boundary where two air masses meet but do not mix. Storms and changeable weather often develop along the front.

The characteristics

The characteristics of an air mass depends on temperature and moisture content (humidity).

Prevailing westerlies

The major wind belts over the continental US push air from west to east.

Isobars

Lines that connect places on the map that have the same air pressure.

Tropical

Masses form in the tropics and have low air pressure.

Polar

Masses form north of 50° North latitude and South 50° South latitude.

Air masses

A huge body of air that has similar temperature humidity and air pressure at any given height.

Cold front

A rapidly moving cold air mass overtakes a warm air mass. When a rapidly moving cold air mass runs into a slow-moving warm air mass the denser cold air slides under the less dense warm air causing it to rise. This results in abrupt weather changes including thunderstorms.

Cyclone

A swirling center of low pressure. The winds spin inward and counter clockwise in the northern hemisphere. As air rises in the cyclone the air cools forming clouds and precipitation. Cyclones and decreasing air pressure brings clouds winds and precipitation.

Occluded front

A warm air mass is caught between two cooler air masses. The weather may turn cloudy and rain or snow may fall.

Continental

Air masses form over land and are drier than maritime air masses.

Maritime

Air masses form over oceans so the air becomes very humid.

Cyclones and anticyclones

As air masses collide to form fronts the boundary between the front sometimes becomes distorted causing air to swirl and develop low-pressure centers.

Jet streams

Bands of high-speed wins about 10 km above Earth surface.

Maritime polar

Called humid air masses form over the icy cold North Pacific and North Atlantic oceans. Even in the summer these masses of cool humid air can often bring fog rain and cool temperatures to the West Coast.

Stationary front

Cold and warm air meet and neither can move the other. Two air masses face each other in a standoff. It may bring days of clouds and precipitation.

What kind of air

Cold is less air has low-pressure so the air sinks. Warm less dense air has low-pressure so the air rises.

Anticyclone

High-pressure centers of dry air. Air spins outward from the center of an anti-cyclone moving toward areas of lower pressure. Air falls in an anti-cyclone and generally brings dry clear weather.

Continental tropical

Hot dry air masses for mostly in the south and bring hot dry air to the Southern Plains.

Continental polar

Large air masses form over Central and Canada and bring cold-weather with very little humidity.

Warm front

Warm air mass overtakes a slower moving cold air mass. Because the cold air is more dense than warm air the warm air moves over the cold air. After warm front passes through an area the weather is likely to be warm and humid.

Maritime tropical

Warm humid air masses that form over tropical oceans. In the summer they usually bring hot humid weather. In the winter and humid air mass can bring heavy rain or snow.