AIR MASSES and FRONTS

Air Masses A large body of air throughout which temperature and moisture are similar.

Air moves from areas of high pressure to areas of low pressure.

Directions of Air Mass Movements in North America

Types of Air Masses Air masses are classified according to their source regions. These regions also determine the temperature and humidity of the air mass Source Region Type of air Symbol Continental Dry c Maritime Moist m Tropical Warm T Polar Cold P

Continental Air Masses Form over large land masses Level of humidity is very low Air mass will remain over its source region for days or weeks

Two Types of Continental Air Masses Continental polar air masses masses move into the northern US, occasionally reach to the Gulf Coast brings cool, dry weather in summer brings very cold weather to the northern US in winter

Continental Tropical bring dry hot weather in the summer Continental Tropical bring dry hot weather in the summer do not form in the winter

Maritime Air Masses Form over oceans or other large bodies of water Humidity tends to be high Commonly bring precipitation and fog

Two Types of Maritime Air Masses Maritime polar air masses bring rain and snow to Pacific Coast in winter move inland and eastward over the Cascades, the Sierra Nevada, and the Rocky Mountains (lose moisture, warm slightly, bring cool, dry weather to the central United States

Maritime tropical air masses bring mild, often cloudy weather in the winter unless it collides with cold polar air bring hot, humid weather and thunderstorms in the summer

FRONTS

FRONTS A front is a boundary between air masses of differing densities. A typical front is several hundred kilometers long, some can be several thousand kilometers long. Fronts do not exist in the tropics because no air masses with significant temperature differences exist there.

TYPES OF FRONTS For a front to form, one air mass must collide with another air mass. The kind of front that forms is determined by how the air masses move in relationship to each other.

COLD FRONTS Cold air mass overtakes a warm air mass. The moving cold air lifts the warm air Brings thunderheads followed by cool, fair weather.

WARM FRONTS Cold air mass retreats from an area Less dense warm air rises over the cooler air Brings gentle rain or light snow followed by warmer, milder weather.

STATIONARY FRONTS Two air masses meet, the cold air moves parallel to the front and neither air mass is displaced Air masses move very slowly or not at all Brings many days of clouds and precipitation

OCCLUDED FRONTS Fast-moving cold front overtakes a warm front The warm air is lifted completely off the ground Brings strong winds and heavy precipitation.