© 2003 Brooks/Cole, a division of Thomson Learning, Inc. Air Masses and Fronts Chapter 9

Air Masses  An extremely large body of air whose properties of temperature and moisture content are similar in any horizontal direction   Form when air stagnates for long periods of time over a uniform surface  Over time, the air mass acquires the temperature and moisture properties of the underlying surface

Naming Air Masses  Air masses are named by combining their temperature and moisture characteristics:  Temperature: –Warm/Hot: Tropical (T) – –Really Cold: Arctic (A)  Moisture: –Dry: –Moist: Maritime (m) formed over ocean  For example, a warm, moist air mass will be named maritime tropical (mT)

4 Types of Air Masses  Maritime polar   Continental polar 

Source Regions  The birthplaces of air masses  Must be dominated by light winds (or none at all)  Must have an extensive, uniform surface  Good regions are found: – –

North American Air Masses  Air masses that strongly affect the weather of North America come from seven regions. 1.Maritime Tropical Pacific 2.Continental Tropical 3.Maritime Tropical Gulf 4.Maritime Tropical Atlantic 5.Maritime Polar Atlantic 6.Continental Polar Canadian 7.Maritime Polar Pacific

Polar Air Masses  Three polar air masses influence weather in N. America Continental Polar Canadian 3.Maritime Polar Pacific

 The cP air masses move SE across Canada into the U.S.  They will occasionally reach as far south as the Gulf Coast.  mP originate over the Pacific waters near Alaska.   They lose much of their moisture as they pass over the Cascades and Sierra Nevada.

 mP form over the Atlantic ocean by Greenland and Iceland.   They bring cold, cloudy weather and precipitation in the winter.  Produce cool weather with low clouds and fog in the summer.

Tropical Air Masses  Four tropical air masses influence weather in N. America. 1.Maritime Tropical Pacific 2.Continental Tropical 3.Maritime Tropical Gulf 4.Maritime Tropical Atlantic

  They bring clear, dry, and very hot weather as they form over the deserts in N. America.  mT form over the Gulf of Mexico.  They move N. across the E. U.S. bringing mild, cloudy weather in the winter and hot humid weather in the summer.

 mT from over the warm areas of the N. Pacific ocean.   In the winter mT air masses may bring heavy precipitation and thunderstorms to the coast and the SW deserts.

Air Mass Modification  When an air mass moves away from its source region, it will begin to change its temperature and moisture properties to that of the new underlying surface  If an air mass is heated from below, that will lead to instability   For example, when cP air moves over the Great Lakes in the late-fall, heat and moisture is gained which results in “lake effect” snow showers  Air masses also may be modified by ascent and descent (mP moves up a mountain, precipitation removes the moisture, the air mass is more like cP on the other side)