© 2003 Brooks/Cole, a division of Thomson Learning, Inc.

Air Masses and FrontsAir Masses and Fronts

Chapter 9Chapter 9

Air MassesAir Masses

An extremely large body of air whose properties of An extremely large body of air whose properties of temperature and moisture content are similar in temperature and moisture content are similar in any horizontal directionany horizontal direction

Form when air stagnates for long periods of time Form when air stagnates for long periods of time over a uniform surfaceover a uniform surface

Over time, the air mass acquires the temperature Over time, the air mass acquires the temperature and moisture properties of the underlying surfaceand moisture properties of the underlying surface

Naming Air MassesNaming Air Masses

Air masses are named by combining their Air masses are named by combining their temperature and moisture characteristics:temperature and moisture characteristics:

Temperature:Temperature:– Warm/Hot: Tropical (T)Warm/Hot: Tropical (T)– – Really Cold: Arctic (A)Really Cold: Arctic (A)

Moisture:Moisture:– Dry: Dry: – Moist: Maritime (m) formed over oceanMoist: Maritime (m) formed over ocean

For example, a warm, moist air mass will be For example, a warm, moist air mass will be named maritime tropical (mT)named maritime tropical (mT)

4 Types of Air Masses4 Types of Air Masses

Maritime polarMaritime polar Continental polarContinental polar

Source RegionsSource Regions

The birthplaces of air massesThe birthplaces of air masses Must be dominated by light winds (or none Must be dominated by light winds (or none

at all)at all) Must have an extensive, uniform surfaceMust have an extensive, uniform surface Good regions are found:Good regions are found:

– –

North American Air MassesNorth American Air Masses

Air masses that strongly affect the weather of Air masses that strongly affect the weather of North America come from seven regions.North America come from seven regions.

1.1.Maritime Tropical PacificMaritime Tropical Pacific

2.2.Continental TropicalContinental Tropical

3.3.Maritime Tropical GulfMaritime Tropical Gulf

4.4.Maritime Tropical AtlanticMaritime Tropical Atlantic

5.5.Maritime Polar AtlanticMaritime Polar Atlantic

6.6.Continental Polar CanadianContinental Polar Canadian

7.7.Maritime Polar PacificMaritime Polar Pacific

Polar Air MassesPolar Air Masses

Three polar air masses influence weather in Three polar air masses influence weather in N. America.N. America.

1.1.

2.2.Continental Polar CanadianContinental Polar Canadian

3.3.Maritime Polar PacificMaritime Polar Pacific

The cP air masses move SE across Canada The cP air masses move SE across Canada into the U.S.into the U.S.

They will occasionally reach as far south as the They will occasionally reach as far south as the Gulf Coast.Gulf Coast.

mP originate over the Pacific waters near mP originate over the Pacific waters near Alaska.Alaska.

They lose much of their moisture as they pass They lose much of their moisture as they pass

over the Cascades and Sierra Nevada.over the Cascades and Sierra Nevada.

mP form over the Atlantic ocean by mP form over the Atlantic ocean by Greenland and Iceland.Greenland and Iceland.

They bring cold, cloudy weather and They bring cold, cloudy weather and

precipitation in the winter.precipitation in the winter. Produce cool weather with low clouds and Produce cool weather with low clouds and

fog in the summer.fog in the summer.

Tropical Air MassesTropical Air Masses

Four tropical air masses influence weather Four tropical air masses influence weather in N. America.in N. America.

1.1.Maritime Tropical PacificMaritime Tropical Pacific

2.2.Continental TropicalContinental Tropical

3.3.Maritime Tropical GulfMaritime Tropical Gulf

4.4.Maritime Tropical AtlanticMaritime Tropical Atlantic

They bring clear, dry, and very hot weather They bring clear, dry, and very hot weather

as they form over the deserts in N. America.as they form over the deserts in N. America. mT form over the Gulf of Mexico.mT form over the Gulf of Mexico. They move N. across the E. U.S. bringing They move N. across the E. U.S. bringing

mild, cloudy weather in the winter and hot mild, cloudy weather in the winter and hot humid weather in the summer.humid weather in the summer.

mT from over the warm areas of the N. mT from over the warm areas of the N. Pacific ocean.Pacific ocean.

In the winter mT air masses may bring In the winter mT air masses may bring

heavy precipitation and thunderstorms to the heavy precipitation and thunderstorms to the coast and the SW deserts.coast and the SW deserts.

Air Mass ModificationAir Mass Modification

When an air mass moves away from its source region, it When an air mass moves away from its source region, it will begin to change its temperature and moisture will begin to change its temperature and moisture properties to that of the new underlying surfaceproperties to that of the new underlying surface

If an air mass is heated from below, that will lead to If an air mass is heated from below, that will lead to instabilityinstability

For example, when cP air moves over the Great Lakes in For example, when cP air moves over the Great Lakes in

the late-fall, heat and moisture is gained which results in the late-fall, heat and moisture is gained which results in “lake effect” snow showers“lake effect” snow showers

Air masses also may be modified by ascent and descent Air masses also may be modified by ascent and descent (mP moves up a mountain, precipitation removes the (mP moves up a mountain, precipitation removes the moisture, the air mass is more like cP on the other side)moisture, the air mass is more like cP on the other side)