© 2015 Pearson Education, Inc.

Air Masses

and Fronts

Chapter 9 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo

© 2015 Pearson Education, Inc.

• Introduction – Air masses contain uniform temperature and humidity

characteristics.

• They affect vast areas.

– Fronts are boundaries between different air masses.

• Fronts are spatially limited and usually linked to midlatitude

cyclones.

Air Masses and Their Source Regions

© 2015 Pearson Education, Inc.

• Source Regions

– The areas of the globe where air masses form are called

source regions.

– Long-term heating or cooling of large bodies of air must

remain over a source region for a substantial length of time

for an air mass to form.

– The air mass will reflect the temperature and humidity

characteristics of the source region.

• These characteristics are categorized using a lowercase and

uppercase letter scheming.

Air Masses and Their Source Regions

© 2015 Pearson Education, Inc.

• Source Regions

Air Masses and Their Source Regions

© 2015 Pearson Education, Inc.

• Source Regions

Air Masses and Their Source Regions

© 2015 Pearson Education, Inc.

• Continental Polar (cP) Air Mass

– Continental polar (cP) air masses form over large,

high-latitude landmasses, such as N Canada and Siberia.

– In winter, these locations have short days, low sun angle,

and are usually snow-covered (reflecting more energy).

– The cold, dry air associated with this air mass is typically

stable.

Air Mass Formation

© 2015 Pearson Education, Inc.

• Continental Arctic (cA) Air Mass

– Continental arctic (cA) air masses are characterized by

extremely cold and dry conditions, as it contains very little

water vapor.

– The boundary between cA and cP air is the arctic front.

– Some meteorologists believe that the distinction between

cA and cP is minor.

Air Mass Formation

© 2015 Pearson Education, Inc.

• Continental Polar (cP) and Continental Arctic (cA) Air Masses

Air Mass Formation

© 2015 Pearson Education, Inc.

• Continental Polar (cP) • Continental Arctic (cA) Air Masses

Air Mass Formation

© 2015 Pearson Education, Inc.

• Maritime Polar (mP) Air Masses

– Maritime polar (mP) air masses form over the North Pacific

and are more moderate than cP.

– mP air masses migrate to the east, crossing the Gulf of

Alaska, west coast of N America.

– Impacts N America primarily in winter.

– mP air masses affect the east cost of N America in the form

of northeasters, bringing cold winds and heavy snowfall to

the US northeast.

Air Mass Formation

© 2015 Pearson Education, Inc.

• Continental Tropical (cT) Air Masses

– Continental tropical (cT) air masses form during the

summer over hot, low-latitude areas (southwestern US and

Mexico).

– cT air masses are hot and dry.

– These air masses are very unstable but due to low availability

of water vapor, do not usually produce precipitation.

– When water vapor is present, thunderstorms form within

these air masses.

Air Mass Formation

© 2015 Pearson Education, Inc.

• Maritime Tropical (mT) Air Masses

– Maritime tropical (mT) air masses form over warm tropical

waters, influencing southeastern US primarily in the summer.

– mT air masses form over the Atlantic and Gulf of Mexico and

migrate toward N America.

– High moisture content and instability create opportunities for

thunderstorms and heavy rains.

– Ideal for precipitation and clouds.

Air Mass Formation

© 2015 Pearson Education, Inc.

• Midlatitude Cyclone

Fronts

© 2015 Pearson Education, Inc.

• Fronts

– Fronts separate air masses and bring about changes in

temperature and humidity as one air mass is replaced by

another.

– There are four general types of fronts associated with

midlatitude cyclones.

• Cold fronts

• Warm fronts

• Stationary fronts

• Occluded fronts

Fronts

© 2015 Pearson Education, Inc.

• Cold Fronts

– A cold front is a mass of cold air advancing toward warm air.

– Typically associated with heavy precipitation, rain, or snow,

combined with rapid temperature drops.

Fronts

© 2015 Pearson Education, Inc.

• Cold Fronts

Fronts

© 2015 Pearson Education, Inc.

• Warm Fronts

– Warm fronts are warm air moving toward cold air.

– This overrunning process produces large amounts of warm,

moist air over cooler, drier air.

– Shallow stratus clouds dominate and bring light precipitation

to affected regions.

– Stable regions above the warmer air create vertically limited

clouds and light precipitation.

– Frontal fogs may occur as rain evaporates in the colder air

near the surface.

Fronts

© 2015 Pearson Education, Inc.

• Warm Fronts

Fronts

© 2015 Pearson Education, Inc.

Fronts

• Warm Fronts