Download Air masses

Chapter 9 Lecture
Understanding
Weather and
Climate
Seventh Edition
Air Masses
and Fronts
Frode Stordal, University of Oslo
Redina L. Herman
Western Illinois University
© 2015 Pearson Education, Inc.
Air Masses and Their Source Regions
• 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.
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Air Masses and Their Source Regions
• 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.
© 2015 Pearson Education, Inc.
Air Masses and Their Source Regions
• Source Regions
© 2015 Pearson Education, Inc.
Air Masses and Their Source Regions
• Source Regions
© 2015 Pearson Education, Inc.
Air Mass Formation
• 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.
© 2015 Pearson Education, Inc.
Air Mass Formation
• 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.
© 2015 Pearson Education, Inc.
Air Mass Formation
• Continental Polar (cP) and Continental Arctic (cA)
Air Masses
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Air Mass Formation
• Continental Polar (cP)
• Continental Arctic (cA) Air Masses
© 2015 Pearson Education, Inc.
Air Mass Formation
• 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.
© 2015 Pearson Education, Inc.
Air Mass Formation
• 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.
© 2015 Pearson Education, Inc.
Air Mass Formation
• 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.
© 2015 Pearson Education, Inc.
Fronts
• Midlatitude Cyclone
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Fronts
• 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
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Fronts
• 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.
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Fronts
• Cold Fronts
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Fronts
• 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.
© 2015 Pearson Education, Inc.
Fronts
• Warm Fronts
© 2015 Pearson Education, Inc.
Fronts
• Warm Fronts
© 2015 Pearson Education, Inc.