Download Meteorology Notes, part 2

Air Masses
4.4
Air masses & Fronts
• An air mass is a large body of air of similar
temperature, humidity, & air pressure.
• Four major types of air masses influence
the weather in North America: maritime
tropical, continental tropical, maritime polar,
& continental polar.
• Tropical: warm air mass, forms in the
tropics, low air pressure
• Polar: cold air mass, forms in the poles,
high air pressure.
Air masses & Fronts
• Maritime: form over oceans, can be very
humid.
• Continental: forms over land, can be very
dry.
• In the continental US, air masses are
commonly moved by the prevailing
westerlies (pushing air masses from west
to east) & jet streams (bands of highspeed winds about 10km above Earth’s
surface).
Fronts-the boundary where the air masses
meet.
• Cold front: a fast-moving cold air mass overtakes
a warm air mass. (represented by triangles)
• Warm front: a warm air mass overtakes a slowmoving cold air mass. (represented by half circles)
• Stationary front: cold & warm air masses meet,
but neither can move the other. (represented by half
circles and triangles on opposite sides of a line)
• Occluded front: a warm air mass is caught
between 2 cooler air masses. (represented by half
circles and triangles on the same side of a line)
Air Masses & Fronts
• When a cold front passes through an
area, colder, drier air moves in, often
bringing clear skies, a shift in wind, &
lower temps.
• After a warm front passes through an
area, the weather is likely to be humid
& warm.
Air Masses & Fronts
• If a stationary front remains stalled
over an area, it may bring many days
of clouds & precipitation.
• As the warm air of an occluded front
cools and its water vapor condenses,
the weather may turn cloudy & rain or
snow may fall.
•
http://www.fcgov.com/oem/oem-images/warm-front.jpg
Air masses & Fronts
• A swirling center of low pressure is a
cyclone.
• Cyclones & decreasing air pressure are
associated with clouds, wind, &
precipitation.
• A swirling center of high pressure is an
anticyclone.
• The descending air in an anticyclone
generally causes dry, clear weather.
Anticyclones & Cyclones