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Weather changes as air masses move
Air masses are large bodies of air
• Air mass- a large volume of air in which
temperature and humidity are nearly the
same in different locations at the same
• Air masses take on the characteristics of
the Earth below them and takes them to
new locations
Characteristics of an Air Mass
• Air mass names contain two words
– First describes the area where the air mass
• Continental-forms over land, dry
• Marine- forms over water, moist
– Second describes how close to the equator
the air mass formed
• Tropical-near the equator, warm land and water
• Polar- far from the equator, cold land and water
Movement of an Air Mass
• Air masses:
– Travel away from the area where they form
– Move west to east along the jet stream
– Fast moving air masses do not take on the
characteristics of the land below
Weather changes where air
masses meet
• Front- boundary between air masses
• Cold Front
– Move into regions quickly
– Push warm air up, water vapor condenses
forming clouds
– Produces precipitation
– Cooler air arrives
• Warm Front
– Move slowly
– Rises above cold front forming clouds
– Produces many hours or rain or snow
– Leaves warmer air behind it
• Stationary Front
– Occurs where air masses first meet or when
cold or warm fronts stop moving
– Air moves sideways or upwards along the
– Produces cloud cover for days
– Becomes either a warm or cold front when
one air mass advances
High Pressure Systems
• Formed when an air moves all the way
around a high pressure system
• Large and change slowly
• Generally brings clear skies and calm air
• Can form air masses (warm or cold)
Low Pressure Systems
• Large weather system that surrounds a
center of low pressure
• Air rises quickly producing stormy weather
• Usually from along the boundary between
a warm and cold front
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