• Weather patterns resulting from movements
of large bodies of air called air masses.
• An air mass is a huge body of air that has
similar temperatures and amounts of moisture
at any given altitude.
• Due to its size, it may take several days for an
air mass to move over an area.
• This causes us to experience fairly constant
weather, often called air-mass weather.
• When an air mass moves out of a region it
carries temperature and moisture
conditions with it.
• As it moves, the characteristics of an air
mass change and so does the weather in
the area over which the air mass moves.
A cold, dry air mass from
northern Canada moves
southward. The initial
temperature of the air
mass is -46°C. It warms
13°C by the time it
reaches Winnipeg. The
air mass continues to
warm as it moves south
through the Great Plains
and into Mexico.
Through its journey, the
air mass warms, but still
brings with it some of the
coldest winter weather to
the places in its path.
What are the four types of air masses?
• The area over which an air mass gets its
characteristic properties of temperature
and moisture is called its source region.
• Air masses are named according to their
– Polar (P) air masses form at high altitudes
towards Earth’s poles.
• Polar air masses are cold.
– Air masses that form at low latitudes are
tropical (T) air masses.
• Tropical air masses are warm.
• Air masses are also classified according to
the surface over which they form.
• Continental (c) air masses form over land.
– Are likely to be dry.
• Maritime (m) air masses form over water.
– Are likely to be humid.
Classifying Air Masses
• Using this classification scheme,
there are four basic types of air
–Continental Polar (cP) = Dry and
–Continental Tropical (cT) = Dry
and warm or hot.
–Maritime Polar (mP) = humid and
–Maritime Tropical (mT) = Humid
• When two air masses meet, they form a
front, which is a boundary that separates
two air masses.
• Fronts can form between any two
contrasting air masses.
• Fronts are often associated with some
form of precipitation.
• Fronts are more narrow than air masses
Types of Fronts
• Fronts are classified according to the
temperature of the advancing front.
• There are four types
1. Warm Fronts:
• Forms when warm air moves into an
area previously covered by cool air.
• Produce light to moderate
precipitation over a large area for an
• A gradual increase in temperature
• A wind shift from the east to the
2. Cold Fronts:
• cold, dense air moves into a region
occupied by warmer air.
• more violent weather associated with a
• Heavy downpours and gusty winds.
• Once the cold front passes,
temperatures drop and wind shifts.
• The weather behind a cold front is
dominated by a cold air mass, so, the
weather clears soon after it passes.
3. Stationary Fronts:
• Surface position of the front does not move,
and a stationary front forms.
– The flow of air on either side of a front is neither
toward the cold air mass nor toward the warm air
– Parallel to the line of the front.
• Sometimes, gentle to moderate precipitation
occurs along a stationary front.
4. Occluded Front:
• When an active cold front overtakes a
• Advancing cold air wedges the warm
• The weather associated with an
occluded front is generally complex.