UDU
Junior Geography Chapter 13
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Chapter 13 - Air Masses, Fronts & Depressions
Air Mass - large body of air in which temperatures, pressure and humidity are
fairly uniform (the same).
Front The place where two different air masses meet. The fronts move as the
air masses move. A front which is followed by a cold air mass is called
a cold front. A front that is followed by a warm air mass is called a
warm front.
Polar Fronts - usually occur between 50° and 60° N. & S. This is where cold
polar air mass meets the warm tropical air mass.
In the N. Hemisphere the cold polar winds blow from a N.E.
direction and they are dry winds. The warm tropical winds blow
from a SW direction and are damp or wet winds.
Depressions - often occur along the polar front in the Northern Hemisphere. This
front is over Ireland, and is the reason for our often wet and
changeable weather.
Formation:
1. The cold Polar air meets the warm tropical air at the polar front.
Diagram page 63:
2. These air masses push against each other and the warm air mass makes a dent
into the cold air mass.
Diagram:
3. The cold air wraps itself around the back of the warm air mass until a large
wedge of warm air is almost surrounded by the colder air. A depression has
been formed.
Diagram.
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UDU
Junior Geography Chapter 13
Depressions and Irish weather:
Depressions develop over the Atlantic Ocean. They move in from the west and
often reach Ireland, especially in winter.
Diagram:page 65
1. Cold Sector - temperature low, high cirrus cloud, dry, pressure falls slowly.
2.Warm Front - stratus clouds, continuous rainfall, pressure still falling.
3. Warm Sector - temperature rises, stratus cloud, drizzle or dry, pressure low.
4. Cold Front - temperature falls, cumulus cloud, blustery showers, pressure rises.
5. Cold Sector - temperature low, some cumulus cloud, becoming dry, pressure
high.
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