Air Masses, Fronts, Cyclones,
and Anticyclones
What causes our weather to
change from day to day?
There are warm masses of air
There are cold masses of air.
These masses of air flow about the earth.
When they meet warm air rises and cold air
sinks.
When air masses of different
temperatures meet; we experience most of
the changes in our weather.
Air Mass
 Large body of air that has similar temperature and
moisture throughout.
 An air mass gets its moisture and temperature
characteristics from the area over which it forms.
Tm air masses come from the south west and
originate over the Azores or the Caribbean. They bring
mild, damp, cloudy weather.
Tc air masses come from the south and originate over
dry northern Africa . They bring hot, dry weather and
summer heatwaves.
Pm air masses come from the north-west and
originate over the north Atlantic. They bring cool, moist
conditions.
Pc air masses come from the east and originate over
Scandinavia and Russia . They bring clear dry
conditions - cold in winter, warm in summer.
What happens when two different
air masses meet?
 A boundary forms between them (due to temperature and
humidity). They do not mix. The weather at a front is
usually cloudy and stormy.
 There are 3 different kinds of fronts:
• cold
• warm
• occluded
Cold Fronts
 When a cold air mass bumps into a warm front the
cold air sinks down and the warm air mass rises up
 When the warm air rises it cools and the water vapour
condenses into droplets of liquid water
 Clouds form- possibly rain, storm or snow
 Cold fronts move quickly
 They cause abrupt weather changes including
thunderstorms
 After a cold front passes, cool dry air moves in often
bring clear skies and cooler temperatures
Warm Fronts
 Clouds, storms and rain also
accompany warm fronts
 Occur when warm air collides
with slowly moving cold air mass
 If the warm air is humid showers and light rain will fall
along the front where the warm and cold air meet.
 If the warm air is dry scattered clouds form.
 Warm fronts move much slower than cold fronts so the
weather may be rainy or foggy for several days.
 When a warm front passes through an area the weather is
likely to be warm and humid
 In winter warm fronts bring snow.
Occluded Fronts
 A warm air mass is caught between two cold air masses
 The denser cold air masses will push up the warm air
mass and push it upward.
 The two cold air masses may meet in the middle and
may mix together.
 The ground may become cooler
 The Warm Air mass is cut off or occluded from the
ground.
 When the warm air cools and its water vapor condenses
the weather may turn cloudy and rainy or snowy
Depressions:
Low Pressure
 Warm air at the center of a depression rises as a result the
air pressure decreases.
 Cooler air blows toward this lower air pressure from nearby
areas where the air pressure is higher.
 Winds spiral inward toward the center of the system.
 Winds in cyclones spin anti-clockwise in the Northern
Hemisphere.
 Depressions play a major role in our weather in the
Northern Hemisphere. As air rises in a Low the air cools,
forming clouds and precipitation.
Anticyclones:
High Pressure
 High pressure centres of dry air
 Winds spiral outward from the center of the high
moving toward areas of lower pressure
 Cool air moves downward from the troposphere.
 As it does the air warms up so its relative
humidity drops.
 The descending air in the high causes dry clear
weather as the water vapour cannot condense
Summer Anticyclones
• Hot days with few or
no clouds.
• Light winds.
• Cooling of ground
leading to morning
mist.
• Warm moist air rising
from the ground
forming thunderstorms.
• Cloud cover over
Eastern England caused
by light winds blowing
over the cooler North
Sea.
Winter Anticyclones
• Cloudless skies but less
radiation due to the low
angle of the sun.
• Temperature drop,
making the days cold
and the nights even
colder due to lack of
cloud cover.
• Fog and frost forming
at night.
• Cold air from Asia
bringing snow to the
East.