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.