Depressions are areas of low atmospheric pressure but unlike anticyclones occur where two air masses meet - one warm the other cold. Depressions consequently have fronts (a boundary between two air masses). Winds always blow anticlockwise (counterclockwise)in a depression.
You will notice that the fronts are shown in different ways and always tell you what the air is like behind the line. So a line with red semi circles tells you that the air behind the front is warm. A cold front has blue triangles and tells you that the air behind the front is cold.
A depression develops where a warm and cold air mass meet. Low pressure at the surface of the earth draws air in (rather like a vacuum sucks air in) and the warm, unstable air rises. As the warm air rises over the cold air, water vapor in the air starts to cool. At the condensation level clouds start to form. This process can eventually lead to precipitation.
ACTIVITIES
- Define these important terms in your own words - Front, Isobar, Air mass.
- Using a map or diagram to draw the position of a warm and cold front.
- Try to describe and explain the changes in weather that would be experienced in any city as a depression passes over (from west to east).
- Try this handy experiment or demonstration for showing how air masses meet.
- You can also get the full Depressions and Frontal Systems lesson in PDF format suitable for classroom printing here.
