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3. Fronts and Cyclones
Types of fronts
Large areas of the earth's surface have the same 'type' of air (e.g. temperature and humidity) sitting over them. These bodies of similar air are called air masses. Fronts on a weather map separate different types of air. The symbols on these lines indicate the direction of movement of the line and the type of air that is behind it. The two main types are cold and warm fronts. A cold front has triangles on one side of the line showing the direction of movement, a warm front has half-circles. Some fronts will have both triangles and half-circles. If they appear on opposite sides of the line then the front is stationary and will be a warm of cold front depending on its future movement. If these symbols are on the same side of the line then it is an occluded front where the cold front and warm front have merged and the warm (lighter) air has been forced off the ground.
The use of these fronts becomes clear if we take an example of a typical weather system. As cyclones are areas of convergence they bring different types of air together. These air masses don't mix easily and are separated by these fronts (notice there are no fronts in high-pressure areas). As the cyclone develops warm air drawn into the system is forced off the ground to be replaced by cold air. This process can be seen as the area between the warm and cold front becomes smaller over time. Where the warm air is completely displaced an occluded front is shown.
