An air mass is a large body of air with a similar temperature, moisture and lapse rate characteristics over thousands of kilometers. The air mass characteristics are acquired in source areas where the air remains for some time allowing it to acquire the characteristics of the surface over which it rests. Air masses are classified on the basis of the latitude and the surface type of the source area. A front is a boundary between one air mass and another. The leading edge of cold air advancing into an area is called a cold front .
Warm air moving into an area of cold air is called a warm front .
When a cold front overtakes a warm front and forces the warm air aloft, it is called an occluded front . An important weather system affecting middle and high latitudes is a traveling low pressure system called a wave cyclone . Wave cyclones move from west to east and involve interaction of warm and cold fronts that often produces cyclonic precipitation. A tornado is an intense low pressure system with very high wind speeds. Tornadoes develop with thunderstorms and hurricanes.