When a warm air mass meets a cold air mass, they do not mix because of their density differences. Instead, they form a front, usually hundreds of miles long. Warm air rises since it is lighter, and at high altitude, it cools, and the water vapor it contains condenses. This type of front is called a warm front. The atmosphere is relatively stable when warm, less dense air moves over the colder, denser air. The two air masses push against each other along a line called a front. As air masses move, pushed by winds, they directly influence the weather in the regions over which they pass. In this way, they help to circulate heat and humidity in the atmosphere.