Air Masses and Fronts

A situation opposite to the preceding is one in which a warm air mass approaches and then slides up and over a cold air mass. The boundary formed in this case is a warm front. As the warm air mass comes into contact with the cold air mass, it is cooled and some of the moisture held within it condenses to form clouds. In most cases, the first clouds to appear are high cirrus clouds, followed sometime later by stratus and nimbostratus clouds.

Warm fronts are designated on weather maps by means of solid lines to which are attached solid half circles. The direction in which the half circles point shows the direction in which the warm front is moving.