When a cold air mass is being replaced by a warm air mass, this transition zone is known as a warm front. The movement of a warm front typically follows the direction of the southwest to northeast and brings a change in weather. On weather maps, a solid red line typically represents a warm front, as the red stands for warm air and blue stands for cold air.
The air behind a warm front is not only typically warmer but is also often moister, than the air ahead of it. A warm front can be hundreds of miles long, and a drop in air pressure is usually also detected.