A warm or cold front is defined by change in temperatures and wind direction. Warm fronts are usually on the leading edge of a low pressure system, which is generally on the northeastern side, bringing warmer temperatures from the south, northward. On weather maps they are defined as red half-circles that point in the direction that the front is moving.
Because warm air is less dense and lighter than cold air, as the warm air moves through it gradually advances over the cold air mass – or gradual frontal lifting. This process allows for clouds and precipitation to develop ahead of the frontal boundary.