Warm Front

A warm front forms when a warm air mass pushes into a cooler air mass. Warm fronts move more slowly than cold fronts because it is more difficult for the warm air to push the cold, dense air across the Earth’s surface. Warm fronts often form on the east side of low-pressure systems where warmer air from the south is pushed north.

You will often see high clouds like cirrus, cirrostratus, and middle clouds like altostratus ahead of a warm front. These clouds form in the warm air that is high above the cool air. As the front passes over an area, the clouds become lower, and rain is likely. There can be thunderstorms around the warm front if the air is unstable.

On weather maps, the surface location of a warm front is represented by a solid red line with red, filled-in semicircles along its length. The semicircles indicate the direction that the front is moving. They are on the side of the line where the front is moving.

Below is a summary of the important aspects regarding a warm front:

Definition: A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass.
Rate of Movement: Warm fronts move slowly relative to a cold front
Frontal Slope: The warm air slides over the top of the cooler air and gradually pushes it out of the area. The slope is shallow when compared to a cold front.
Cloud Formation: Warm fronts contain warm air that often has very high humidity. As the warm air is lifted, the temperature drops and condensation occurs. Clouds typically do not have as much vertical development as those associated with a cold front.
Type of Precipitation: Often produces widespread continuous precipitation near the front.

	Before Passage	While Passing	After Passage
Winds	SSE	Variable	SSW
Temperature	Gradual Warming	Steady rise	Warmer then steady
Pressure	Usually Dropping	Levelling Off	Slight rise followed by fall
Clouds	Ci → Cs → As → Ns → St	Stratus	Clearing with scattered Sc
Precipitation	-RA, RA, SN, DZ	DZ or NIL	Usually none, sometimes -RA
Visibility	Poor	Poor but improving	Fair
Dew Point	Steady Rise	Steady	Rise then Steady