What is a Weather Front?

Fronts Signal the Arrival of Warm Air, Cold Air, and Precipitation

Known as the colorful lines that move across weather maps, weather fronts are boundaries that separate air masses of different air temperatures and moisture content (humidity).

A front takes its name from two places: it is the literal front, or leading edge, of air that's moving into a region; it is also analogous to a war battle front, where the two air masses represent the two clashing sides. Because fronts are zones where temperature opposites meet, weather changes are usually found along their edge.

Fronts are classified depending on what kind of air (warm, cold, neither) is advancing onto the air in its path. The main types of fronts include:

Warm Fronts

warm front diagram
UK ECN, http://www.ecn.ac.uk/what-we-do/education/tutorials-weather-climate

If warm air moves in such a way that it advances onto and replaces the cooler air in its path, the leading edge of the warm air mass found at the earth's surface (the ground) is known as a warm front.

When a warm front passes through, the weather becomes noticeably warmer and more humid than it was before.

The weather map symbol for a warm front is a red curved line with red semi-circles. The semi-circles point in the direction the warm air is moving.

Cold Fronts

cold front diagram
UK ECN, http://www.ecn.ac.uk/what-we-do/education/tutorials-weather-climate

If a cold air mass spills onto and overtakes a neighboring warm air mass, the leading edge of this cold air will be a cold front.

When a cold front passes through, the weather becomes significantly colder and drier. (It isn't uncommon for air temperatures to drop 10 degrees Fahrenheit or more within an hour of a cold frontal passage.)

The weather map symbol for a cold front is a blue curved line with blue triangles. The triangles point in the direction the cold air is moving.

Stationary Fronts

Stationary Front icon
In a stationary front, neither the warm nor cold air "wins" out. NOAA

If a warm and a cold air mass are next to one another, but neither is moving strongly enough to overtake the other, a "stalemate" occurs and the front remains in one place, or stationary. (This can happen when winds blow across the air masses rather than toward one or the other.)

Since stationary fronts move very slowly, or not at all, any precipitation that occurs with them can stall out over a region for days on end and cause a significant flood risk along the stationary front boundary.

As soon as one of the air masses pushes ahead and advances onto the other air mass, the stationary front will begin to move. At this point, it'll become either a warm front or a cold front, depending on which air mass (warm or cold) is the aggressor.

Stationary fronts appear on weather maps as alternating red and blue lines, with blue triangles pointing towards the side of the front occupied by warmer air, and red semi-circles pointing towards the cold air side.

Occluded Fronts

occluded front diagram
UK ECN, http://www.ecn.ac.uk/what-we-do/education/tutorials-weather-climate

Sometimes a cold front will "catch up" to a warm front and overtake both it and the cooler air out ahead of it. If this happens, an occluded front is born. Occluded fronts get their name from the fact that when the cold air pushes underneath the warm air, it lifts the warm air up from the ground, which makes it hidden, or "occluded." 

Occluded fronts usually form with mature low pressure areas. They act like both warm and cold fronts.

The symbol for an occluded front is a purple line with alternating triangles and semi-circles (also purple) pointing in the direction the front is moving.


Dryline on a NOAA SPC mesoscale discussion map
NOAA Storm Prediction Center

Up until now, we've talked about fronts that form between air masses having contrasting temperatures. But what about boundaries between air masses of different humidity?

Known as drylines, or dew point fronts, these weather fronts separate warm, moist air masses found ahead of the dryline from hot, dry air masses found behind it. In the US, they're most often seen east of the Rocky Mountains across the states of Texas, Oklahoma, Kansas, and Nebraska during spring and summer. Thunderstorms and supercells often form along drylines, since the drier air behind them lifts up the moist air ahead, triggering strong convection.

On surface maps, the symbol for a dryline is an orange line with semi-circles (also orange) that face toward humid air.