Weather Fronts Explained

The lines, triangles, and half-circles on a weather map show where one kind of air ends and another begins. They mark where the weather is about to change: where storms tend to form, where the wind will shift, and where it's about to get warmer or colder. This guide explains what each front is, what the red and blue symbols mean, and what weather to expect before, during, and after one passes.

What is a front?

Air comes in large bodies called air masses, each with a fairly even temperature and humidity across hundreds of miles: cold, dry air from Canada; warm, humid air from the Gulf of Mexico; hot, dry air from the desert Southwest. A front is the boundary where two of them meet.

The two air masses don't mix easily. Warm air is lighter than cold air, so along the boundary the warm air is pushed up and over the cold air. As it rises it cools, and its moisture condenses into clouds, rain, snow, or thunderstorms. That is why fronts usually come with a band of clouds and precipitation, and why the temperature, humidity, and wind direction change once one passes.

Most fronts trail out from a center of low pressure, like spokes from a hub. In the Northern Hemisphere, air turns counterclockwise around a low, pulling warm air north on its east side and cold air south on its west side. The edge of that warm air is the warm front; the edge of the cold air is the cold front.

What the colors and symbols mean

The colors show which air is moving in, and the shapes show which way.

• Blue means colder air is advancing. Blue triangles mark a cold front.
• Red means warmer air is advancing. Red half-circles mark a warm front.
• The symbols point the way the front is moving. They sit on the side of the line the front is heading toward, so they point at the places that are about to get the new air.
• Purple is an occluded front, where cold and warm fronts have merged. Dashed brown is a trough.
• Blue H is a high-pressure center and red L is a low-pressure center.

In the drawings below, each front is moving toward the top of the page.

Cold fronts

A cold front is the leading edge of colder air pushing in and replacing warmer air. Cold air is dense, so it wedges under the warm air and lifts it sharply. That fast, steep lift is what builds tall thunderstorm clouds, so cold fronts bring the most dramatic weather of any front, usually in a fairly narrow band.

• Before it arrives: warm and often humid, with a south or southwest wind and falling pressure. A line of storms may build along or just ahead of the front.
• As it passes: a sudden wind shift to the west or northwest, gusty wind, and a short burst of heavy rain or thunderstorms. In winter it can be a quick round of snow squalls.
• After it passes: the temperature drops, often 10–20°F within hours, and the air dries out. Pressure rises, the sky usually clears, and a cool, breezy day often follows.

Cold fronts typically move at 20–30 mph, faster in winter. In spring they are the main trigger for severe weather in the Plains and Southeast: squall lines, damaging wind, hail, and tornadoes. A strong winter cold front can also cause a flash freeze, when rain on the roads freezes as the temperature crashes behind it.

Warm fronts

A warm front is the leading edge of warmer air moving in to replace cooler air. Instead of pushing under, the light warm air slides gently up and over the cool air on a long, shallow slope. The lift is gentle and spread over a wide area, so warm fronts bring widespread, steady weather rather than a sharp line of storms.

• Before it arrives: clouds thicken and lower over a day or more, starting with thin, high wispy clouds. Light to moderate rain or drizzle can last for hours, often several hundred miles ahead of the front line, with an east or southeast wind, cool air, and sometimes fog.
• As it passes: the rain tapers off and the wind turns to the south or southwest.
• After it passes: warmer and noticeably more humid, with partly cloudy skies. Afternoon thunderstorms can pop up in this warm, muggy air.

Warm fronts move more slowly, typically 10–20 mph. In winter, the zone just ahead of a warm front is where you most often get freezing rain and sleet: rain falls out of the warm air above into a shallow layer of below-freezing air at the ground.

Cold front vs. warm front at a glance

Stationary fronts

A stationary front is a boundary that has stalled: neither the cold nor the warm air is pushing the other out of the way. Its symbols point in opposite directions, the blue triangles toward the warm side and the red half-circles toward the cold side, because the front isn't going either way.

The weather along a stationary front can sit over the same area for days: clouds, on-and-off rain, and sometimes storms that form again and again over the same places. That repeated rain is a common cause of flash flooding. A stationary front ends when one side starts moving again; it is then redrawn as a cold or warm front.

Occluded fronts

Because cold fronts move faster than warm fronts, the cold front often catches up with the warm front near the low-pressure center. When it does, the warm air between them is lifted completely off the ground, and the merged boundary is called an occluded front. It is drawn in purple, with triangles and half-circles both on the side it's moving toward.

An occluded front usually means the storm system is mature and near its strongest, and it often starts to weaken soon after. The weather is a mix of both front types: clouds, steady rain or snow that can wrap around the low, and a wind shift as it passes. In winter this wrap-around zone is often where the heaviest snow falls.

Troughs

A trough is a long, stretched-out area of lower pressure, like a valley in the pressure map, without the clear temperature contrast of a true front. Air tends to come together and rise along it, so troughs can still set off clouds, showers, and thunderstorms, especially on warm afternoons, and the wind often shifts as one passes. In the summer, troughs are frequently where the day's storms line up.

Highs and lows

• H, high pressure (blue): air sinks under a high, which dries it out and stops clouds from forming. Highs usually bring fair weather, light wind, and clear skies: cool, crisp nights in fall and winter, or hot, hazy, still days in summer. A high that sits in place for days can trap smoke and pollution near the ground.
• L, low pressure (red): air comes together and rises over a low, building clouds and precipitation. Lows bring unsettled weather, stronger wind, and the fronts that trail from them. The deeper the low, the stronger the wind around it.

In the Northern Hemisphere, wind turns clockwise around a high and counterclockwise around a low. That's why the wind ahead of a low, on its east side, usually blows from the south and brings warmer air north.

How fronts move across the U.S.

Across most of the country, weather systems travel from west to east, steered by the jet stream high above. A typical cycle takes a few days: a low forms or moves in from the west, its warm front lifts north with rain and rising humidity, the cold front sweeps through with a line of showers or storms, and a high builds in behind with cooler, drier, clearer weather. Then the cycle starts again.

• Spring: the strongest contrasts between lingering cold air and returning Gulf moisture make spring cold fronts the main severe-weather season in the Plains and Southeast.
• Summer: the jet stream retreats north and fronts weaken. They often stall across the middle of the country and fade, and many fronts never reach the Gulf Coast at all.
• Fall: the first strong cold fronts of the season bring the first real relief from summer humidity, and in the Southeast the first one is often a welcome event in late September or October.
• Winter: fronts are frequent and strong. Arctic cold fronts can drop temperatures dramatically, and fronts are where winter storms bring snow on the cold side and ice near the boundary.

A few regional patterns are worth knowing. Along the Gulf Coast and in Florida, a cold front in the cool season means a few days of dry, pleasant air, and a strong one can bring a freeze. In the Northeast, a backdoor cold front moves in from the northeast instead of the west, bringing a sudden chill and low clouds off the ocean in spring. On the Pacific coast, fronts arrive straight off the ocean and bring long periods of rain to the coast and snow to the mountains. In the southern Plains, storms often form along a dryline, a boundary between moist and dry air that this layer doesn't draw; see the Tornado Alley Weather Guide.

Using fronts with the other layers

• Radar: a band of rain or a line of storms on radar that lines up with a blue front is the front's weather arriving. Watch which way the triangles point to see where it's heading next.
• Watches and warnings: severe thunderstorm and tornado watches often lie just ahead of a cold front in spring. See NWS Watches vs. Warnings Explained.
• Temperature and humidity: the temperature and humidity badge will show the change once a front passes your location.
• Pollen and air quality: the wind ahead of a cold front can stir up pollen, rain behind it can wash it out, and the drier, breezier air after it often clears haze and smoke. A stalled high does the opposite. See Pollen Types, Seasons & Counts Explained.

To show fronts, turn on NWS fronts, highs & lows in the map settings. Pollen mode turns them on automatically. The fronts, highs, and lows come from the NOAA/NWS Weather Prediction Center, which updates them several times a day, and RadarBug checks for new ones every 30 minutes.