What a weather front is, and how to read one
Every 'a front moves through Thursday' in a forecast describes the same physical thing: the boundary between two air masses. The four types, what each does to your day, and the tells that show up in the hourly forecast before the map does.
A weather front is the boundary between two air masses of different temperature and humidity. Air masses do not mix easily, so the boundary stays sharp, often just a few miles wide, and most of the interesting weather in the mid-latitudes happens along it: the wind shift, the temperature drop, the line of storms, the day of steady rain. The blue and red lines on a surface map are those boundaries drawn at ground level. Fronts belong to the large low-pressure systems that cross the United States every few days from west to east, which is why "a front comes through" is such a common phrase in forecasts and why, when the models disagree about the timing of a front, the whole forecast for that day is in play.
Cold front
Drawn as a blue line with triangles pointing in the direction of travel. Cold, dense air pushes under warmer air and lifts it sharply, so cold fronts produce a narrow band of weather: a line of showers or thunderstorms, sometimes severe, then rapid clearing. The sequence as one passes is reliable enough to feel by hand. Wind backs to the south or southwest and strengthens ahead of it; pressure falls; the front arrives with a gust and a wind shift to the west or northwest; temperature and dew point drop, sometimes 20°F in an hour; pressure rises; skies clear. In the Plains a strong autumn cold front is sometimes called a "norther" or "blue norther". Severe thunderstorms along a cold front are the subject of Severe Thunderstorm Warning vs Watch.
Warm front
A red line with semicircles. Warm air advances over retreating cold air, but because warm air is lighter it slides gently up and over rather than shoving, so the weather is spread out: high cirrus a day ahead, thickening mid-level cloud, then hours of steady light-to-moderate rain or snow before the front itself passes. Behind it the air is warmer and more humid and the wind shifts from easterly or southeasterly to southerly. In winter a warm front is the classic setup for freezing rain: warm air aloft melts the snow, which refreezes on the cold ground below. Warm fronts move slowly and often stall, which is one reason a "rain ending by noon" forecast so often becomes "rain ending by 4".
Stationary front
Alternating blue triangles and red semicircles on opposite sides of the line. Neither air mass is winning, so the boundary sits in place, and moisture streaming along it can produce days of cloud and rain on one side and clear, warm weather on the other, sometimes only 50 miles apart. Summer stationary fronts across the Southeast and the Midwest are behind many flash-flood events, because storms keep forming and moving along the same line, a process forecasters call training.
Occluded front
A purple line with both symbols on the same side. Cold fronts move faster than warm fronts, and late in a storm's life the cold front catches the warm front and lifts the warm air off the ground entirely. Occlusions bring a mix of both fronts' weather, often a long period of rain or snow with a wind shift, and they mark a low-pressure system that is past its peak. Two cousins worth knowing: the dry line, a moisture boundary across Texas and Oklahoma in spring that is the trigger for many tornado outbreaks, and the sea breeze front, the small daily boundary that fires Florida's afternoon storms.
Spotting a front in the hourly forecast
You do not need the map. In an hourly forecast, a front shows up as three things changing in the same hour or two: the wind direction swings (usually from south to west for a cold front), the temperature or dew point jumps or drops, and precipitation starts or stops. When the three line up, that hour is the frontal passage. The forecast models frequently agree that a front will pass and disagree by three to six hours on when, and that is usually the whole argument on a given day: whether the storms arrive before or after the commute, whether the rain ends before the game. On our city pages, Kansas City or Atlanta for example, the NWS blend, the American GFS and the European model show their hourly wind, temperature and rain side by side, so a split on the front's timing is visible at a glance. Why weather forecasts disagree explains where those splits come from, how weather forecasting works covers what the models are doing, and our accuracy scoreboard shows which one has been right most often in your city. Once the front is close, the radar shows the line itself.