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A warm front is a weather front located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than , and move more slowly than the cold fronts which usually follow because cold air is denser and less easy to remove from the Earth's surface. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly Stratus cloud, and rainfall generally increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage thundershowers may continue. On weather maps, the surface location of a warm front is marked with a red line of semicircles pointing in the direction of travel.
As the air mass rises into regions of lower pressure, it expands and cools. As it cools, any water vapor that is present will Condensation and form extensive cloud cover. The first clouds that indicate an approaching warm front tend to be mostly high Cirrus cloud at first, changing to cirrostratus as the front approaches. However, if cirrocumulus also appears, there is greater airmass instability approaching ahead of the front. When these high clouds progressively invade the sky and the barometric pressure begins to fall, precipitation associated with the disturbance is likely about 6 to 8 hours away. A thickening and lowering of these high clouds into middle-stage altostratus or altocumulus is a good sign the warm front or low has moved closer and precipitation may begin within less than six hours. Once the clouds have thickened to from the earth’s surface, precipitation can begin to fall from heavy nimbostratus. If unstable altocumulus castellanus accompanies or takes the place of the main altostratus layer, cumulus congestus or cumulonimbus producing showers or thunderstorms may follow. Low stratus and stratocumulus commonly form underneath the main precipitating clouds.
A warm front is also defined as the transition zone where a warmer air mass is replacing a cooler air mass. Warm fronts generally move from southwest to northeast. If the warmer air originates over the ocean, it is not only warmer but also more moist than the air ahead of it.
In the northern hemisphere, a warm front causes a shift of wind blowing from southeast to southwest, and in the southern hemisphere a shift from winds blowing from northeast to northwest. Common characteristics associated with warm fronts include:
| Temperature | Cool | Warming suddenly | Warmer, then leveling off |
| Atmospheric pressure | Decreasing steadily | Leveling off | Slight rise followed by a decrease |
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| Precipitation | Usually none, but in summer or warm temperatures, cumulus congestus may continue to exist under cirrostratus and altostratus creating light to moderate showers. | Persistent rain, usually moderate with some lighter periods and some heavier bursts. In winter, snow may turn to rain after passing through ice pellets and freezing rain. | Light drizzle, gradually ceasing. |
| Clouds | Cirrus cloud, cirrostratus, altostratus, nimbostratus, then stratus cloud. Other clouds can also often be seen, including cirrocumulus amongst the approaching cirrus, altocumulus with or instead of altostratus (particularly if the front is weak), and occasionally cumulonimbus along with or instead of nimbostratus in summer. Additionally, stratocumulus often appears underneath the main altostratus deck and stratus fractus typically forms in precipitation falling from the thick nimbostratus layer. Often in warm temperatures, rain bearing cumulus congestus clouds can appear under the cirrostratus, and more rarely altocumulus castellanus clouds if convection is sufficient. In cold humid conditions, low airmass stratus or fog may obscure the main frontal clouds. | Nimbostratus, sometimes cumulonimbus | Clearing with scattered stratus cloud and stratocumulus. If the warm front is part of a depression, there is often a sheet of altostratus (often broken in places to altocumulus) above this which thickens when the cold front approaches. |
| Visibility | Poor | Poor, but improving | Sunny |
| Dew point | Steady rise | Steady | Rise, then steady |
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