Characteristics
If the air mass is relatively stable, and the warm front is a katafront, rainfall will increase until the front reaches the location, at which time the clouds can extend all the way to the earth’s surface as fog. Once the front passes, the location experiences some warming and clearing. If the air mass is unstable, and the warm front is an anafront, thunderstorms may precede and follow the front and temperature changes will be larger.
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:
| Weather phenomenon | Prior to the Passing of the Front | While the Front is Passing | After the Passing of the Front |
|---|---|---|---|
| Temperature | Cool | Warming suddenly | Warmer, then leveling off |
| Atmospheric pressure | Decreasing steadily | Leveling off | Slight rise followed by a decrease |
| Winds |
|
Variable |
|
| Precipitation | Showers, snow, sleet, or drizzle | Light drizzle | Usually none, sometimes light rain or showers |
| Clouds | Cirrus, cirrostratus, altostratus, nimbostratus, then stratus (pilots use the acronym CCANS) and fog; occasionally cumulonimbus in summer. Other clouds can also often be seen including cirrocumulus amongst the approaching cirrus, the altostratus becoming broken in places into altocumulus (particularly if the front is weak) and the very common occurrences of stratocumulus under the cirrostratus and altostratus and stratus fractus under the nimbostratus. | Stratus, sometimes cumulonimbus | Clearing with scattered stratus 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 | Fair in haze |
| Dew Point | Steady rise | Steady | Rise, then steady |
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