When large masses of warm air and cold air meet, they do not mix because of density differences. Instead, they form a front, usually hundreds of miles long. A Warm Front forms when a relatively moist, warm air mass slides up and over a cold air mass. As the warm air mass rises, it often condenses into a broad area of clouds. The warm air at the surface, behind the warm front, advances slowly, replacing the cold air at the surface. Depending on the amount of moisture available and the intensity of lifting being produced, light to heavy amounts of rain or snow can occur ahead of the surface warm front. Convective showers and thunderstorms are even possible if the warm air mass is unstable. Severe weather, however. is unlikely with colder air near the surface. Following warm frontal passage, temperatures will rise.
Cloud types associated with a Warm Front
The first clouds that indicate an approaching warm front tend to be mostly high cirrus 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 to be about six to eight 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 2,500 metres (8,200 ft) 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 can produce snow for a period, as warm, moist air overrides below-freezing air and creates precipitation at the boundary. Often, snow transitions to rain in the warm sector behind the front.
If the air mass is relatively stable, 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, thunderstorms may precede and follow the front.
In the northern hemisphere, a warm front causes a shift of wind blowing from the east to the south, and in the southern hemisphere a shift from winds blowing from northeast to northwest.
How Freezing Rain Occurs
Freezing Rain occurs when precipitation, in the form of rain, passes from a warm air mass into a relative cold air mass with an air temperature less than 0°C. The rain maintaining its liquid state in sub-zero temperatures renders it super-cooled. These super-cooled rain droplets freeze when they come into contact with the ground or other exposed surface, if the surface temperature is below 0°C. For further information see the separate article Freezing Rain.
Depiction on weather maps
On weather maps, the surface location of a warm front is marked with a red line of half circles pointing in the direction of the frontal movement.
A mature low pressure system showing a warm front ahead of the warm mid latitude air mass streaming towards the Pole where it meets the retreating cold polar air mass