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When large masses of warm air and cold air meet, they do not mix because of density differences. In World War I, meteorologists in the “Norwegian School of Meteorology” referred to this boundary as a “front”, a military analogy to a battle line where in this case the warm and cold air masses were “fighting” for supremacy.
Fronts are usually hundreds of miles long. A cold front forms when a cold, dense air mass pushes under a warm, lighter air mass, forcing the warm air to rise. The cold air advances, replacing the warm air at the surface. Rain and even thunderstorms can form as the moisture in the warm air mass rises, cools, and condenses. With a cold frontal passage, the winds will typically shift from the south to the west or north. As the front moves through, cool, fair weather is likely to follow.
The temperatures behind a cold front vary depending on the type of air mass moving in. A polar air mass will bring in cold air in the winter and cool air at other times. An Arctic air mass will be associated with bitterly cold temperatures in the winter. An air mass coming off high latitude ocean areas will be unusually cold in summer but more moderate in winter.
If the cold front is highly unstable, cumulonimbus clouds producing thunderstorms commonly form along or ahead of the front. Anvil cirrus clouds may spread a considerable distance downwind from the thunderstorms. The other cloud types associated with a cold front depend on atmospheric conditions such as air mass stability and wind shear. As the front approaches, altostratus and low-level stratocumulus with intermittent light precipitation may form if the warm airmass being displaced by the cold front is mostly stable.
After the passage of the cold front, the sky usually clears as high pressure builds in behind the system, although significant amounts of cumulus or stratocumulus, often in the form of long bands called cloud streets may persist if the air mass behind the front remains humid from a source of moisture. Small and unchanging amounts of cumulus or cirrus clouds in an otherwise clear sky are usually indications of continuing fair weather as long as the barometric pressure remains comparatively high.
The amount of precipitation associated with a cold front depends on the amount of moisture in the air and the amount of lifting the front produces. You need both abundant moisture and strong lifting to produce significant precipitation. Sometimes bands of precipitation can occur well ahead of the front especially if the warm air mass is unstable.
The cold front itself commonly brings a narrow band of precipitation that follows along the leading edge of the cold front. These bands of precipitation can be very strong and can bring severe thunderstorms, hailstorms, snow squalls, and/or tornadoes. In the spring, these cold fronts can be very strong, and can bring strong winds when the pressure gradient is higher than normal. Cold fronts sometimes come through an area with little or no precipitation. These dry cold fronts can cause wildfire control problems. Wider rain bands can occur behind some shallow cold fronts which tend to have more stratiform, and less convective, precipitation. These rainstorms sometimes bring flooding, and can move very slowly when the storm steering it is strong and embedded within a meridional flow pattern (with more pole to equator motion rather than west to east motion). If the low-level cold air behind such a front has below freezing temperatures, ice storms due to freezing rain are possible.
In the winter, cold fronts can bring cold spells, and occasionally snow. In the spring or summer in temperate latitudes, hail may occasionally fall along with the rain. If moisture is not sufficient, such as when a system has previously moved across a mountain barrier, cold fronts can pass without cloudiness.
A cold front, can produce frontal snowsqualls—an intense frontal convective line, when temperature is near freezing at the surface. The strong convection that develops has enough moisture to produce whiteout conditions at places which line passes over as the wind causes intense blowing snow. This type of snowsquall generally lasts less than 30 minutes at any point along its path but the motion of the line can cover large distances. Frontal squalls may form a short distance ahead of the surface cold front or behind the cold front where there may be a deepening low-pressure system or a series of trough lines which act similar to a traditional cold frontal passage. In situations where squalls develop post-frontally it is not unusual to have two or three linear squall bands pass in rapid succession only separated by 25 miles (40 kilometers) with each passing the same point in roughly 30 minutes apart. In cases where there is a large amount of vertical growth and mixing the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed "thundersnow".
On weather maps, the surface location of a cold front is marked with a blue line of triangles pointing in the direction of the front.
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 cold polar air mass
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