Development of Cold Front
The cooler and denser air wedges under the less-dense warmer air, lifting it. This upward motion causes lowered pressure along the cold front and can cause the formation of a narrow line of showers and thunderstorms when enough moisture is present. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel. A cold front's location is at the leading edge of the temperature drop off, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough. Cold fronts move faster than warm fronts and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary.
In the northern hemisphere, a cold front usually causes a shift of wind from southwest to northwest clockwise, also known as veering, and in the southern hemisphere a shift from northeast to southwest, in a clockwise manner. Normally, cold fronts can be marked by these characteristics:
| Weather phenomenon | Prior to the Passing of the Front | While the Front is Passing | After the Passing of the Front |
|---|---|---|---|
| Temperature | Warm | Cooling suddenly | Steadily cooling |
| Atmospheric pressure | Decreasing steadily | Lowest, then sudden increase | Increasing steadily |
| Winds |
|
Gusty; shifting |
|
| Precipitation/conditions* | Light patchy rain can be produced by stratocumulus or stratus in the warm sector. | Prolonged rain (nimbostratus) or thunderstorms (cumulonimbus): depends on conditions. | Showers, then clearing |
| Clouds* | Usually takes the form of a line of cumulonimbus and/or cumulus congestus clouds, but can also be nimbostratus or stratus/stratocumulus if the front is weak. Approach often obscured by stratocumulus or stratus in the warmer air approaching it, but if the air is clear usually a small sharply edged altostratus sheet can be seen marking the edge of the approaching front. | Cumulonimbus or nimbostratus | Patchy altocumulus and altostratus along with stratus fractus then cumulus and cumulonimbus |
| Visibility* | Fair to poor in haze | Poor, but improving | Good, except in showers |
| Dew Point | High; steady | Sudden drop | Falling |
*Provided there is sufficient moisture.
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