A cold front is followed by an area of cold air. Thick, dark clouds, heavy rain and sometimes violent storms arrive immediately. If seen from the side, a cold front looks much steeper than a warm front. Cold air pushes beneath the warm air and rising water vapour condenses into clouds and then rain. Showers of rain will often follow as the front passes over.
The cold front is depicted on a weather map as a blue line with triangles or as simply a blue line (Figure 9.28). A cold front, as discussed in the previous section, is the leading edge of colder air brought southward by winds around an area of low pressure. These fronts are most common during the active weather times of fall, winter, and spring.
Winds ahead of the cold front are southwesterly in the warm sector of the mid-latitude cyclone. After the cold front passes a point, winds turn to the west, northwest, or north. Since the cold air is very dense it is very effective at displacing the warm air ahead of it. The dense cold runs under the warm air lifting it. The lifting of warm moist air usually causes cloudiness at the least. If the air is moist and unstable enough, rain and thunderstorms can accompany the passage of the front. Air pressure usually falls as a cold front approaches, rising rapidly after passage as the dense cold air moves in. The dew point falls indicating the change to a dry air mass. Usually there is little local observational evidence of a cold front approaching.
A cold weather front is defined as the changeover region where a cold air mass is replacing a warmer air mass. Cold weather fronts usually move from northwest to southeast. The air behind a cold front is colder and drier than the air in front. When a cold front passes through, temperatures can drop more than 15 degrees within an hour.
On a weather forecast map, a cold front is represented by a solid line with blue triangles along the front pointing towards the warmer air and in the direction of movement.
There is usually an obvious temperature change from one side of a cold front to the other. It has been known that temperatures east of a cold front could be approximately 55 degrees Fahrenheit while a short distance behind the cold front, the temperature can go down to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.
Again, there is typically a noticeable temperature change from one side of the warm front to the other, much the same as a cold front.
If colder air is replacing warmer air, it is a cold front, if warmer air is replacing cold air, then it is a warm front.
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