A front in which cold air is invading the warm-air zone is termed a cold front. The colder air mass, being heavier, remains in contact with the ground and forces the warmer air mass to rise over it. The slope of the cold front surface is greatly exaggerated in the figure, being actually of the order of slope of 1 in 40 to 1 in 80 (meaning that the slope rises 1 foot vertically for every 80 feet of horizontal distance). Cold fronts are associated with strong atmospheric disturbance; the warm air thus lifted often breaks out in violent thunderstorms. These may also occur along a line well in advance of the cold front, a squall line.
The cold air mass remains in contact with the ground, and the warm air mass is forced to rise as if ascending a long ramp. Warm fronts have lower slopes than cold fronts, being of the order of 1 in 80 to as low as 1 in 200. Moreover, warm fronts are commonly attended by stable atmospheric conditions and lack of the turbulent air motions of the cold front. Of course, if the warm air is unstable, it will develop convection cells and there will be heavy showers and thunderstorms.
Cold fronts normally move along the ground at a faster rate than warm fronts. Hence, when both types are in the same neighborhood, as they are in the cyclonic storm, the cold front may overtake the warm front. An occluded front then results. The colder air of the fast-moving cold front remains next to the ground, forcing both the warm air and the less cold air to rise over it. The warm air mass is lifted completely free of the ground.
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