Secondary cold fronts occur within cold air masses and may follow in intervals of several hundred miles to the rear of rapidly moving cold fronts. Usually, as a secondary cold front develops, the primary front tends to dissipate. The secondary front then becomes the primary front. Secondary fronts are far more common during the winter when temperature discontinuity is greatest. Active warm fronts are generally located in pressure troughs on the surface charts.
Pressure usually falls for an appreciable length of time before the fron-tal passage.
Normally, it is steady after passage. The wind in advance of a warm front in the Northern Hemisphere is usually from the southeast, shifting to southwest after passage. The wind speed normally increases as the front approaches. The wind shift accompanying a warm front is seldom as abrupt as with a cold front. Warm fronts are nearly always well defined by typical stratified clouds.