However, it is not only the formation of clouds that is important, but the sequence in which the clouds approach. When a cold front approaches, the colder air, which is much heavier than the warm air ahead of it, forces its way under the warm air mass. The result is an abrupt jump from clear skies to mountainous thunderheads and heavy rain squalls as the warm, moist air is rapidly forced to move into the cold upper atmosphere. The sequence that indicates an advancing cold front begins with thunderheads, lightning and thunder and violent squalls. As the worst of it passes, the sky will lighten, the air will be colder and the storm will give way to lighter rain and eventually high cirrus clouds and then clear skies.
Warm fronts bring warm, moist air that rides gradually up over the top of the cooler air ahead.
Because of this gradual approach, storms associated with the actual frontal boundary may be several hundred miles behind the wispy cirrus clouds, which are the first indication that the front is approaching. The cloud sequence that indicates an approaching warm front begins with very high cirrus and proceeds to lower and lower cloud decks that become increasingly dark, finally bringing rain from low nimbostratus clouds. First, keep your eye on the sky. Violent weather has a way of being very localized. Monitor temperature, humidity and barometric pressure.