When warmer air replaces colder air a Warm Front is the result. In contrast to the cold front the warm front generally moves at about 15 knots or slower and has a shallower slope. With warm fronts there is generally a broad cloud system found 500-700 miles ahead of front. In stable air, the clouds encountered in order are cirrus, cirrostratus, altostratus, nimbostratus. In unstable moist air, the cloud pattern is cirrus, cirrocumulus, altocumulus, cumulonimbus with thunderstorms often embedded in other cloud layers ahead of the front. The general characteristics of a warm front are stratus clouds if enough moisture present, little turbulence except if the airmass is unstable, precipitation ahead of the front, gradual pressure drop levelling off as the front passes, temperature rise after passage, a wind shift, poor visibility, overcast, drizzly precipitation.
When there is little movement between two airmasses we see what are known as
Stationary Fronts. Winds tend to flow parallel to the front at
the surface and the weather is similar to a warm front. If one type of front
catches up with another and traps air aloft we see what are termed
Occluded Fronts. These fronts may come in the form of a cold
front occlusion when air behind the cold front is colder than air in advance of
the warm front. Cold air replaces cool air at the surface and the weather
resembles that of a regular cold front. The warm front occlusion occurs when air
ahead of the warm front is colder than the overtaking cold front. In this case
cool air is lifted over cold air.