From Chapter 20.2 Fronts, Earth Science, Tarbuck & Lutgens
Recall that air masses have different temperatures and amounts of moisture, depending on their source regions. Recall also that these properties can change as an air mass moves over a region. What do you think happens when two air masses meet? When two air masses meet, they form a front, which is a boundary that separates two air masses. Fronts can form between any two contrasting air masses. Fronts are often associated with some form of precipitation,
A warm front forms when warm air moves into an area formerly covered by cooler air
The slope of the warm front is very gradual, as shown in Figure 10.
As warm air rises, it cools to produce clouds, and frequently precipitation.
The sequence of clouds shown in Figure 10 typically comes before a warm front. The first sign of the approaching warm front is the appearance of cirrus clouds. As the front nears, cirrus clouds change into cirrostratus clouds, which blend into denser sheets of altostratus clouds. About 300 kilometers ahead of the front, thicker stratus and nimbostratus clouds appear, and rain or snow begins
On a weather map, the surface position of a cold front is shown by a blue line edged with blue triangles
that point toward the warmer air mass.
Figure 11 shows how a cold front develops. As this cold front moves, it becomes steeper. On average, cold fronts are about twice as steep as warm fronts and advance more rapidly than warm fronts do.
These two differences—rate of movement and steepness of slope— account for the more violent weather associated with a cold front.
The forceful lifting of air along a cold front can lead to heavy downpours and gusty winds. As a cold front approaches, towering clouds often can be seen in the distance. Once the cold front has passed, temperatures drop and wind shifts. The weather behind a cold front is dominated by a cold air mass. So, weather clears soon after a cold front passes. When a cold front moves over a warm area, low cumulus or stratocumulus clouds may form behind the front.
Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front. In such cases, the surface position of the front does not move, and a stationary front forms.
When an active cold front overtakes a warm front, an occluded front forms. It develops as the advancing cold air wedges the warm front upward. The weather associated with an occluded front is generally complex. Most precipitation is associated with the warm air’s being forced upward. When conditions are suitable, however, the newly formed front is capable of making light precipitation of its own.
It is important to note that the descriptions of weather associated with fronts are general descriptions. The weather along any individual front may or may not conform to the idealized descriptions you’ve read about. Fronts, like all aspects of nature, do not always behave as we would expect.