of fronts move in the Southern Hemisphere is the mirror image of how they move in the
Northern Hemisphere. Fronts can be slowed or stopped by a barrier such as a mountain
range.
The rest of this section will be devoted to four types of fronts. Three of these fronts move
and one is stationary. With cold fronts and warm fronts, the air mass at the leading edge of
the front gives the front its name. In other words, a cold front is right at the leading edge
of moving cold air and a warm front marks the leading edge of moving warm air.
Stationary Front
Most fronts move across the landscape, but atstationary fronts( 3 ) the air masses do not
move. A front may become stationary if an air mass is stopped by a barrier. For example,
cold air masses may be stopped by mountains, because the cold air mass is too dense to rise
over them.
A region under a stationary front may experience days of rain, drizzle and fog. This weather
may be present over a large area. Winds usually blow parallel to the front, but in opposite
directions. This results in shear stress. Shear stresses result when objects are pushed past
each other in opposite directions.
After several days, the front will break apart. Thetemperature gradientor temperature
difference across the front may decrease, so the air masses start to mix. Shear stresses
may force the front to break apart. Conditions may change so that the stationary front
is overtaken by a cold front or a warm front. If the temperature gradient between the air
masses increases, wind and rainy weather will result.
Cold Fronts
When a cold air mass takes the spot of a warm air mass, there is acold front( 1 ) (Figure
16.16). Since cold air is denser than the warm air, the cold air mass slides beneath the warm
air mass and pushes it up. As the warm air rises, there are often storms.
When cold air moves underneath warm air, the ground temperature drops. The humidity
may also decrease since the colder air may also be drier. Winds at a cold front can be strong
because of the temperature difference between the two air masses. When a cold front is on
its way, there may be a sharp change in dew point, changes in wind direction, changes in air
pressure, and certain characteristic cloud and precipitation patterns.
Cold fronts often move rapidly across the landscape. Fast-moving cold fronts create a line of
intense storms over a fairly short distance. Asquall line( 6 ) is a line of severe thunderstorms
that forms along a cold front (Figure16.17). If the front moves slowly, the storms may form
over a larger area.