If the air mass is very different from the ground beneath it, storms may form. For example,
when a colder air mass moves overwarmer ground, the bottom layer of air is heated. That air
rises, forming clouds, rain, and sometimes thunderstorms. When a warmer air mass travels
over colder ground, the bottom layer of air is cooled. This forms a temperatureinversion,
since the cold air near the ground is trapped. Inversions may form stratus clouds, advection
fogs, or they may trap a layer of pollution over a city.
In general, cold air masses tend to flow toward the equator and warm air masses tend to
flow toward the poles. This brings heat to cold areas and cools down areas that are warm.
It is one of the many processes that act towards balancing out the planet’s temperatures.
Fronts
Two air masses meet at a front. Because the two air masses have different temperature and
humidity, they have different densities. Air masses with different densities do not easily mix.
Ordinarily, when fronts meet, one air mass is lifted above the other. Rising air creates a low
pressure zone. If the lifted air is moist enough, there will be condensation and precipitation.
Fronts usually also have winds in them. If the temperature difference between the two air
masses is high, then the winds will be strong. Fronts are the main cause of stormy weather.
The map symbols for the different types of fronts are shown in (Figure16.15): ( 1 ) cold
front, ( 2 ) warm front, ( 3 ) stationary front, ( 4 ) occluded front, ( 5 ) surface trough, ( 6 ) squall
line, ( 7 ) dry line, ( 8 ) tropical wave.
Figure 16.15: The map symbols for different types of fronts. ( 3 )The direction that fronts move is guided by pressure gradients and the Coriolis Effect. In
the Northern Hemisphere, cold fronts and occluded fronts tend to move from northwest to
southeast. Warm fronts move southwest to northeast. The direction the different types