Chapter 6
Weather Systems
This chapter relates atmospheric circulation phenomena to the daily variations in
temperature, humidity, cloudiness, windiness, and precipitation. These parameters taken
together that are revealed as weather or the average state of the atmosphere at a given
instant at a particular location.

An air mass is a large body of air with a similar temperature, moisture, and lapse rate
characteristics over thousands of kilometers.

The air mass characteristics are acquired in source areas where the air remains for
some time allowing it to acquire the characteristics of the surface over which it rests.

Air masses are classified on the basis of the latitude and the surface type of the source
area. The main air mass classes are:
 mT
maritime tropical
 mE
maritime equatorial
 cT
continental tropical
 mP
maritime Polar
 cP
continental Polar
 cA
continental Arctic
 cAA
continental Antarctic

A front represents a boundary between two air masses with different temperature and
moisture characteristics. The leading edge of cold air advancing into an area of
warmer air defines a cold front. Alternatively, warm air advancing into an area of
colder air defines a warm front.

An occluded front develops when a cold front overtakes an advancing warm front
and forces the warm air aloft away from the Earth’s surface.

Cyclonic precipitation can occur during convergence when moist air is forced aloft
and cooled adiabatically in relation to the inward flow of a cyclone.

An important weather system affecting middle and high latitudes is a traveling low
pressure system called a wave cyclone that develops along the polar front.

Wave cyclones move from west to east and the interaction of warm and cold fronts
within the cyclone often produces cyclonic storms.

A tornado is an intense low pressure system with very high wind speeds. Tornadoes
occur in association with thunderstorms that develop along cold fronts and with
hurricanes.

A common weather system associated with tropical areas is the easterly wave which
a low pressure trough into which air converges and is lifted often producing
precipitation.

A polar outbreak occurs when cold polar air forces its way into very low latitudes,
bringing storms followed by cold, clear weather.

Tropical cyclones, hurricanes, and typhoons are all names for powerful storms
which develop over warm ocean surfaces between 8° and 15° latitude. These storms
tend to migrate westward, and eventually curve toward the poles.

Tropical cyclones often result in tremendous damage due to high winds, high waves
(storm surge), flooding, and heavy rains.

The atmospheric circulation transfers heat and moisture from equatorial regions
toward the Polar Regions by the Hadley cell circulation and Rossby waves.

The thermohaline circulation within the oceans is another important mechanism by
which heat is transferred from the equatorial to the polar regions of the Earth.

An important element of climatic change studies is the positive and negative
feedbacks between surface temperature and cloud cover.