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Chapter 6
Weather Systems
This chapter examines the way in which atmospheric circulation processes generate the
daily variations in temperature, humidity, cloudiness, windiness, and precipitation that we
know as weather.

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 is a boundary between one air mass and another. The leading edge of cold air
advancing into an area is called a cold front. Warm air moving into an area of cold
air is called a warm front.

An occluded front develops when a cold front overtakes a warm front and forces
warm air aloft.

Cyclonic precipitation can occur when moist air is forced aloft and adiabatically
cooled in the convergent, upward 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 weather system associated with tropical areas is the easterly wave, a low pressure
trough into which air converges and is lifted 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, migrate
westward, and curve toward the poles.

Tropical cyclones often create tremendous damage due to high winds, high waves,
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.