Chapter 6

Weather Systems

This chapter relates atmospheric circulation phenomenato the daily variations in temperature, humidity, cloudiness, windiness, and precipitation. These parameters taken together that are revealedas 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
  • cAAcontinental Antarctic
  • A frontrepresents a boundary between two air masseswith 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.