Chapter 13 — CIRCULATION OF THE ATMOSPHERE 



Climate Defined 



Climate is defined as the average or collec- 

 tive state of the earth's atmosphere at any 

 given location or area within a specified period 

 of time. We think of weather as the day-to- 

 day changes in the atmosphere. On the other 

 hand, the climate of an area is determined 

 over periods of many years and represents 

 the general weather characteristics of an area 

 or locality. 



Classification of Climate 



The climate of a given region or locality 

 is determined by a combination of several 

 meteorological elements, and not just one 

 element alone. For example, two regions may 

 have similar temperature climates but very 

 different precipitation climates. Their climatic 

 difference therefore becomes apparent only if 

 more than one climatic factor is considered. 



Since the climate of a region is composed 

 of all the averages of the various climatic 

 elements, such as dew, ice, rain, temperature, 

 wind force, and wind direction, it is obvious 

 that no two locations have exactly the same 

 climate. However, it is possible to place 

 similar areas into a grouping known as a 

 climatic zone. 



Climatic Zones 



The basic grouping of climatic zones con- 

 sists of classifying climates into five broad 

 belts based on astronomical or mathematical 

 grounds. Actually they are zones of sunshine, 

 or solar climate. The five basic regions or 

 zones are the Torrid or Tropical Zone, the 

 two Temperate Zones, and the two Polar Zones. 

 The Tropical Zone is limited on the north by 

 the Tropic of Cancer and on the south by the 

 Tropic of Capricorn, which are located at 

 23 1/2° N and S lat, respectively. The Temperate 

 Zone of the Northern Hemisphere is limited 

 on the south by the Tropic of Cancer and on the 

 north by the Arctic Circle, which is located 

 at 66 1/2° N lat. The Temperate Zone of the 

 Southern Hemisphere is bounded on the north 

 by the Tropic of Capricorn and on the south 

 by the Antarctic Circle, which is located at 

 66 1/2° S lat. The two Polar Zones are the 

 areas in the polar regions which have the Arctic 

 and Antarctic Circles as their boundaries. The 

 Polar Zones are sometimes called the Frigid 

 Zones. 



A glance at any chart depicting the iso- 

 therms over the surface of the earth will show 

 that the isotherms do not coincide with latitude 

 lines. In fact, at some places the isotherms 

 parallel the longitude lines more closely than 

 they parallel the latitude lines. The astro- 

 nomical or light zones therefore differ from 

 the zones of heat. A closer approach to the 

 understanding of climate can be made if the 

 climatic zones are limited by isotherms rather 

 than by parallels of latitude. (See fig. 13-7.) 



Climatic Types 



Any classification of climate depends to a 

 large extent on the purpose of the classification. 

 A classification for the purpose of establishing 

 air stations, for instance, where favorable flying 

 conditions are important, would differ con- 

 siderably from one for establishing the limits 

 of areas that are favorable for the growing of 

 crops. Consequently, several classification 

 systems have evolved and are described in 

 detail in climatology texts. 



Climatic Controls 



The variation of climatic elements from 

 place to place and from season to season is 

 caused by several factors called climatic con- 

 trols. The same basic factors that cause 

 weather in the atmosphere also determine the 

 climate of an area. These controls, acting in 

 different combinations and with varying 

 intensities, act upon temperature, precipitation, 

 humidity, air pressure, and winds to produce 

 many types of weather and therefore climate. 



Four factors largely determine the climate 

 of every ocean and continental region. They 

 are as follows: 



1. Latitude. 



2. Land and water distribution. 



3. Topography. 



4. Ocean currents. 



LATITUDE. — Perhaps no other climatic 

 control has such a marked effect upon climatic 

 elements as does the latitude, or the position 

 of the earth relative to the sun. The angle at 

 which rays of sunlight reach the earth and the 

 number of "sun" hours each day depend upon 



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