AEROGRAPHER'S MATE 3 & 2 



south polar area is completely in sunshine, and 

 the north polar area is completely in darkness. 

 Since the revolution of the earth around the 

 sun is a gradual process, the changes in the 

 area receiving the sun's rays and the changes 

 in seasons are gradual. However, it is 

 customary and convenient to mark these changes 

 by specific dates and to identify them by specific 

 names. These dates are as follows: 



1. March 21. The VERNAL EQUINOX, when 

 the earth's axis is perpendicular to the sun's 

 rays. Spring begins in the Northern Hemisphere 

 and fall begins in the Southern Hemisphere. 



2. June 21. The SUMMER SOLSTICE, when 

 the earth's axis is inclined 23 1/2° toward the 

 sun and the sun has reached its northernmost 

 zenith at the Tropic of Cancer. Summer 

 officially commences in the Northern Hemi- 

 sphere, and winter begins in the Southern 

 Hemisphere. 



3. September 22. The AUTUMNAL EQUI- 

 NOX, when the earth's axis is again perpen- 

 dicular to the sun's rays. This date marks the 

 beginning of fall in the Northern Hemisphere 

 and spring in the Southern Hemisphere. It is 

 also the date, along with March 21, when the 

 sun reaches its highest position (zenith) directly 

 over the Equator. 



4. December 22. The WINTER SOLSTICE, 

 when the sun has reached its southernmost 

 zenith position at the Tropic of Capricorn. It 

 marks the beginning of winter in the Northern 

 Hemisphere and the beginning of summer in 

 the Southern Hemisphere. 



In some years, the actual dates of the 

 solstices and the equinoxes vary by a day from 

 the dates given here because the period of 

 revolution is 365 1/4 days, and the calendar 

 year is 365 days, except for leap year when 

 it is 366 days. 



Because of its 23 1/2° tilt and its revolution 

 around the sun, the earth is thus marked by 

 five natural light (or heat) zones according to 

 the zone's relative position to the sun's rays. 

 Since the sun is ALWAYS at its zenith between 

 the Tropic of Cancer and the Tropic of Capri- 

 corn, this is the hottest zone. It is called the 

 Equatorial Zone, the Torrid Zone, the Tropical 

 Zone, or simply the Tropics. 



The zones between the Tropic of Cancer 

 and the Arctic Circle and between the Tropic 

 of Capricorn and the Antarctic Circle are the 



Temperate Zones. These zones receive sun- 

 shine all year, but less of it in their respective 

 winters and more of it in their respective 

 summers. 



The zones between the Arctic Circle and 

 the North Pole and between the Antarctic Circle 

 and the South Pole receive the sun's rays only 

 for parts of the year. (Directly at the poles 

 there are 6 months of darkness and 6 months 

 of sunshine.) This, naturally, makes them the 

 coldest zones. They are therefore known as 

 the Frigid or Polar Zones. 



RADIATION 



The term RADIATION refers to the process 

 by which electromagnetic energy is propagated 

 through space. Radiation moves at the speed 

 of light (186,000 miles per second) and travels 

 in straight lines without the aid of a material 

 medium through which to pass. All of the heat 

 received by the earth is through this process. 

 It is the most important means of heat transfer. 



SOLAR RADIATION is defined as the total 

 electromagnetic energy emitted by the sun. The 

 sun's 6000° K surface emits gamma rays, 

 X-rays, ultraviolet, visible light, infrared, heat, 

 and electric waves. Even though the sun radiates 

 in all wavelengths, about half of the radiation 

 is visible light with most of the remainder 

 being infrared. 



The earth receives the sun's SHORT WAVE 

 radiation, converts it to LONG WAVE radiation, 

 and reradiates it out to space. 



INSOLATION 



Insolation (an acronym for INcoming SOLar 

 radiATION) is the rate at which solar radiation 

 is received by a unit horizontal surface at any 

 point on or above the surface of the earth. 

 Henceforth, in this training manual, insolation 

 is used when speaking about incoming solar 

 radiation. 



There is a wide variety of differences in 

 the amounts of radiation received over the 

 various portions of the earth's surface. These 

 differences in heating are important and must 

 be measured or otherwise calculated to deter- 

 mine their effect on the weather. 



DISPOSITION OF INSOLATION 



We will now discuss insolation by describing 

 some of its characteristics and the means of 



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