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 sunshine 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, which is 186,000 miles per second (297,600 km per second) and travels in straight lines in a vacuum. All of the heat received by 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 surface  emits  gamma  rays,  x-rays,  ultraviolet,  visible light,    infrared,    heat,    and    electromagnetic    waves. Although  the  Sun  radiates  in  all  wavelengths,  about half  of  the  radiation  is  visible  light  with  most  of  the remainder being infrared. (See figure 1-5.) Energy   radiates   from   a   body   by   wavelengths, which vary inversely with the temperature of that body. Therefore,   the   Sun,   with   an   extremely   hot   surface temperature,  emits  short  wave  radiation.  Earth  has  a much cooler temperature (15°C average) and therefore reradiates  the  Sun’s  energy  or  heat  with  long  wave radiation. 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 Earth. In this manual, insolation is used when speaking about incoming solar radiation. There   are   a   wide   variety   of   differences   in   the amounts of radiation received over the various portions of  Earth’s  surface.  These  differences  in  heating  are important and must be measured or otherwise calculated to determine their effect on the weather. 1-8 SCHEMATIC DIAGRAM  0F THE DISTRIBUTION OF ENERGY IN THE SOLAR SPECTRUM. (NOT TO SCALE).  THE NUMBERS ARE PERCENTAGES OF THE SOLAR CONSTANT . THE FIGURE FOR THE RADIO ENERGY IS FOR THE OBSERVED BAND FROM 15 TO 30,000 MHZ. EXTREME U.V. AND RAYS NEAR U.V. VISIBLE INFRARED RADIO 41 52 7 10^-3 10 10 10 10 10 10 10 10 10 10 10 -6 -4 -2 2 4 6 8 10 12 14 16 10^-10 WAVELENGTHS   IN  MILLIMICRONS VISIBLE SPECTRUM AGf0105 COSMIC RAYS GAMMA RAYS X-RAYS ULTRA- VIOLET RAYS INFRA- RED RAYS HERTZIAN WAVES RADIO WAVES LONG ELECTRICAL OSCILLATIONS 400mu 700mu 1 Figure 1-5.—Electromagnetic spectrum.