The sun is a mass of incandescent gas, where hydrogen is turned into helium at temperatures of millions of degrees. The fusion of hydrogen to helium (two hydrogen equals one helium) releases a lot of energy in the form of waves which are sent out in all directions. Some of the waves (one 2-billionth, or 1/2,000,000,000) intersect the earth. These are called insolation or INncoming SOLAr radiaTION. The waves range from very short to very long (see the figure below and Figure 2.6 in your text), but each wave length (distance from trough to trough) carries approximately the same amount of energy. Therefore, as you can see in Figure 2.5 of your text, short-wave radiation (the purple lines) have much more energy per unit time than long wave radiation (red lines).
Figure 1.5.1 Types of waves
The figure above shows the approximate real-world wavelength of each type of wave. Gamma rays are very, very, very small, about the size of an atomic nuclei. X-rays are about the size of an atom. The waves that cook your popcorn in the microwave and bring you music over the radio in your car are fundamentally the same as gamma rays and x-rays; they just have much less energy per unit area.
Light is also the same kind of wave, but it occupies a very small portion of the electromagnetic spectrum (see figure 2.6). However, our atmosphere filters out (by absorbing or reflecting) almost all of the other wavelengths of incoming solar radiation. As a result, the majority of the electromagnetic energy received by the surface of the earth is in the visible spectrum (see Figure 2.7). Because of this, our eyes have evolved to respond to this tiny portion of the spectrum.
All exogenic systems (those that function above the earth’s surface) require energy to function, and allof that energy comes from the sun. This is a very important point – the energy from the sun drives all systems on earth. For example, people (open systems) require the input of food (energy) and water. Food can come from plants (or animals who eat plants) which are themselves open systems requiring water and sun energy to drive photosynthesis, the process by which they produce the sugars that they require to grow and sustain themselves. The water cycle (which we will learn about in more detail in week 5) gives us fresh water through rain or groundwater sources. The driving force behind the water cycle is sun-energy.
K. Allison Lenkeit-Meezan (Foothill College)