The earth receives only a small amount of the total energy emitted by the sun. Most of the sun's energy is blocked by our atmosphere but some passes through--around 5 %-- to benefit life on earth. For instance, sunlight is necessary for plants' photo

The earth receives only a small amount of the total energy emitted by the sun. Most of the sun's energy is blocked by our atmosphere but some passes through--around 5 %-- to benefit life on earth. For instance, sunlight is necessary for plants' photosynthesis, and human bodies use sunlight to help make Vitamin D.

The Electromagnetic Spectrum

Radiant energy is comprised of electromagnetic waves, which are arranged on the electromagnetic Spectrum (array) from the longest wavelength--radio waves-- to the shortest--gamma rays. The shorter the wave, the higher the frequency, which means, for example, that an x-ray wave, crest to crest, containing a shorter wavelength, passes a certain point more often than a radio wave with a longer wavelength.

Ultraviolet Radiation

Radiation from sunlight that reaches the earth is grouped within the electromagnetic spectrum into three forms: visible, infrared, and ultraviolet (UV) energy. Infrared energy is the range below the visible color of red, and UV light is the energy range beyond the visible color of violet.

Our eyes are sensitive to visible light, which we can see. However, we cannot see infrared light, long wave radiation that we can feel as heat. Furthermore, we cannot see UV light, short wave radiation that can cause sunburn. UV light can be further subdivided into UVA, UVB and UVC. UVC, the shortest wavelength, does not reach the earth. However, we are concerned about UVA and UVB light because they do reach the earth and can cause skin damage and skin cancer.

Sunsets and Seasons

Around noon, when the sun hits the earth at a more direct angle, the sunlight is more intense. However, earlier or later in the day, the sunlight strikes the earth at a lower angle and passes through more atmosphere. This causes sunlight to scatter or be absorbed by the atmosphere, which in turn attenuates (weakens) the sunlight even more. When all of the visible light is absorbed except for the red, for instance, you see a red sunset.

The angle of the sun to the earth changes throughout the day. And the number of hours in a day receiving sunlight also changes with the seasons. We have seasons because the earth in orbit around the sun is not perpendicular (direct)to the plane of its orbit but is tilted on its axis at an angle of 23.5 degrees.

In the Northern hemisphere, the sun is more intense during the summer when the sun's angle to the earth is higher because that part of the earth is tipped toward the sun. Also, the day is longer and the sun appears higher above the horizon. Vertical rays mean a higher noon sun.

Yet in the Southern Hemisphere, the reverse is true. The sun is more intense during the winter. But the closer to the North or South Pole you travel, the weaker the sunlight becomes.

In contrast, people who live nearer to the equator receive more sunlight because the sun hits the earth there at a higher angle.

What's more, sunlight is reflected from surfaces depending upon the texture, moisture, temperature and color. For instance, fresh snow reflects 72%-95% and sand reflects 15%-45% of sunlight, whereas grass or rocks reflect around 10%-30% and a plowed field reflects around 5%-20% of sunlight.

About the Author

Diana Clarke is a California credentialed teacher and freelance writer. Her sun protection articles have appeared in publications, such as the San Jose Mercury News, Saratoga News and a high school health magazine, Listen Magazine.