The earth constantly tries to maintain an energy balance with the atmosphere. Most of the energy that reaches the Earth’s surface comes from the Sun. About 44 percent of solar radiation is in the visible light wavelengths, but the Sun also emits infrared, ultraviolet, and other wavelengths. When viewed together, all of the wavelengths of visible light appear white. But a prism or water droplets can break the white light into different wavelengths so that separate colors appear.

Of the solar energy that reaches the outer atmosphere, UV wavelengths have the greatest energy. Only about 7 percent of solar radiation is in the UV wavelengths. The three types are:

• UVC: the highest energy ultraviolet, does not reach the planet’s surface at all.
• UVB: the second highest energy, is also mostly stopped in the atmosphere.
• UVA: the lowest energy, travels through the atmosphere to the ground.

The remaining solar radiation is the longest wavelength, infrared. Most objects radiate infrared energy, which we feel as heat. Some of the wavelengths of solar radiation traveling through the atmosphere may be lost because they are absorbed by various gases. Ozone completely removes UVC, most UVB and some UVA from incoming sunlight. Oxygen, carbon dioxide, and water vapor also filter out some wavelengths.

Solar Radiation on Earth

Different parts of the Earth receive different amounts of solar radiation. Which part of the planet receives the most insolation? The Sun’s rays strike the surface most directly at the equator. Different areas also receive different amounts of sunlight in different seasons. What causes the seasons? The seasons are caused by the direction Earth’s axis is pointing relative to the Sun.

The Earth revolves around the Sun once each year and spins on its axis of rotation once each day. This axis of rotation is tilted 23.5 degrees relative to its plane of orbit around the Sun. The axis of rotation is pointed toward Polaris, the North Star. As the Earth orbits the Sun, the tilt of Earth’s axis stays lined up with the North Star.

Northern Hemisphere Summer

The North Pole is tilted towards the Sun and the Sun’s rays strike the Northern Hemisphere more directly in summer. At the summer solstice, June 21 or 22, the Sun’s rays hit the Earth most directly along the Tropic of Cancer (23.5 degrees N); that is, the angle of incidence of the sun’s rays there is zero (the angle of incidence is the deviation in the angle of an incoming ray from straight on). When it is summer solstice in the Northern Hemisphere, it is winter solstice in the Southern Hemisphere.

Northern Hemisphere Winter

Winter solstice for the Northern Hemisphere happens on December 21 or 22. The tilt of Earth’s axis points away from the Sun. Light from the Sun is spread out over a larger area, so that area isn’t heated as much. With fewer daylight hours in winter, there is also less time for the Sun to warm the area. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere.

Equinox

Halfway between the two solstices, the Sun’s rays shine most directly at the equator, called an equinox. The daylight and nighttime hours are exactly equal on an equinox. The autumnal equinox happens on September 22 or 23 and the vernal or spring equinox happens March 21 or 22 in the Northern Hemisphere.