The greenhouse effect is more discussed today for its role in warming the earth than in warming greenhouses, but it does both. The effect depends on the fact that objects emit energy at a wavelength that depends on their temperatures. For example, the surface of the sun is about 9,300 degrees The sun emits most of its energy in the visible part of the electromagnetic spectrum. The surface of the earth is about 57 degrees. Because of its temperature, the earth emits radiation in the infrared part of the spectrum.
Warming the Earth
The earth’s atmosphere passes most of the energy from the sun to the earth’s surface, which is heated by this energy. The earth radiates its energy back to space. However, most of its energy is radiated in the infrared range, and gasses in the atmosphere absorb some of this energy, trapping it in the atmosphere. The gasses trap just enough heat to keep the earth at its current temperature.
However, man’s activity has increased the release of carbon dioxide and other “greenhouse gasses” to the atmosphere. These greenhouse gasses absorbs much of the infrared radiated by the earth. More heat is being trapped in the atmosphere, and the earth is slowly becoming warmer.
Warming the Greenhouse
In a greenhouse, glass performs the same function as carbon dioxide in the atmosphere: it allows free passage to the visible light coming in, but blocks some of the infrared trying to exit. This effect helps to warm the greenhouse.
However, the “greenhouse effect” isn’t the only effect that warms greenhouses, or even the most important one. More important is the fact that surfaces inside the greenhouse are warmed by the sun, then warm the air in the greenhouse. The warm air is trapped, raising the temperature inside. Gardeners with solar or unheated greenhouses can take advantage of this effect by painting surfaces dark colors to absorb more heat, and by having more absorbent surfaces inside the greenhouse to warm the air.