How Greenhouses Work
Greenhouses work by the differing transparency of glass to solar and thermal radiation. Glass is transparent to most of the wavelengths of solar radiation, but is thick to the much longer (thermal infrared) wavelengths released by the plants and soil inside the greenhouse. Solar radiation is able to enter the greenhouse, where it is absorbed by the plants and heats them up. The longer thermal wavelengths emitted by the heated plants cannot get out through the glass so heat keeps building up. Plastic greenhouses, however, appear to work as well as glass ones. The plastic is not nearly as transparent to thermal infrared radiation as it is to solar radiation. This leads us to wonder why a greenhouse gets so hot on a sunny day verses a cloudy one. Whether covered or uncovered by a greenhouse, soil naturally absorbs radiation from the sun and becomes warm. The air molecules adjacent to the soil also become warm. As air gets warmer, the molecules begin to become more active.
Greenhouses work by the differing transparency of glass to solar and thermal radiation. Glass is transparent to most of the wavelengths of solar radiation, but is thick to the much longer (thermal infrared) wavelengths released by the plants and soil inside the greenhouse. Solar radiation is able to enter the greenhouse, where it is absorbed by the plants and heats them up. The longer thermal wavelengths emitted by the heated plants cannot get out through the glass so heat keeps building up. Plastic greenhouses, however, appear to work as well as glass ones. The plastic is not nearly as transparent to thermal infrared radiation as it is to solar radiation. This leads us to wonder why a greenhouse gets so hot on a sunny day verses a cloudy one.
by: Michael Cook Greenhouses work by the differing transparency of glass to solar and thermal radiation. Glass is transparent to most of the wavelengths of solar radiation, but is thick to the much longer (thermal infrared) wavelengths released by the plants and soil inside the greenhouse. Solar radiation is able to enter the greenhouse, where it is absorbed by the plants and heats them up. The longer thermal wavelengths emitted by the heated plants cannot get out through the glass so heat keeps building up. Plastic greenhouses, however, appear to work as well as glass ones. The plastic is not nearly as transparent to thermal infrared radiation as it is to solar radiation. This leads us to wonder why a greenhouse gets so hot on a sunny day verses a cloudy one. Whether covered or uncovered by a greenhouse, soil naturally absorbs radiation from the sun and becomes warm. The air molecules adjacent to the soil also become warm.
