How Do Greenhouses Work?
If you've ever heard an explanation of how a greenhouse works, it was most likely based on the differing transparency of glass to solar and thermal infrared radiation. Glass is transparent to most of the wavelengths of solar radiation, but is effectively opaque to the much longer (thermal infrared) wavelengths emitted by the plants and soil inside the greenhouse. Solar radiation can get into the greenhouse, where it is absorbed by and heats whatever is inside the greenhouse. The longer wavelengths emitted by the heated surfaces cannot get out through the glass, however, so heat keeps building up -- at least that's how the conventional explanation goes. Polyethylene greenhouses, however, seem to work just about as well as glass ones -- and polyethylene is nearly as translucent to thermal infrared radiation as it is to solar radiation. So why does a greenhouse get so hot on a sunny day?
Whether bare or covered by a greenhouse, the ground absorbs radiation from the sun and heats up. The increase in temperature is conducted to the air next to the earth; that air then warms and expands, thus becoming less dense than the air higher up. The lighter air rises, allowing cooler and denser air to take its place at the surface and absorb more heat from the warmed ground. Thus the radiation absorbed by the ground goes into heating a deepening layer of air. Above open ground on a sunny day in summer, the heated layer of air may easily be a mile or more deep, and since the warming is spread over such a large mass (a deep layer of air plus a very thin layer of soil and vegetation), the temperature rise is diluted by the sheer amount of stuff that must be heated. (Think of how slowly the temperature rises in a large kettle full of water which is set on a hot stove for five minutes.)
In a greenhouse, this mixing is confined to the layer of air trapped under the roof, so there is a much smaller mass to be heated. Essentially, the large kettle full of water has been replaced by one with half an inch of water on its bottom, and as a result the water will warm up much faster. This also explains why ventilation is so important in keeping a greenhouse from overheating. (A closed car in the sun heats up due to the same mechanism.)
Ironically, the notorious "greenhouse effect" really does work the way a gardener's greenhouse was (inaccurately) thought to operate: carbon dioxide, methane, water vapor and other such gasses in the atmosphere let the solar radiation pass to the earth's surface but impede the reradiation of thermal infrared wavelengths back to space.
The old explanation of how a greenhouse works also has some relevance in explaining why the nighttime temperatures in a greenhouse are warmer than those in the unprotected garden. The stored heat of the day continues to be radiated by the ground after the sun goes down, so the ground and plants cool. Cooling of the air near the ground does not spread through the atmosphere, as the cooled air is denser than that above. But in the greenhouse, the cooling starts from the greenhouse roof. The stored heat from the full height of the greenhouse is available overnight, and the plants never get much colder than the air. The process can be made more efficient, and at the same time the overheating during the day minimized, by increasing the thermal mass of the greenhouse, perhaps by storing black-painted barrels of water under the plant benches.
The night-time radiative effect works even with a polyethylene greenhouse, because the first thing that happens when the air starts to cool is that some of the water from the moist air of the greenhouse condenses on the inside of the greenhouse roof. Water, like glass, is opaque to thermal infrared, so the polyethylene and glass greenhouses are effectively identical in their night-time behavior -- provided you've remembered to water the greenhouse plants!