Why Is a Greenhouse Warm and How Does a Greenhouse Trap Heat?

Greenhouses, sometimes called hothouses or glasshouses, are always naturally warmer, without any artificial heating. But believe it or not, the main reason is not what everybody refers to as “the greenhouse effect.”

A greenhouse is just a closed, glass container for plants. The glass lets in sunlight, which the plants need for growth, while keeping out damaging wind, hail, and animals. It also prevents the loss of moisture and keeps the humidity high, which is part of what hit you in the face when you entered. But mainly it acts as a heat valve, limiting the loss of heat from the plants to the cold, cruel world outside.

A plant, or anything else for that matter, can get cold, that is, can lose heat, in any of three ways: by conduction, by convection, and by radiation. Conduction isn’t a problem because the leaves aren’t in contact with anything, such as a mass of metal, that could conduct their heat away. That leaves convection and radiation. The greenhouse cuts down both.

Convection is the circulation of warm air or water. Because warm air rises, it can carry heat up and away from a plant leaf. Anything that prevents that warm air from escaping completely will prevent the loss of heat along with it, and any closed-in building will serve that purpose. That’s the major effect of the greenhouse: It simply prevents heat loss by moving currents of air. Of course, no farmer would dream of constructing a plant enclosure building without letting in lots of sunlight, and that’s why glass-walled and glass-ceilinged enclosures were born.

A secondary effect of the glass, which nobody knew about when they invented greenhouses, is that it cuts down on heat loss by radiation. That’s where the so-called greenhouse effect comes in, and here’s how it works.

The photosynthesis reactions that keep plants living and growing utilize ultraviolet radiation from sunlight. After using some of the energy of this radiation, they emit lower-energy “waste radiation,” infrared radiation, which can then be absorbed by other objects. But when an object absorbs infrared radiation, its molecules become more energetic and the object grows warmer. We can therefore think of the infrared radiation from the plants as if it were heat, traveling through the air in search of something to warm up.

What happens when the infrared radiation hits a glass wall or ceiling? Although glass lets in ultraviolet light pretty well, it is not completely transparent to infrared. So the glass blocks some of the infrared radiation from getting out of the greenhouse, and this trapped radiation gradually warms up everything inside.

Clearly this heating can’t go on forever; greenhouses have not been known to suffer spontaneous meltdowns. After a certain point, the inevitable leakage of heat out of the house balances the infrared buildup inside, and the temperature levels off at a moderately warm level, warmer than if the glass were completely transparent to infrared radiation.