In fact, the atmosphere does not behave like the air in a greenhouse. But since this expression is used here and there, its actual mechanism must be explained.

Greenhouse operation

It goes approximately like this:

• The incoming sunlight passes through the glass of the greenhouse roof. Special glass is chosen to minimise reflection and absorption of UV and to maximise reflection and absorption of IR radiation.
• The ground of the glasshouse is heated by the incoming radiation.
• By convection and radiation the heated ground warms the air inside of the greenhouse.
• As there is no escape the hot air remains within the enclosure and warms the walls and the roof of the glasshouse by convection and conduction.
• An equilibrium is reached when the temperature of the external surface of the glasshouse reaches a level at which all incoming heat can be transferred to the atmosphere by radiation and convection. This is the forcing analogy.
• To cool down the inside of the greenhouse it suffice to open the roof and let escape hot air (and enter cooler one). Another way of preventing too high temperatures in the glasshouse is to block or to reflect the incoming sunlight by whitewashing the roof.

The same mechanism applies in cars parked in the sun, where dark painted ones will be hotter than those having a white body. Bodyworks made of perfect mirrors would be even better, but not for other drivers’ dazzlement …

Since there is no roof over the Earth, the climate analogy is only a partial one in which the ground warms the air by IR radiation and, mostly, convection. Cloud cover plays somehow the role of a roof, but it rarely rains within a greenhouse.