Grades 3–5 · Systems
Why can a garden greenhouse be warmer than outside?
A garden greenhouse lets sunlight reach the plants and soil inside. Those surfaces absorb some of the light and become warmer. They then transfer energy to the surrounding air.
In the diagramSunlight warms an enclosed growing spaceSunlightVents closedWarm air120 in − 40 other loss − 20 air lossNet energy transfer: 60 wattsSupplied instant: losses change in a real greenhouse.
The walls and roof keep that air from mixing freely with cooler air outdoors. Heat still escapes through the covering, leaks and thermal radiation. Some coverings also reduce the escape of thermal radiation, but the enclosure’s control of air movement is a major part of the explanation.
Opening vents lets warm air leave and outside air enter. In the invented picture, absorbed sunlight supplies 120 watts. Other losses take 40 watts, and air exchange takes 20 with vents closed or 70 with them open. That leaves a net gain of 60 or 10 watts. A watt measures energy transferred each second; these numbers explain the account rather than predict a temperature.
Without enough incoming energy a greenhouse can cool, especially at night. Its name is related to the planetary greenhouse effect, but Earth has no glass roof: greenhouse gases change thermal radiation exchange instead of enclosing air behind walls.
About this explanation
Starting knowledge: Know that sunlight can warm a surface and that warm air can carry energy away.
Sources
- University of Georgia Extension — Greenhouses: Heating, Ventilation, and Cooling
- University of Minnesota Extension — Prepare your high tunnel for a hot summer