How does a greenhouse work?
Have you wondered?
In a way, it’s really rather amazing how a greenhouse works to convert light into warmth.
It actually seems kind of magical when I walk into one of my greenhouses on a cooooold winter’s day.
Though the outside air may be ferociously frigid with the thermometer showing well below freezing, if it’s a sunny day, then I can count on it being toasty warm inside the greenhouse.
It seems miraculous because all that separates the inside of that greenhouse from the outside is a sheet of plastic only a few thousandths of an inch thick.
But with only that thin plastic membrane holding winter’s wrath at bay, the greenhouse works so efficiently at converting the sunlight to heat that the ventilation fans must occasionally cycle on to prevent the greenhouse from becoming too warm.
So just how does a greenhouse work?
Whether or not you believe in man-made global warming, the ‘greenhouse effect’ is a term that you hear quite frequently these days.
This is how the greenhouse effect works: the so-called greenhouse gases in the atmosphere, such as water vapor and carbon dioxide, allow sunlight to pass through the atmosphere to the earth.
When the sunlight strikes the surface of the earth, much of the energy of the sunlight is converted into a different form of energy – heat. Since the greenhouse gases do a good job of trapping much of the heat and preventing it from escaping back out into space, the net result is a warming of the earth’s atmosphere.
If the amounts of greenhouse gases in the atmosphere are increased, then even more of the heat energy of the sunlight is preventing from escaping into space, causing a further warming of the atmosphere.
The term ‘greenhouse effect’ was coined, obviously, because of how a greenhouse works. Problem is, that’s NOT how a greenhouse works...
In both the greenhouse effect and an actual greenhouse, there are similarities. Sunlight is allowed to pass through the greenhouse covering or atmosphere relatively unimpeded. And in both cases, the sunlight is converted into heat.
But in the atmospheric greenhouse effect, the earth warms because the solar energy is re-radiated back towards the earth by the greenhouse gases.
While in a greenhouse, the warming of the interior is simply a result of the heat energy from the sunlight heating the air. The covering of the greenhouse prevents the heated air from escaping the house. And since the air is trapped inside as additional solar radiation continues to stream into the greenhouse, the air gradually becomes warmer and warmer.
So on one hand (the atmosphere), it’s the energy itself that is re-radiated and prevented from escaping, and on the other hand (an actual greenhouse) air that has been warmed by the energy is prevented from escaping.
Splitting hairs? Perhaps. But apparently not as far as physicists are concerned! (And I am most certainly NOT a physicist!)
But if the above is clear as mud, then consider this…
When I enter the fantasy-like world of a greenhouse on a frigid February day, and work with my plants in shirtsleeve comfort, I get a bit of a reprieve from winter. Sort of a suspension of reality.
I can grow summertime goodies like tomatoes and strawberries, and never have a concern about freezing or frosts.
So when I make the walk out to a greenhouse through the bone-chilling wintertime wind, and step into that artificial cocoon of springtime warmth with nose dripping and fingers numbed from the cold, I’m not really all that concerned about the physics of how a greenhouse works.
I’m just really glad that it does!