How do greenhouses actually work?

Submitted by _Dnikeb t3_11tln6y in askscience

I remember my teacher explaining that greenhouses keep the environment inside them warm because glass lets most visible light in but no infrared out, making new heat, and that's also what CO2 and lots of other gasses do, hence the greenhouse effect we all know and love.

However, some people say that this theory is outdated and infrared has nothing to do with it, but instead greenhouses work by limiting air exchange, so if you keep the same air in the same sun-exposed enclosure for a long time it's going to get warmer simply because it can't mix with outside air. So no new heat is made, it's just trapped in place, and therefore the term "greenhouse effect" as applied to the earth's atmosphere is a misnomer.

So who's right?

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DrOmega2468 t1_jcnvtne wrote

The temperature of an object is the result of the balance between heat being added and heat being lost. In the case of greenhouses, both effects you described are in play, but stopping the air exchange has a far bigger role than blocking infrared light. More details below:

Suppose you're standing in an open-air garden on a cold but sunny day. The majority of the heat reaching you comes in the form of solar radiation, i.e. sunlight. But because the environment around you is fairly cold, that heat is quickly lost to the cold air.

In a greenhouse, you still have roughly the same amount of heat incoming. But because the air is (mostly) trapped, heat loss is greatly reduced. More specifically, instead of the heat being lost by convection in the previous case, heat entering the greenhouse has to be conducted through the glass walls in order to escape. This is much harder, so the equilibrium temperature inside the greenhouse is higher than the garden.

Side note: regular glass is actually pretty high transmissivity in the infrared. Although the stuff inside your greenhouse aren't at a temperature where they're producing a lot of infrared radiation anyways (I hope).

In the case of Earth, incoming heat is radiation in the visible spectrum, while outgoing heat is radiation in the infrared. Having more greenhouse gases in the atmosphere block more of the infrared heat from leaving, so we end up with higher average global temperature.

So in the broad sense of reducing heat loss, the term greenhouse effect is correct and easy to understand. It's just reducing radiative heat loss (for Earth) vs convective heat loss (for greenhouses).

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elegance78 t1_jcoaeqz wrote

Replying to Jarlentium (#2,260,639)

https://youtu.be/oqu5DjzOBF8 For the first time I actually understood what earth scale "greenhouse" effect is. The stupid name does so much damage.

Actual greenhouses - infrared escapes through the glass easily and heat losses are very high at night. To the extent that growers measure this with pyrgeo sensors and take actions to prevent it (closing thermal screens so that plants "see" the warm screen material instead of the -50C clear night sky.

They work by trapping warmed air (warmed either by sun or warmed by hot water pipes). Actual trapping works by greatly limiting air exchange. In fact, there is not enough air exchange capacity to even out inside temperature of an empty greenhouse and outside temperature on warm summer day. We rely on the moisture transpired by the plants to cool down the greenhouse. To achieve this we limit the air exchange - it may be 40C outside but it can be 31-33C inside.

We also enrich air with co2 to 1000ppm. Only way this affects the inside temperature is the effect it has on plant transpiration.

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loki130 t1_jcoq0lp wrote

Replying to EarthHuman0exe (#2,261,245)

Those are the most important in Earth's atmosphere, but methane, ozone, and NO2 also contribute, and there are numerous other gasses that could act as greenhouse gasses (and some cases where certain combinations of gasses can have a stronger greenhouse effect than either alone).

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KauaiCat t1_jcp2xwe wrote

A "greenhouse" gas or "greenhouse" effect really has nothing to do with a literal greenhouse.

A molecule can only absorb IR if its vibrational modes cause a change in its dipole moment. No matter how you configure the two nitrogens of N2, it will always have zero dipole. Therefore, N2 is not a greenhouse gas, but if you bend a CO2 molecule, it will have a net dipole.

This means it will absorb IR photons of certain wavelengths and become excited where the wavelengths are associated with vibrational modes of the molecules.

Once a greenhouse gas has absorbed an IR photon it could: transfer the photon's energy to another gas molecule during a collision, relax and re-emit the photon, or lose the energy by combination of collision and emisssion which will result in emission of a photon with a different wavelength than the original photon.

As greenhouse gas concentration increases, re-emitted photons are more likely to be absorbed by another molecule, providing resistance to the energy leaving the earth.

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