# How much water evaporates in 1 hour from 1 m² of water at a temperature of 30 °C?

Jaroslav Kores, Ph.D.

Unfortunately, there is no simple answer to such a simple question. The rate of evaporation of any substance depends on many factors and only a few are listed. Specifically, the rate of evaporation depends on the material (which in this case is water), temperature (entered), area (also entered), pressure and vapour suction.

Even if we consider that the evaporation takes place at normal atmospheric pressure, we have here a crucial (and also the most difficult to measure) variable — the suction of vapours/saturation of the air above the liquid with vapours. And I can easily prove that this is an important parameter: if we have water in a pot without a lid and with a lid, we will see that the water is still evaporating from the pot without the lid, while the evaporation is lower in the pot with the lid closed. If we seal the lid, the evaporation will be zero. Why is that so? Evaporation is a process in which molecules of a liquid (in our case water) escape from the surface into the surrounding gas. I will point out that the molecules are still the same, only their arrangement changes (a molecule of ice, water and water vapour is still the same molecule, it is only bound differently to neighbouring molecules). Thus, vaporized molecules accumulate above the surface of the liquid. If we do not remove them, they will fill the space above the liquid so that there will be no free space for another vaporized molecule and if one molecule evaporates, another will return (condense) to the liquid. This is what happens with a pot with a lid. With an open pot, the vaporized molecules will freely continue beyond the surface and do not take up space where other molecules can vaporize. If we even help the vaporized molecules (we blow above the surface), the freeing of space is even faster. So if there’s wind, evaporation is faster than when there’s no wind (which we can easily say when we get out of water — we’re colder when there's wind blowing than when there’s no wind). But to give at least some number — swimming pool manufacturers state that roughly 1 litre evaporates from a 1 m2 surface per hour. A similar example is also solved in Feynman’s physics lectures (but with a result of about 0.1 litres per hour).

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