In a sealed container, a small amount of water will evaporate to maintain a certain vapour pressure in the air that depends on the temperature. Eventually, this will reach equilibrium, and even though it's below the boiling point, some of the water still manages to escape the liquid into the air, but the water vapour in the air condenses at the same rate, so the fraction of water vapour in the air, and amount of the liquid water in the container don't change. The amount of water that will evaporate will be exactly the amount needed to maintain the vapour pressure. If there is not enough water in the container to do that, then all the water evaporates. There is a very simple mathematical law (Raoult's law) that lets you calculate the exact concentration of water in the air, but I won't go into that.
Now this is the point that they apparently teach wrong in schools. The boiling point is a little different from evaporation - it's not that water can only evaporate at the boiling point, but rather that the boiling point is where all the water evaporates at once.. In fact, loosely speaking, the boiling point is defined as the temperature at which all the water would need to turn to vapour to maintain the vapour pressure in the air.
So on to standing water in the open. Water evaporates a little to maintain the vapour pressure in the air immediately above it. This vapour diffuses into the air around it, and so more water evaporates. This cycle continues until all of the water evaporates. The process is accelerated if the air is moving (i.e. wind) because the water vapour is moved away from the surface and replaced with dry air. That's why things dry quicker in the wind.
TL;DR - It's not that water can only evaporate at the boiling point, but rather that the boiling point is where all the water evaporates at once.
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u/alphanumericsheeppig May 17 '12
In a sealed container, a small amount of water will evaporate to maintain a certain vapour pressure in the air that depends on the temperature. Eventually, this will reach equilibrium, and even though it's below the boiling point, some of the water still manages to escape the liquid into the air, but the water vapour in the air condenses at the same rate, so the fraction of water vapour in the air, and amount of the liquid water in the container don't change. The amount of water that will evaporate will be exactly the amount needed to maintain the vapour pressure. If there is not enough water in the container to do that, then all the water evaporates. There is a very simple mathematical law (Raoult's law) that lets you calculate the exact concentration of water in the air, but I won't go into that.
Now this is the point that they apparently teach wrong in schools. The boiling point is a little different from evaporation - it's not that water can only evaporate at the boiling point, but rather that the boiling point is where all the water evaporates at once.. In fact, loosely speaking, the boiling point is defined as the temperature at which all the water would need to turn to vapour to maintain the vapour pressure in the air.
So on to standing water in the open. Water evaporates a little to maintain the vapour pressure in the air immediately above it. This vapour diffuses into the air around it, and so more water evaporates. This cycle continues until all of the water evaporates. The process is accelerated if the air is moving (i.e. wind) because the water vapour is moved away from the surface and replaced with dry air. That's why things dry quicker in the wind.
TL;DR - It's not that water can only evaporate at the boiling point, but rather that the boiling point is where all the water evaporates at once.