Re: 'AVP at room temperature = SVP at dew point' Why?

The question: "AVP at room temperature = SVP at dew point. Why? (AVP- actual vapour pressure; SVP-saturated vapour pressure) question from hygrometry"

The vapor pressure of the partial pressure of a gas in contact with the liquid (or solid) surface of the substance, assuming the vapor and liquid (or solid) are in equilibrium, depends primarily on the temperature of the substance. Please look up the following information:
"vapor pressure" at Wikipedia
"vapor pressure" at Hyperphysics
"vapor pressure" at Davidson

At the atomic level it has to do with the dynamic equilibrium between the vapor and liquid (or solid). The dynamic equilibrium means that the number of particles (atoms or molecules) leaving the liquid (or solid) phase is the same as the number leaving the vapor phase (and going back to the liquid (or solid) phase).

So let's say you want to know the vapor pressure of water, in equilibrium with its liquid surface, at a temperature of, for instance, 20^oC. One can look up the vapor pressure of water at various temperatures, such as this table at Wikipedia, or one can use a calculator such as the one here.

If we are dealing with a closed container then we are finished with the discussion, but what we usually want to know is the vapor pressure of the atmosphere on any particular day. If the air temperature is above the dew-point temperature then the air is not at 100% relative humidity because the higher vapor pressure of water at the higher temperature would cause a higher equilibrium concentration of water vapor in the air (more water molecules per unit volume) if there were a source of water in which to be in equilibrium. Unless some water vapor is added to the atmosphere it has the same partial pressure of water as it did when it was saturated, so the AVP is the same as the SVP from the saturation temperature (the dew point).

If the air temperature falls below the dew point we obtain dew or frost until the dynamic equilibrium is re-established at the SVP.

John Link, MadSci Physicist