Weather Conditions

Hi,

search for a "Mollier i-x diagram", (in thermodynamics book)

French 18th century Mollier did solve this by measurement of equilibra between evaporated, fluid and solid status as a function of pressure and temperature.

This is at conditions far away from ideal gas: the steam may be wet or near condensation.

The process of evaporation is to be considered separately as the first step of evaporation is a molecule of sufficient velocity leaving the fluid (or the reverse: a gas molecule hitting the surface). As these molecules have very small distances (mean free path length) before hitting its neighbours it is not likely that it can escape into the surrounding gas or steam but will re-condense or stay very near the evaporating surface.

This situation is changed by wind velocity which carries away the freshly evaporated molecules. (Same situation on drying clothes).

So in short:

you need high enough temperature or low enough pressure to get efficient evaporation, some evaporation is existing at any temperature. At high enough temperature you get boiling (vapor-pressure as function of temperature above ambient pressure).

You need wind to carry away the evaporates.

At elevations up in atmosphere clouds are forming where the solubility of water in air is lowered and no longer capable of holding in solution this water (by cooling down and lower pressure) that droplets = condensing water is forming. The condensed droplets will refract the sunlight so we see white clouds.

RHABE