Water evaporating from my hand in the cold.

[wasteofo2]

Recently I was playing in the snow without gloves. I made a snowball and the residual water on my hand began evaporating and was visible, much the way your breath is in the cold.

Why did the water do this?

 

[russ_watters]

Water evaporated there for the same reason it evaporates anywhere - due to the random motion of the water molecules, some reach the energy needed to boil.

 

[Integral]

There are a couple of things going on. First your hand is a source of heat, so you were heating the water, then I am betting that it was pretty cold out and the humidity was pretty low. So in essence the water diffused from the high concentration in your hand to the lower concentration in the air.

 

[wasteofo2]

So then why is the boiling point of water always given as an absolute number instead of there being some sort of formula to decide when water boils/evaporates?

 

[Bystander]

b.p. --- vapor pressure equals one atmosphere. Vapor pressure increases with temperature; lower than b.p., evaporation occurs without boiling so long as the partial pressure of water in a one atmosphere total gas pressure atmosphere above liquid water is less than the vapor pressure; higher than b.p. requires that pressure be maintained at least as great as the vapor pressure for the liquid phase to exist.

 

[russ_watters]

Originally posted by wasteofo2
So then why is the boiling point of water always given as an absolute number instead of there being some sort of formula to decide when water boils/evaporates?

No formula, but there is a chart: PHASE DIAGRAM

 

[Integral]

Do some googling and you will find a lot of information on the variation of boiling point with elevation and barametric pressure.