[dentstudent (OP)]

Here is a picture of a Steaming lake.

I'm having delivered to Neil's place next Tuesday 'cos I'm buggered if I'm going to look after it. Anyway, the air temperature was around 0°C and clearly the water was warmer, as it had that lovely steamy effect. So, how big does the temperature diffference have to be for this to happen? I guess that the humidity of the air has a role to play too.....but it seems that with large bodies of water, this only occurs in colder air temperatures?

All insights welcome!

[RD]

The molecules of water are not all at the same temperature.
The value given by a thermometer is an average value, the temperature is more accurately described by a "bell distribution curve".
A small minority of the liquid water molecules will have sufficient kinetic energy to become water vapour (gas).


If the air temperature is particularly cold then the water vapour will condense back into liquid water droplets close to the water surface which is the "steam" on the lake.

[lyner]

The molecules of water are not all at the same temperature*.

I think you mean 'KE' here*, bearing in mind the definition of temperature i.e. the mean KE of a large number of particles.

I think the conditions for maximum steaminess could involve cooler, moist, air drifting over the (fractionally warmer) lake. This happens in the evening as the land cools quicker than the water.

Strong sunlight on a shallow lake could cause it, too; the water temperature could rise quite quickly - particularly if it were muddy, absorbing the energy in the top cm or so of the water. (It certainly happens on puddles after a shower).
It always must involve moist air, though.

[dentstudent (OP)]

Thanks SC - so only "fractionally" warmer, then? I guess that there are always water molecules evaporating from the surface, and as the temperature difference increases favourably for the water, then there is a greater rate of evaporation.

So it is unlikely to occur when the water temp. is lower than the air temp.?

[RD]

A relevant webpage animation... http://www.saskschools.ca/curr_content/chem30_05/1_energy/energy1_4.htm
 

[lyner]

Thanks SC - so only "fractionally" warmer, then? I guess that there are always water molecules evaporating from the surface, and as the temperature difference increases favourably for the water, then there is a greater rate of evaporation.

So it is unlikely to occur when the water temp. is lower than the air temp.?

Yes, I should have thought so. The vapor pressure of the water would be less than the vapor pressure of any droplets formed in the air above .Even if the air were saturated, I'd have expected condensation onto the water.

[dentstudent (OP)]

Thanks RD for the link and SC for comments!

[Madidus_Scientia]

To be pedantic, it's fog not steam. Steam is invisible.

[dentstudent (OP)]

To be pedantic, it's fog not steam. Steam is invisible.

No it isn't.

(yes it is)

No, it isn't.

Ah, yes it is. I didn't know that! Thanks MS.

Wiki: "In the spout of a steaming kettle, the spot where there is no condensed water vapor, where there appears to be nothing there, is steam."

[dentstudent (OP)]

Also, if you wanted to FOG about steam being fog, then you need to "FOG". Ok?

[Madidus_Scientia]

I'm unfamiliar with the acronym

[dentstudent (OP)]

WHAT?! Where have you been?

Are you a FOG?

[Madidus_Scientia]

I see

I'm not that old yet

[dentstudent (OP)]

I see

I'm not that old yet

It's more a question of pedancy than age - you seem to qualify for the former very well (which is by no means an insult!).