Re: Why does a sauna or shower room get so steamy when the hot water is on?

Sorry, this is a bit long, but explains lots of neat things about water and air...

The white fog that most people call "steam" is really a cloud of tiny liquid water droplets floating in the air. This same suspension of droplets forms summertine clouds, fog, "steam" from a boiling kettle, and the "steam" in the shower.

Engineers use the word "steam" to mean water in gas form -- also known as "water vapor". Water vapor is a transparent, colorless, odorless gas.

Certainly, water vapor is formed when you boil water. But water vapor is also released into dry air by liquid water at much lower temperatures. This is what happens when water evaporates from a dish left out in the sun, and why laundry hung on a line becomes dry.

The amount of water vapor in air is described by its partial pressure, which is the total air pressure times the percentage of water molecules in the air. The Clausius-Claperyon equation states that the amount of water vapor air can hold depends exponentially on the temperature. The partial pressure is very small when air is very cold, and reaches 1 bar (standard atmospheric pressure) at 100 celsius.

If you take cool air (which can't have much water vapor in it) and heat it up, it can suddenly hold much more water, so it evaporates liquid water off anything it touches. This is how a clothes dryer works, and why your skin feels so dry in the wintertime.

Conversely, if you take warm air (which has lots of water vapor in it) and cool it down, it suddenly has too much water vapor and has to get rid of it. It does this by spontaneously forming zillions of tiny liquid water droplets in midair. (liquid water doesn't count toward the partial pressure: only water vapor does.) This is called "condensation".

This is what happens in a cloud (as warm air from the ground mixes with the cooler air higher in the atmosphere, it cools and forms droplets), in fog (same thing, but at ground level), in a boiling teakettle (the gas leaving the kettle is almost 100% water vapor, which mixes with the kitchen air and cools), and in your bathroom (where warm moist air heated by the shower water mixes with cool air from under the door, or air cooled by the window, the walls, etc.)

So, to answer your question succinctly, the white billowy clouds in the shower are formed whenever warm, humid air mixes with cooler air. No boiling is required.

I answered a similar question last year, giving the Clausius-Claperyon and other technical equations relating to water vapor.