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From a physics perspective, is there a difference between fog and steam? What is it and how does is manifest in properties of fog vs properties of steam?

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I've considered asking a similar question myself. Obviously there is the concept of humidity, which formalizes the capacity of air to hold water at a certain temperature. Steam, however, requires being over the boiling point to be called "steam". I wonder about the physics of how steam changes into humidity and the molecular energy distribution changes through the process. –  Alan Rominger Aug 26 '11 at 2:43

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Steam is water in a gas phase, while mist, or fog, are small droplets of water in the condensed phase, but small enough to be kept in the air by thermal Brownian motion. There is a huge difference in their properties.

steam at 1 atmospheric pressure is defined to be water at a temperature of 100 degrees celsius or more. It is an invisible gas, and contains a large latent energy which is released when the steam condenses into droplets. It is highly dangerous, since its high energy content can scald very effectively per unit mass.

Mist, on the other hand, is a harmless droplet state with only marginally more energy than bulk water. The extra energy is all due to the surface tension of the droplet formed, and is orders of magnitude smaller than the latent heat required to boil water. To make a mist or a fog, you can charge water electrically, and the electric repulsion will form droplets for you.

Humidity is water dissolved in air, and it is a different issue. Air will come to statistical equilibrium with liquid water when it has a certain water content, defined as 100% humidity. At any lower level, water will evaporate into the air, and at any higher level, water will spontaneously condense into droplets in the air.

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Thanks. Does this also mean that fog when coming into contact with e.g. a Window will not condense on the window in the way that steam is famous for doing? –  user1264 Aug 26 '11 at 6:00
@Ron: Steam is defined to be water at the temperature that corresponds to the boiling temperature of water at the given pressure. –  Martin Gales Aug 26 '11 at 6:23
@user1264: The fog might stick to the window when it comes close, I don't know. That depends on the details. But any condensation is probably due to the fact that fog can only exist when the air around it is 100% humidity, so any colder area, like near a cold glass will lead to condensation. –  Ron Maimon Aug 26 '11 at 7:05

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