Re: Why are scientists definitions of steam and fog in disagreement?

Hi, Constanza.

There are problems with the ways that scientists use words. And some of these 
problems come out worst when scientists are trying to answer ordinary questions 
from ordinary people.

You see scientists need "technical terms" so that they can talk about things in 
very exact ways. Often they take over words from everyday language, and use them 
to mean something that is nearly, but not exactly the same. Or they choose one 
particular meaning of a word that might have 5 or 6 other slightly different 
meanings in ordinary language.

As an example take the word "energy". A scientist might say something like "the 
internal energy change in this chemical reaction is 123.45 kilojoule per mole". 
An ordinary person might say "I'm feeling really crook today; I just have no 
energy" (at least that's what an Australian might say!) You can see that the word 
"energy" is being used in quite a different way, and has a different meaning.

What you are seeing over the lake is a cloud of water droplets (or possibly ice 
particles, if it is really cold). If people around where you are call that 
"steam" in ordinary language, then that is OK. But when a scientist uses the term 
steam, it means the vapour or gaseous form of water. If you want to use the word 
steam like a scientist, then you would not use it for what you see over the lake. 
But you would not call the cloud you see coming out of a kettle steam either!

Boil a kettle and look closely at it. Near where the water is boiling, you cannot 
see anything. As you look further away from the water (or the spout of the 
kettle) you see white clouds. Those clouds are water droplets, liquid water, so 
they are not what a scientist would call steam. The steam is the invisible 
gaseous bit right next to the boiling water (or right at the tip of the spout).

"Fog" as a technical term can refer pretty much to any cloud at ground level. So 
it is quite OK to call what you see over the lake a fog.

By the way, water does evaporate (that is, turn into a gas or vapour) without 
having to boil or even get very hot. It does so rather slowly. The gas that comes 
off is usually called vapour, and is not usually called "steam", but there is no 
good reason why it should not be.