The foehn wind effect
describes the phenomeneon,
that air, which is crossing a high mountain, after the passage has a
higher temperature than before. A necessary requirement is here, that
the
crossing air lose humidity on the front side, for example by raining.
To understand this phenomeneon, it is important to know,
that in
the air not all molecules/atoms have the same energy/speed. Some have
under average and some have over average energy.
When water molecules unite in a water drop, it is necessary, that they
have a relative low amount of energy to hold together. So it can be
asumed, although without measuring, that this molecules have under
average energy. But if under average individuals leave a system, it is
logical, that the remaining rest has a higher level then before.
The new created water drop can not be in an equilibrium with the gas
atmosphere. A higher material density lead to a higher energy density
and so the probability of energy lost is higher than the opposite
process. The equilibrium is reached, when the drop has lost enought
energy to the gas, which is called condensation heat.
But what has this to do with the concept of irreveribility ? When
growing water drops reach a critical mass, they fall down to the earth.
But the down fall is an irreversible process, possible only from up to
down. Would the water drops remain in the air and pass
the mountain, they would vaporize at the back site and no foehn wind effect
would appear.
