The
weather machine
The radiation from the sun heats the surface of the Earth, which
in turn heats the air 
above
it, causing it to rise.
Air above a warm part of the planet will rise faster than the
surrounding cooler air, so more air slides in to take its place
in the low-pressure void left behind.
So wind is born. And that is the heart of the weather machine:
heat and pressure differences bring cyclones, depressions, thunderstorms
and hurricanes, all blown across the surface of the globe by the
wind.
But if wind is the engine of the weather, water is the fuel. Our
weather consists of a constant interplay of water as vapour, liquid
and solid ice.
As we will see in later chapters describing the formation of rain,
snow and storms, so much depends on water molecules changing from
one state to the other that it is not surprising to find that
our planet lies at precisely the right distance from the sun for
water to exist in all three states at the same time.
Alone among the planets of the solar system, Earth has average
temperature and pressure that are very close to what scientists
call the ‘triple point’ of water — the combination of temperature
and pressure at which water can exist in all three of its physical
states. This property of our planet is one which allows us to
exist.
Warm air, dry air, cold air and wet air all behave in different
ways, and it is those qualities of air that make it move and drive
the engine of the weather.
Although it may not seem obvious, moist air is less dense than
dry air, and so will always tend to rise above it. This is because
of a very simple piece of physics.
A water molecule is made up of one oxygen atom and two hydrogen
atoms (H20). Because hydrogen atoms are the smallest and lightest
atoms that exist, a molecule of H20 is much lighter than a molecule
of either oxygen (02) or nitrogen (N2), which are what dry
air
consists of. In moist air, water-vapour molecules have replaced
some of the oxygen or nitrogen molecules, one for one, so any
given volume of moist air contains more lightweight molecules
than does the same volume of dry air, and therefore it is lighter
and less dense.
Warm air is less dense than cold air because, when they are warm,
all air molecules are more active, move around more and take up
more space — the air expands.
Thus a given volume~of warm air will simply contain fewer molecules
than that of cold air, so it will be less dense.
In a nutshell, if there is warm, hioist air it is going to move
up and, at its very simplest, all of our various forms of weather
have that single action at their heart: the air rises, in comes
the wind below; as the air rises further it cools, the water vapour
condenses to cloud and down comes the rain That’s all there
is to it.
