as air cools, it is able to hold much less moisture than warm air
this means that if the air temperature falls to a low enough level, then it will get to a temperature at which the air is saturated. this means that it cannot hold any more moisture.
the point at which the air becomes saturated is known as the dew point temperature.
at this point, excess water in teh air will then be converted to liquid water in the process of condensation.
water molecules cannot just condense, they need something to form around. this can be tiny particles such as smoke, salt, dust etc. collectively such particles are known as condensation nuclei.
condensation can also happen on surfaces e.g. leaves, grass stems, windows etc. these surfaces need to be below the dew temperature.
crucially, if the surface in question is below the freezing point, then water vapour sublimates. sublimation is where a gas turns straight into a solid without going through the liquid state. hoar frost is produced as a result of this process.
condensation is the direct cause of all forms of precipitation.
precipitation can happen in the following conditions:
when the temperature of the air is reduced to dew point, but the volume of the air remains constant. this happens when:
warm air passes a cold surface,
on a clear winters night heat is radiated out to space and the ground gets colder, which can cool the air directly in contact with it,
it also happens when the volume of air increases, but there is no addition of heat. this is known as adiabatic cooling, this happens when air rises and expands in the lower pressure of the upper atmosphere. this happens in the following types of rainfall- relief, frontal and convectional rainfall.