Can someone explain humidity please?

Surely its just a percentage of how saturated the volume of air can be against the point where it can hold no more vapour?

It changes with the temperature of the air so you can't use Absolute.
 
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Partial pressure with gasses involves splitting a total pressure value (which conincides with the amount of gas and the volume) into the pressure values for each type of gas in the mixture.

The saturated vapour pressure is the value at which the boiling point of water is lowest for a given temperature.

You don't use absolute humidity because temperature varies, which alters the behaviour of the gasses.
 
Well partial pressure - is essentially as it says, the part of the total pressure of the air (in this case) exerted by the water vapour.

Absolute humidity is OK, but with RH it also depends on the temperature of the surrounding environment (the air) and the water vapour concentration.

For a fixed quantity of water vapour the RH goes down as the temp increases as the air can 'hold' more water (laymans terms)

The reason we sweat is that the sweat evaporates from the skin taking with it a certain amount of heat and that is what cools us down. The rate at which this happens depends on the difference in water concentrations between what the air can absorb and the sweat. So if the RH is 0% then there is no water in the air and sweat will evaporate quickly cooling you down (this is what Aircon does)but if the RH is high (esp near 100%) then the sweat will have trouble evaporating so you will feel hot.
 
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