[ian123]

Why does the air humidity increases if the temperature of liquid water is greater than the temperature of air? Please explain. Thanks.

[P]

the higher the temperature of the water the faster the molecules are moving. This means that more will come off as vapour and the humidity of the air will increase.

There maybe something about vapour pressures involved and their links to temperature, but I do not know off the top of my head, I can't remember.

[AWK]

https://www.wikiwand.com/en/Humidity
https://www.wikiwand.com/en/Vapour_pressure_of_water

[ian123]

But isn't the boiling point of water 100 degrees celsius. How, can it evaporate if the pressure is constant and the boiling point of water is 100 degrees celsius?

According to Wikipedia (Water vapour- chemical reactions) or visit https://en.wikipedia.org/wiki/Water_vapor)
'If the reactions take place at temperatures higher than the dew point of the surrounding air the water will be formed as vapor and increase the local humidity, if below the dew point local condensation will occur.'

[P]

Water doesn't need to be at 100C to evaporate though does it?

The temperature is measure of the average thermal energy of the system (don't take that as a definition) - some molecules are moving fast and some slow. The temperature is the measure of the whole system which will be a distribution of molecular speeds and energies throughout the sample. So - at room temperature even there are molecules in the water moving fast enough to escape the surface and leave the system leaving the slower ones behind (thus you get cooling during evaporation). At 100C you will see that nearly all of the molecules move fast enough/have enough energy to escape the surface and the system turns to steam.

I hope that helps - sorry, it started to get a bit wordy and I do not know if I have explained it clearly.

[Enthalpy]

Air doesn't have to be saturated with humidity, and often isn't. Very dry air would receive more humidity even if it's hotter than the water.

[jasongnome]

But isn't the boiling point of water 100 degrees celsius. How, can it evaporate if the pressure is constant and the boiling point of water is 100 degrees celsius?

Boiling and evaporation are two entirely different processes. If water needed to get to 100°C to evaporate, you'd get serious burns taking your washing off the line...