what is the percentage of water vapour in air?
The water-vapour content of the atmosphere varies from place to place and from time to time because the humidity capacity of air is determined by temperature. At 30 °C (86 °F), for example, a volume of air can contain up to 4 percent water vapour. At -40 °C (-40 °F), however, it can hold no more than 0.2 percent.