Question

how is water vapour added to the air ​

Answer 1

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Heat from sun causes water to evaporate and water becomes water vapour and they are in the form which we cant see and they were added to atmosphere.

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.
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Answer 2

water vapour is light in weight...

it can easily carried in air...

so that's the reason for existing of water in air....

hope it help you