Steam.
A five letter word which means to convert into vapor is steam. It is the process of heating or boiling water and turning it into vapor.
sublimate
Steam
vapor, steam
humidity
Humidity means the amount of water vapor in the air. It begins with the letter H.
steam, vapor
SublimateImprovedActually, sublimate is to change from a solid to a gas while skipping the liquid state. However, the 9 letter word changing WATER into vapor is evaporate.
Fog
touchdown
water vapor or vapor
Vapor