Question: what is the percentage composition of water vapour in air by volume?
Answer: Gaseous water represents a small but environmentally significant constituent of the atmosphere. The percentage water vapor in surface air varies from 0.01% at -42 °C (-44 °F) to 4.24% when the dew point is 30 °C (86 °F). Approximately 99.13% of it is contained in the troposphere.