The property which primarily controls how much water can be present as a gas is:

The temperature of the air is the property which primarily controls how much water can be present as a gas. 

Meteorology is the study of the atmosphere. It primarily focuses on weather which is defined as the condition of the atmosphere at a certain time and place. It changes every day depending on earth location. There are five factors that affect weather namely, temperature, pressure, humidity, wind and precipitation.

The question above primarily talks about humidity-the amount of water vapor in the air. As the  temperature increases, the air or gas can hold more water. The warmer the air is, the more water it can possibly hold as vapor.