Which physical factor, related to the formation of severe weather, has to do with the amount of water vapor in the air?

humidity
pressure
temperature
wind

The correct answer is Humidity

Humidity is the amount of water vapor in the air. These places are usually difficult to be in if you're unaccustomed as the locals are because the heat because of the water vapor is dreadful. For example, Japan has high humidity which makes it extremely hot, while Egypt has lower humidity in many places and although the temperature his higher, it is more bearable than some parts of Japan.