Humidity is an important aspect of the atmosphere because it affects both weather and climate. The amount of water vapour present in the atmosphere is referred to as humidity. Humidity of the atmosphere is high when it has large quantities of water vapour. The amount of water vapour in the atmosphere is called absolute humidity. The ratio between the amount of water vapour in the atmosphere and the amount of water vapour it can hold is relative humidity Hot air can hold more water vapour then cold air. Relative humidity increases when air gets cold and decreases when air gets heated up.