This is the ratio between the actual water vapour content of the atmosphere and the maximum water vapour content possible at that given temperature. If the temperature of a given parcel of air rises, the amount of moisture it can hold increases and as such its relative humidity decreases. If there is no change in temperature but the moisture content decreases then the relative humidity will again decrease because the ratio of actual water vapour present is less than the maximum amount the air could hold. Conversely, if the water content does not change but the temperature falls, the relative humidity increases until saturation and possible precipitation occurs.