The ratio of the amount of moisture or water vapour (vapour pressure) in the air (the humidity) to the total moisture that the air could hold at that temperature, i.e. Saturated with water vapour (saturation vapour pressure), is the relative humidity. It is usually expressed as a percentage and abbreviated to rh. Air saturated with water vapour has a relative humidity of 100% and when only half saturated the rh is 50%. In UK experience, an rh of 69 to 70% is satisfactory for cold smoking.