What is ‘Relative Humidity’?

Relative Humidity, or ‘RH’, measures the amount of water in the air in vapor form, comparing it to the maximum amount of water that can be held at a given temperature. For example, if the RH is quoted as being 50% at 73°F, this would imply that the air contains 50% of the maximum level of water vapor it could hold at 73°F. 100% RH indicates that the air is at maximum saturation. When humid air comes into contact with cooler air, or a cooler surface, the water vapor will turn into water droplets. When this occurs on a surface it is referred to as the ‘Dew Point’. Never apply paint above a maximum relative humidity of around 85% as at that level you will reach the dew point regardless of surrounding temperatures. Relative Humidity can be measured using a hygrometer; a hygro thermometer will measure both temperature and humidity.