What is Relative Humidity?
Relative humidity is a measure of the amount of moisture (held invisibly as water vapour) that a volume of air is presently holding relative to the full amount that it is capable of holding. When a volume of air holds as much water vapour as it can (that is to say it is “saturated”) then it has a relative humidity of 100%. When it holds only half of this amount then the relative humidity (often abbreviated to RH) is 50%.
The warmer the air, the greater the amount of moisture that the air can hold as vapour. It follows therefore that on a day when the temperature is 20 degrees Celsius a 100% humidity figure will represent a lower amount of moisture in the air than will be present for a 100%RH figure on a day where the temperature is 30 degrees Celsius. This also implies therefore that if the temperature increases then the relative humidity will decrease (as the amount of moisture that represents 100% will increase with the temperature). This of course assumes no change to the absolute humidity (the amount of moisture in the air in terms of grams of water per Kg of air)
It is worth noting that in reality the water is not in any sense dissolved in the air. The water vapour exists within the gases that make up the air but are not dissolved in them.
How is Relative Humidity Measured?
Relative humidity is most commonly measured either manually using a whirling hygrometer or automatically using either automated wet and dry air temperature sensors or using an electronic humidity sensor.
Wet and dry air temperature sensors are exactly what their names imply. The dry air temperature sensor is the one used to report air temperature for any standard weather report. The wet air temperature has a thin gauze wick wrapped over the temperature sensing element. The temperature sensor (with one end of the wick wrapped around it) is exposed in the air but the other end of the wick runs down into a bottle of distilled water. This draws water up from the reservoir of distilled water, it then evaporates from the surface of the air temperature sensor and causes a drop in the measured temperature (due to the heat lost in evaporating the water from the wick).
A set of dry and wet air temperature sensors from Munro Instruments. The wet air sensor is on the right hand side (the wick descending into the bottle of water can be clearly seen).
Unless the relative humidity is at 100% the wet air temperature sensor will read less than the dry air temperature sensor because of the evaporative cooling of the water in the wick. If the RH=100% then the water in the wick cannot evaporate (as the air around it can hold no more water vapour) and the wet air temperature = dry air temperature. The difference between the wet and dry can be used to calculate (or look up in a table) the humidity.
Most electronic sensors work by measuring the capacitance of a volume of air, the capacitance will vary with the amount of moisture in the air.
An Oregon Scientific wireless relative humidity sensor for the home.
Why is Relative Humidity of Interest?
To us as humans relative humidity has a lot to do with how hot or cold we feel. The human body naturally cools by the evaporation of moisture from the surface of the skin. Heat from our bodies is used to evaporated this moisture and through this loss of generated heat we feel cooler.
When the air is more humid the rate of evaporation is slowed and so we feel warmer. Conversely if the humidity is low then the rate of evaporation is faster and the cooling effect that much more efficient. This is why people often talk about a dry heat being more bearable than a humid one, because it is easier to cool in low humidity conditions.
The relationship between the amount of water vapour in air and the air temperature is not a linear one, at cooler air temperatures the amount of water vapour that can be held increases relatively slowly but at higher temperatures the rate of increase is far higher. For example, in very rounded figures, at 5 degrees Celsius, 100% RH represents around 5g of water per Kg of air) at 10 degrees this is around 9g. At 40 degrees Celsius the water amount is around 50g per Kg of air but at 45 degrees Celsius it is around 70g. This is why in tropical areas the high heat and humidity can feel so oppressive when people are not accustomed to it.
When the RH reaches 100% then the air can carry no more water vapour and any excess moisture now forms water droplets resulting in condensation. Condensation can ruin some essential work operations such as industrial painting and the laying of road surfaces. For these and many other activities it is important to monitor the relative humidity and the dew point in order to choose a suitable time for the activity such that condensation can be avoided.
How Can You Measure Relative Humidity at Home?
There are now many inexpensive weather stations available for the home which can measure relative humidity or a simple whirling hygrometer will do the job easily and well.