What is Humidity?

The question should actually be - What is Relative Humidity, Absolute Humidity and Dew Point?

How Humidity is Measured

Humidity can be measured in several ways: either by relative humidity, absolute humidity or dew point. Below we try to explain the different ways of measuring humidity.

what is ABSOLUTE HUMIDITY

Relative humidity is the most common measurement. However, in order to understand relative humidity, it is helpful to first understand absolute humidity. Absolute humidity is the water content of air. In other words, the mass of water vapor divided by the mass of dry air in a volume of air.  It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C.  The hotter the air is, the more water it can contain. Absolute humidity is measured in g/kg. If you take an average spring day at 22°C and 55% relative humidity (RH) you would have an absolute humidity of 9 g/kg.

What is relative humidity?

Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum for that temperature (which depends on the current air temperature). A reading of 100% relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain in the atmosphere and condensation in a factory or storage warehouse.

Further, when the moisture content remains constant and temperature increases, relative humidity decreases. Example, if we have air at 22°C and 55% RH, we would have an absolute humidity level of 9 g/kg. Warmer air can hold more humidity. So, if we increase the temperature to 30°C but keep the absolute humidity the same (9 g/kg) then RH would now be 34%. Similarly, if we decrease the temperature from 22°C to 12 °C and kept the absolute humidity the same we would now have a relative humidity of 100%. 

DEW POINT

The dew point is the saturation temperature for water in air. The dew point is associated with relative humidity. A high relative humidity implies that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. For example, at 22°C and 100% RH the air will have a dew point of 22°C also. Likewise, to take our previous example, air at 22°C and 55% RH has a dew point of 12°C. So if cooled this air down, 12°C is the temperature at which moisture will be released from the air and condense on surrounding surfaces. 

WHAT MEASUREMENT SHOULD I USE?

Different measurements are used for different applications. Relative humidity is the most common option and is typically used for most applications.

• If you have changing temperatures, absolute humidity may be more suitable. This measurement is used for spray drying and blower applications.

• Dew point is typically used when trying to prevent condensation or ice from forming on a cold surface which is operating in a warmer area. 

Talk to us about your application today and we will be able to discuss the right measurement with you.