Especially with the advent of summer months, the phrase "the humidity is very high" begins to be frequently heard, and people complain as much about humidity as they do of heat. This is particularly a common occurrence in coastal regions. So, "What is humidity?"; and why do we say "The heat is fine, but the humidity is awful".


In short, humidity is the term used for water vapor suspended in air. There are 3 separate humidity measurements in HVAC industry(or terminology) which denote different concepts. These are:

  • Absolute humidity
  • Specific humidity
  • Relative humidity

Absolute humidity denotes the mass of water vapor per unit volume of air. Measured with the aid of devices called hygrometers, absolute humidity is a value which changes with climate and geography. Therefore it decreases as one moves from the equator to the poles, and from coastal regions to inland and highland areas.

Specific humdity denotes the ratio of the mass of water vapor in the air to the total mass of air. In other words, it is a simple mixture ratio. It is generally used to indicate how many grams of water vapor is contained by 1 kg of air. In the psychrometric chart shown below, the figures on the left indicate specific humidity.

Relative humidity is a ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that the air could hold at a temperature. 100% relative humidity means that the air is fully saturated with water and that it cannot hold any more water vapor. That is, any more water vapor added to the air condenses into liquid form or is suspended in the air in the form of fog. Based on this, it can also be said that the odds of rainfall is higher with high relative humidity. The value that is sometimes expressed as humidity in weather forecasts is in fact relative humidity. In contrast with specific and absolute humidity, we people can sense relative humidity, which directly influences our comfort.

In the psychrometric chart above, the figures on the bottom represent dry bulb temperature, while the figures on the left represent specific humidity, and the blue lines relative humidity. As can be easily seen from the chart, keeping specific humidity, i.e. the mass of water vapor in the air constant, relative humidity drops as temperature rises. This is because, as mentioned above, the amount of vapor that can be held by air increases with increasing temperature.


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