In this discussion, we are going to explore humidity and how it is measured. The amount of water vapor in the air is measured by its humidity. It can be measured in two ways, specific humidity and relative humidity.

Specific humidity is the actual weight (measured in grains) of water in one pound of dry air. Relative humidity is the amount of water vapor in the air compared to the maximum amount of water vapor the air can hold at that temperature. Once the water vapor surpasses the maximum amount that can be held at a given temperature, it will condense back into a liquid form. A good example of this concept is a sweating cold drink in a glass. The warm air comes in contact with the cool surface of the glass and condenses the water vapor into liquid form.

In general terms, the warmer the air, the more water vapor it can hold and conversely the cooler the air, the less water vapor it can hold before it condenses into liquid form. Taking it a step further, if air holds a specific amount of water vapor, lowering its temperature raises the relative humidity and raising the temperature lowers its relative humidity. In central Florida because of the hot temperatures, our air can hold much more water vapor than places in northerly climates. As a result our air conditioning and heat pump (in the cooling mode) systems need to be effective at dehumidifying and cooling the air to achieve indoor comfort. In the next installment we’ll discuss the relationship between relative humidity and temperature and how it impacts indoor air comfort. Please give us a call if you have any questions on humidity or how it is measured.