What Is Relative Humidity?

Relativity, in our understanding, depends on the value of one thing compared to another. For example, a mouse is small relative to the size of a larger animal, and a person traveling in a vehicle moves relatively faster than someone riding a horse. A similar logic applies when we try to compare the amount of moisture present in a volume (or parcel) of air on a cool day with that on a warm day. When we say that air is humid, it’s an acknowledgment that it contains moisture that we can feel; on hot, humid days, we often describe the air as feeling uncomfortable or even muggy and oppressive. Although relative humidity is the measure often used in weather reporting to convey a sense of moisture in the air, it is necessary to consider the roles of both atmospheric pressure and air temperature in order to understand what relative humidity actually represents.

Humidity, in general, is the amount of water vapor (or moisture) held by the air at a given place and time. It can be measured as absolute humidity (a straightforward representation of the density of water vapor in the air) or as specific humidity (the ratio of the water’s mass to the air’s mass within a given volume of air). However, these measures alone do not explain how humidity makes us feel.

Here’s where relative humidity may be able to assist us, because it is a representation of the amount of moisture in the air compared to the maximum amount of moisture that the air can hold at its saturation point. Relative humidity is a ratio, often expressed as a percentage, that shows us how close the air is to saturation. Fully saturated air has a relative humidity of 100 percent. If we were to add even more moisture to this volume of air, the excess water vapor would undergo a phase change and condense into water droplets, so saturation is the upper limit. In contrast, on the lower end of its range, relative humidity near Earth’s surface rarely falls below 30 percent, though humidity values of less than 1 percent do occur in some of Earth’s deserts from time to time.

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The “relative” nature of relative humidity arises from its dependence on the characteristics of a given parcel of air, and the values of two parcels of air are rarely the same. Air parcels can vary by temperature and pressure, and these factors can greatly affect relative humidity. Near sea level, where atmospheric pressure can be higher than on the side of a mountain, the weight of the atmosphere forces water vapor molecules into a smaller space, which has the effect of increasing relative humidity by building up the density of overall moisture. In addition warmer air has a greater potential to hold water vapor than cooler air, so if atmospheric pressure and the amount of water vapor in the air are constant, a warmer parcel of air would have a lower relative humidity than a cooler one. For example, a cubic meter (about 35 cubic feet) of air that has a temperature of about 30 °C (86 °F) can hold a maximum of about 30 grams (about 1 ounce) of water vapor. The same volume of air heated to only 20 °C (68 °F) holds only about 17 grams (0.6 ounce) at its saturation point.

There are a few common ways to determine relative humidity. One method uses an instrument called a sling psychrometer, which is made of a dry-bulb thermometer that measures ambient air temperature and a wet-bulb thermometer that uses a wet cloth wick to measure the cooling effect of evaporation on temperature. The difference between these two thermometer measurements is charted against the actual air temperature (the temperature shown on the dry-bulb thermometer) to determine relative humidity (see also hygrometer). You can also calculate relative humidity if you know the air’s dew point temperature—that is, the temperature at which saturation occurs without changing the air pressure or adding or subtracting water vapor—and the actual temperature of the air. Generally speaking, as the dew point temperature approaches the actual air temperature, the relative humidity increases, with saturation taking place when both temperatures are equal.

Although relative humidity can be used to help us understand how we experience wet and dry air, it’s imperfect because of the influence of temperature and pressure discussed above. To our bodies a cool day with high relative humidity may not feel uncomfortable or may even go unnoticed, whereas a hot day with similar or even lower humidity levels could feel oppressive. For that reason, dew point temperature is a better measure of how comfortable the air will feel when considering weather forecasts. The air typically begins to feel uncomfortable on days when temperatures surpass about 21 °C (roughly 70 °F) and dew points rise to match them.