What Is Relative Humidity and How Does it Affect How I Feel Outside?


A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain. Jose Luis Raota/Getty Images

If you've ever been to southern Arkansas on a hot July day, you're familiar with incredible mugginess, a borderline hallucinatory experience where you feel like you're walking a stew of your own perspiration. But there's more to understanding that clammy, gross feeling than simply glancing at a weather forecaster's humidity reading. To really get a grasp on how humidity affects your health, home, and your sanity, you need insights on the types of humidity, as well as the concept of dew point.

Humidity can be measured in several ways, but relative humidity (RH) is the most common. In order to understand RH, it is helpful to first understand absolute humidity.

Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Absolute humidity is expressed as grams of moisture per cubic meter of air (g/m3).

Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain. This doesn't mean that the relative humidity must be 100 percent in order for it to rain — it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less [source: University of Illinois].

Humans are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature. If the air is at 100 percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. Your shirt may become saturated with perspiration that doesn't go anywhere, leaving you feeling like a swampy bog monster of revolting proportions.

If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling us off. For example, if the air temperature is 75 degrees Fahrenheit (24 degrees Celsius) and the relative humidity is zero percent, the air temperature feels like 69 degrees Fahrenheit (21 C) to our bodies. If the air temperature is 75 degrees Fahrenheit (24 C) and the relative humidity is 100 percent, we feel like it's 80 degrees (27 C) out, and you start praying that you had the air conditioner serviced last fall.

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