Relative humidity is a meteorological term which describes how much water vapor is present in the atmosphere. Water naturally evaporates and condenses as it gains and loses energy, and a certain amount is present in the air at all times. As the temperature changes, it can alter the relative humidity as well as trigger weather effects.
Evaporation and Condensation
When you expose a quantity of water to air, it naturally begins to lose water molecules through evaporation. At the same time, water vapor in the air condenses, re-entering a liquid phase. When these two processes are balanced, the air is at saturation point; it is holding the maximum amount of water vapor possible under the current conditions.
Temperature Effects
When the temperature increases, two things happen. The rate of evaporation increases and the amount of water the air can hold increases. Eventually, the system will reach a new saturation point reflecting the higher temperature. When the temperature falls, condensation increases relative to evaporation, the amount of vapor in the air decreases, and the system reaches another level of equilibrium.
Relative Humidity
While meteorologists can measure in absolute terms, grams of water per kilogram of air, in most cases they express humidity as a percentage. Relative humidity is the ratio of water in the air compared to the amount of water needed to reach the saturation point. If the air contains half as much water vapor as it could hold under the current conditions, the relative humidity is 50 percent.
Humidity Effects
As humidity increases in hot weather, it can make the air feel sticky and uncomfortable. Hotter temperatures exacerbate the problem of high relative humidity, as the air can hold more water the hotter it gets. In cold weather, an increase in humidity can make the air feel damp and even colder, causing condensation on exposed skin and clothing that can help transfer heat away from the body.
Condensation Point
If the amount of water present in the air remains constant but the temperature drops, it can cause water to condense out of the air. This can cause moisture on exposed surfaces, such as the dew found on grass on cold mornings. When the air temperature drops suddenly in saturated conditions, it can cause water to precipitate out of the air as fog, restricting visibility. Rain occurs when areas high in the atmosphere reach 100 percent relative humidity, forcing water to condense around dust and other particles and fall out of the sky.
References
- USA Today: FAQ on Humidity and Water in the Atmosphere
- Temperatures.com: Dewpoint Temperatures and Humidity
- University of Nebraska at Lincoln: Air Properties--Temperature and Relative Humidity
- Palomar College: How Humidity is Expressed, Jane R. Thorngren
- The Encyclopedia of Earth: Atmospheric Humidity
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