InTeGrate Modules and Courses >Water Science and Society > Student Materials > Section 1: Fresh Water: Scarcity or Surfeit? > Module 2: Climatology of Water > Relative Humidity
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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
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Initial Publication Date: March 31, 2017

Relative Humidity

The explanation for spatial variations in precipitation centers on the concept of relative humidity. Relative humidity describes the amount of water vapor actually in the air, relative to the maximum amount of water the air can possibly hold. It is expressed as a percentage:

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If the relative humidity (RH) is 100%, this means that condensation would occur. On a typical hot muggy summer day, RH might be around 60-80%. In a desert, RH is commonly around 15-25%. What makes things interesting is that air can hold more water vapor when it is warm, and less when it is cool, as shown below.

One important consequence is that when air masses change in temperature, the relative humidity can change, even if the actual amount of water vapor in the air does not (the numerator in our equation, which is defined by the saturation curve, stays the same, but the denominator changes with temperature). Figures 11-13 below show an example of this process. As the air cools, the relative humidity increases. If the air mass were cooled enough to become saturated, condensation would occur. This temperature is called the dew point.

In the same way, changes in relative humidity occur when warm moist air is forced to rise or, conversely, when cool dry air descends. For example, when an air mass moves over mountains, it cools as it rises, and when it reaches the dewpoint, water will condense. This forms clouds, and if the air mass cools enough, the condensation becomes rapid enough to form precipitation.


These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »