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:

RH=H2OactualH2OmaxThis equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers.

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.

×

Figure 10. When an air mass contains the maximum amount of water it can hold, it is saturated with water vapor. This is shown graphically in the plot above. With increasing temperature, the air can hold more water vapor, as indicated by higher saturation values. In general, it is not possible to have water contents that exceed saturation (in other words, the maximum relative humidity is 100%). Another way to think about relative humidity is that it describes how close the air is to saturation. In the example shown, the actual water vapor content is about 40% of that at saturation (i.e. the blue point is about 40% of the way to saturation) – meaning the RH = 40%.
Source: Michael Arthur and Demian Saffer

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.

×

Figure 11.
Source: Mike Arthur and Demian Saffer

×

Figure 12.
Source: Mike Arthur and Demian Saffer

×

Figure 13
Source: Mike Arthur and Demian Saffer

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.

• Formative Assessment 2: Relative Humidity and Dewpoint
• The Orographic Effect
• Atmospheric Convection: Hadley Cells
• Energy Balance
• Global Wind