What Is Relative Humidity Relative to Anyway?

Article excerpt

Byline: Terry Carroll, Erin Martin, and Jon Carroll

When describing the amount of moisture present in the air, the term relative humidity is frequently not understood. The general term humidity is defined as "a measure of the water vapor content of the air" (Branick 2003). Absolute humidity is more specific and is referred to as "the ratio of the mass of water vapor to a unit volume occupied by a mixture of water vapor and dry air" (NSIDC 2004). In order to grasp the concept of relative humidity, it helps to ask the question, "Relative to what?" Relative humidity is defined as "a ratio, expressed as a percent, of the amount of water vapor present relative to the amount that would be present if the air were saturated (the point at which air cannot hold more water vapor at a given temperature and pressure)" (Branick 2003).

The ability of air to hold moisture is directly affected by air temperature. As temperature increases, so does the air's capacity to hold moisture. A simple demonstration using water and a few beakers can quickly clear up any lack of understanding students may have concerning relative humidity.

The materials needed to do this demonstration include at least five different-sized beakers, preferably with each beaker marked in even increments (not every beaker is marked in the same increments, but each individual beaker in and of itself has even increments on it), a large container filled with water, and temperature labels for each beaker. For example, you may want to use a 100 mL beaker labeled 40[degrees]F, a 250 mL beaker labeled 50[degrees]F, a 400 mL beaker labeled 60[degrees]F, a 600 mL beaker labeled 70[degrees]F, and a 1000 mL beaker labeled 80[degrees]F.

Each beaker represents the maximum volume of water a unit volume of air can hold at each specific temperature. In order to avoid misconceptions, make sure you point out to students that the beakers are not proportional and do not represent the actual amount of water a given volume of air would hold at that temperature.

Suppose you pour 50 mL of water into the 100 mL beaker (representing the amount of moisture the unit volume of air can hold at 40[degrees]F). How full is the beaker? The beaker is holding half the amount of water it is capable of holding, so it is 50% full. This 50% is referred to as the relative humidity of the air at that temperature.

Now, suppose you pour the 50 mL of water from the 100 mL beaker (air at 40[degrees]F) into the 250 mL beaker (air at 50[degrees]F). What is the relative humidity of the air at 50[degrees]F? While the 250 mL beaker is holding the same amount of water (50 mL) that was in the 100 mL beaker, the 250 mL (air at 50[degrees]F) beaker is now only 20% full, so this would represent a relative humidity of 20% for the air at 50[degrees]F.

In addition to the change in temperature (as demonstrated above), relative humidity is also affected by changes in the amount of water vapor added to a volume or parcel of air due to evaporation (a phase change from a liquid state to a vapor state). The evaporation of water from oceans, lakes, plants, animals, and the ground due to higher temperatures can cause a significant increase in the amount of moisture in the air.

In order to tie the concept of evaporation into the demonstration above, suppose the temperature is increasing and, at the same time, additional water is evaporating into the parcel (beaker) of air. How could we demonstrate that? As you are pouring the initial amount of water into increasingly larger beakers (representing an increase in temperature and therefore, a decrease in relative humidity), pour additional water into each beaker to represent the moisture increase in the air due to evaporation. Continuing with our example above, the air's temperature is now at 50[degrees]F (250 mL beaker) and the relative humidity is at 20%. If evaporation causes an additional 100 mL of water to be added to the 50[degrees]F air (add 100 mL of water to the 50 mL of water already in the 250 mL beaker for a total of 150 mL of water), this would cause the relative humidity to increase to 60% at that temperature (50[degrees]F). …

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