Water vapor constitutes only a small fraction of the atmosphere, ranging from as little as 1/10 of 1 percent to about 4 percent by volume. But the importance of water in the air is far greater than these small percentages would indicate. Indeed, scientists agree that water vapor is the most important gas in the atmosphere for understanding atmospheric processes.

Humidity is the amount of water vapor in air. Meteorologists employ several methods to express the water-vapor content of the air. Here we examine three: mixing ratio, relative humidity, and dew-point temperature.

Saturation

Before we consider these humidity measures further, it is important to understand the concept of saturation. Imagine a closed jar that contains water overlain by dry air, both at the same temperature. As the water begins to evaporate from the water surface, a small increase in pressure can be detected in the air above. This increase is the result of the motion of the water-vapor molecules that were added to the air through evaporation. In the open atmosphere, this pressure is termed vapor pressure and is defined as the part of the total atmospheric pressure that can be attributed to the water-vapor content.

In the closed container, as more and more molecules escape from the water surface, the steadily increasing vapor pressure in the air above forces more and more of these molecules to return to the liquid. Eventually, the number of vapor molecules returning to the surface will balance the number leaving. At that point, the air is saturated: It can hold no more water vapor. However, if we add heat to the container, which would increase the temperature of the water and air, more water will evaporate before a balance is reached. Consequently, at higher temperatures, more moisture is required to reach saturation. The amount of water vapor required for saturation at various temperatures is shown in the table below.

Mixing Ratio

Not all air is saturated, of course. Thus, we need ways to express how humid a parcel of air is. One method is to specify the amount of water vapor contained in a unit of air. The mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry air:

Because the mixing ratio is expressed in units of mass (usually in grams per kilogram), it is not affected by changes in pressure or temperature—the quantity of water vapor in the air remains the same. However, measuring the mixing ratio by direct sampling is time-consuming. Thus, meteorologists commonly use other methods to express the moisture content of the air. These include relative humidity and dew-point temperature.

Relative Humidity

The most familiar and, unfortunately, the most misunderstood term used to describe the moisture content of air is relative humidity. Relative humidity is the ratio of the air’s actual water-vapor content to the amount of water vapor required for saturation at that temperature (and pressure). Thus, unlike the mixing ratio, relative humidity indicates how near the air is to saturation, rather than the actual quantity of water vapor in the air.

To illustrate, note in the table ▲ that at 25°C (77°F) air is saturated when it contains 20 grams of water vapor per kilogram of dry air. Thus, if the air contains 10 grams of water vapor per kilogram of dry air on a 25°C (77°F) day, the relative humidity is expressed as 10/20, or 50 percent. If air with a temperature of 25°C (77°F) has a water-vapor content of 20 grams per kilogram, the relative humidity would be expressed as 20/20, or 100 percent. When the relative humidity reaches 100 percent, the air is saturated.

Because relative humidity depends both on the air’s water-vapor content and on the amount of moisture required for saturation, it can be changed in either of two ways. First, relative humidity can be changed by adding or removing water vapor. Second, because the amount of moisture required for saturation is a function of air temperature (the warmer the air, the more water is required to saturate it), relative humidity varies with temperature.

How Changes in Moisture Affect Relative Humidity

In nature, moisture is added to the air mainly via evaporation from the oceans. However, plants, soil, and smaller bodies of water also make substantial contributions.

Notice in the figure below ▼ that when water vapor is added to a parcel of air, the relative humidity of the parcel increases until saturation occurs (100 percent relative humidity). What if even more moisture is added to this parcel of saturated air? Does the relative humidity exceed 100 percent? Normally, this situation does not occur. Instead, as in the closed container described previously, the excess water vapor condenses to form liquid water.

You may have experienced such a situation while taking a hot shower. The water is composed of very energetic (hot) molecules, which means that the rate of evaporation is high. As long as you run the shower, the process of evaporation continually adds water vapor to the unsaturated air in the bathroom. If you stay in a hot shower long enough, the air eventually becomes saturated, and the excess water vapor begins to condense on the mirror, window, tile, and other cool surfaces in the room.

How Changes in Temperature Affect Relative Humidity

The second condition that affects relative humidity is air temperature. Examine the figure below ▼ carefully. Note in Figure ▼A that when air at 25°C contains 10 grams of water vapor per kilogram of air, it has a relative humidity of 50 percent. The table near the top of this page verifies that at 25°C, air is saturated when it contains 20 grams of water vapor per kilogram of air. Because the air in ▼A contains 10 grams of water vapor, its relative humidity is 10/20, or 50 percent.

When the air in the flask is cooled from 25°C to 15°C, as shown in ▲B, the relative humidity increases from 50 to 100 percent. We can conclude that when the water-vapor content remains constant, a decrease in temperature results in an increase in relative humidity.

But there is no reason to assume that cooling would cease the moment the air reached saturation. What happens when the air is cooled below the temperature at which saturation occurs? ▲C illustrates this situation. Notice from the table near the top of this page that when the flask is cooled to 5°C, the air is saturated at 5 grams of water vapor per kilogram of air. Because this flask originally contained 10 grams of water vapor, 5 grams of water vapor will condense to form liquid droplets that collect at the bottom of the container. In the meantime, the relative humidity of the air inside remains at 100 percent.

Similarly, when rising air reaches an elevation where it is cooled below its dew-point temperature, some of the water vapor condenses to form clouds. Because clouds are made of tiny liquid droplets (or ice crystals), this moisture is no longer part of the water-vapor content of the air.

We can summarize the effects of temperature on relative humidity as follows: When the water-vapor content of air remains at a constant level, a decrease in air temperature results in an increase in relative humidity, and an increase in temperature causes a decrease in relative humidity. The following figure ▼ illustrates the variations in temperature and relative humidity during a typical day and the relationship described above.

Dew-Point Temperature

The dew-point temperature, or simply the dew point, of a given parcel of air is the temperature at which water vapor begins to condense. The term dew point stems from the fact that at night, objects near the ground often cool below the dew-point temperature and become coated with dew. You have undoubtedly seen “dew” form on an ice-cold drink on a humid summer day ▼. In nature, cooling air below its dew-point temperature typically generates dew, fog, or clouds when the dew point is above freezing and frost when it is below freezing (0°C [32°F])..

Dew point can also be defined as the temperature at which a parcel of air reaches saturation and, hence, is directly related to the actual moisture content of that parcel. Recall that the saturation vapor pressure is temperature dependent. In fact, for every 10°C (18°F) increase in temperature, the amount of water vapor needed for saturation approximately doubles. Therefore, saturated air at 0°C (32°F) contains about half the water vapor of saturated air at 10°C (50°F) and roughly one-fourth that of saturated air at 20°C (68°F). Because the dew point is the temperature at which saturation occurs, we can conclude that high dew-point temperatures indicate moist air and, conversely, low dew-point temperatures indicate dry air (see the table ▼).

More precisely, based on what we have learned about vapor pressure and saturation, we can state that for every 10°C (18°F) increase in the dew-point temperature, air contains about twice as much water vapor. Therefore, we know that when air over Fort Myers, Florida, has a dew-point temperature of 25°C (77°F), it contains about twice the water vapor as the air over St. Louis, Missouri, with a dew point of 15°C (59°F) and four times that of air over Tucson, Arizona, with a dew point of 5°C (41°F).

Because the dew-point temperature is a good measure of the amount of water vapor in the air, it commonly appears on weather maps. When the dew point exceeds 65°C (18°F), most people consider the air to feel humid; air with a dew point of 75°C (24°F) or higher is considered oppressive. Notice on the map in the figure below ▼ that much of the southeastern United States has dew-point temperatures that exceed 65°C (18°F). Also notice in ▼ that although the Southeast is dominated by humid conditions, most of the remainder of the country is experiencing drier air.

Instruments called hygrometers (hygro = moisture, metron = measuring instrument) are used to measure the moisture content of the air.

Psychrometers

One of the simplest hygrometers, a psychrometer (called a sling psychrometer when connected to a handle and spun), consists of two identical thermometers mounted side by side (▼A). One thermometer, called the dry bulb, measures air temperature, and the other, called the wet bulb, is used to determine relative humidity of the air. The wet bulb has a thin cloth wick tied at the bottom that is saturated with water. A continuous current of air is passed over the wick, either by swinging the psychrometer or by using an electric fan to move air past the instrument (▼B,C). As a result, water evaporates from the wick, absorbing heat energy from the wet-bulb thermometer, which causes its temperature to drop. The amount of cooling that takes place is directly proportional to the dryness of the air: The drier the air, the greater the evaporation and the greater the cooling. Therefore, the larger the difference between the wet- and dry-bulb temperatures, the lower the relative humidity. By contrast, if the air is saturated, no evaporation will occur, and the two thermometers will have identical readings. By using a psychrometer and the tables provided in Appendix B, you can easily determine the relative humidity and the dew-point temperature.

Electric Hygrometers

Today, a variety of electric hygrometers are widely used to measure humidity. A common type of electric hygrometer works on the principle of capacitance—a material’s ability to store an electrical charge. The sensor consists of a thin hygroscopic (water-absorbent) film that is connected to an electric current. As the film absorbs or releases water, the capacitance of the sensor changes at a rate proportional to the relative humidity of the surrounding air. Thus, relative humidity can be measured by monitoring the change in the film’s capacitance. Higher capacitance means higher relative humidity.

• Humidity is the amount of water vapor in the air. The methods used to express humidity quantitatively include (1) mixing ratio, the mass of water vapor in a unit of air compared to the remaining mass of dry air; (2) relative humidity, the ratio of the air’s actual water-vapor content to the amount of water vapor required for saturation at that temperature; and (3) dew-point temperature.

• Relative humidity can be changed in two ways: by adding or subtracting water vapor or by changing the air’s temperature.

• The dew-point temperature (or simply dew point) is the temperature to which a parcel of air must be cooled to reach saturation. Unlike relative humidity, dew-point temperature is a measure of the air’s actual moisture content.

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