Section 4.4:
Relative Humidity &
Dew-Point Temperature

Learning Objective

List and describe the ways relative humidity changes in nature. Compare relative humidity to dew-point temperature.

Section Glossary

The most familiar and, unfortunately, the most misunderstood term used to describe the moisture content of air is relative humidity. Relative humidity is a ratio of the air’s actual water vapor content compared with the amount of water vapor required for saturation at that temperature (and pressure). Thus, relative humidity measures how near the air is to saturation rather than the actual quantity of water vapor in the air (Box 4.1). Relative humidity can be expressed as follows:

Box 4.1

Dry Air at 100 Percent Relative Humidity?

A common misconception is the notion that air with a higher relative humidity has greater water-vapor content than air with a lower relative humidity. This is not always the case (Figure 4.A). Compare a typical January day in Chicago, Illinois, to one in the desert of Death Valley, California. On this hypothetical day, the temperature in Chicago is a cold −10°C (14°F), and the relative humidity is 100 percent. By referring to Table 4.1, we can see that saturated −10°C air has a water-vapor content (mixing ratio) of 2 grams per kilogram (g/kg). By contrast, the desert air at Death Valley on this January day is a warm 25°C (77°F), and the relative humidity is just 20 percent. A look at Table 4.1 reveals that 25°C air has a saturation mixing ratio of 20 g/kg. Therefore, with a relative humidity of 20 percent, the air at Death Valley has a water-vapor content of 4 g/kg (20 grams × 20 percent). Consequently, the “dry” air in Death Valley actually contains twice the water vapor as the air in Chicago, with a relative humidity of 100 percent.

Figure 4.A

Moisture content of frigid air versus hot air. Frigid air with a high relative humidity (A) generally has a lower water-vapor content than hot desert air with a low relative humidity (B).

This example illustrates that very cold places are also very dry. The low water-vapor content of frigid air (even when saturated) helps explain why many arctic areas receive only meager amounts of precipitation and are referred to as “polar deserts.” This also helps us understand why people frequently experience dry skin and chapped lips during the winter months. The water-vapor content of cold air is low, even compared to some hot, arid regions.

How Relative Humidity Changes

Because relative humidity is based on the air’s water-vapor content, as well as the amount of moisture required for saturation, it can change in one of two ways. First, relative humidity changes when water vapor is added to or removed from the atmosphere. Second, because the amount of moisture required for saturation is a function of air temperature, relative humidity varies with temperature.

How Changes in Moisture Affect Relative Humidity

Notice in Figure 4.9 that when water vapor is added to air through evaporation, the relative humidity of the air 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? In the lower atmosphere, this situation is rare. Instead, the excess water vapor condenses to form liquid water. (Note that supersaturation—RH above 100 percent—often occurs in the middle and upper troposphere, and is addressed in Chapter 5.)

Figure 4.9
At a constant temperature (in this example, it is 25°C), the relative humidity will increase as water vapor is added to the air

The saturation mixing ratio for air at 25°C is 20 g/kg (see Table 4.1). As the water-vapor content in the flask increases, the relative humidity rises from 25 percent in A to 100 percent in C.

You may have experienced saturation conditions 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 that hot water runs for enough time, the air eventually becomes saturated, which makes the air foggy.

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. Unlike with your shower, however, the natural processes that add water vapor to the air generally do not operate at rates fast enough to cause saturation to occur directly. One exception is when you exhale on a cold winter day and “see your breath”: The warm, moist air from your lungs mixes with the cold outside air. Your breath has enough moisture to saturate a small quantity of cold outside air, producing a miniature “cloud.” Almost as fast as the “cloud” forms, it mixes with the surrounding dry air and evaporates.

How Relative Humidity Changes with Temperature

The second condition that affects relative humidity is air temperature. Examine Figure 4.10A carefully, and note that when air at 25°C contains 10 grams of water vapor per kilogram, it has a relative humidity of 50 percent. When the flask in Figure 4.10A is cooled from 25° to 15°C, as shown in Figure 4.10B, 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. In the shower example, the bathroom gets foggy when the air is saturated, but the mirror becomes foggy more quickly than the air in the bathroom. This is because the mirror is cooler than the moist air in the room and cools the adjacent air sufficiently to cause condensation directly on the mirror.

Figure 4.10
Relative humidity varies with temperature

When the water-vapor content (mixing ratio) remains constant, the relative humidity will change when the air temperature either increases or decreases. In this example, when the temperature of the air in the flask was lowered from 25°C in A to 15°C in B, the relative humidity increased from 50 to 100 percent. Further cooling from 15°C in B to 5°C in C causes one-half of the water vapor to condense. In nature, when saturated air cools, it causes condensation in the form of clouds, dew, or fog.

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? Figure 4.10C illustrates this situation. Notice from Table 4.1 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 on the walls of the container. In the meantime, the relative humidity of the air inside remains at 100 percent. This illustrates an important concept: When air aloft is cooled below its saturation level, some of the water vapor condenses to form clouds. Since clouds are made of liquid droplets (or ice crystals), this moisture is no longer part of the water-vapor content of the air.

Conversely, an increase in temperature results in a decrease in relative humidity. For example, assume that the flask in Figure 4.10A containing 10 grams of water vapor is heated from 25°C to 40°C. Table 4.1 indicates that at 40°C, saturation occurs at 47 grams of water vapor per kilogram of air. Consequently, when the air is heated from 25° to 40°C, the relative humidity will drop from 10/20 (or 50 percent) to 10/47 (or about 21 percent).

Natural Changes in Relative Humidity

In nature there are three major ways that air temperatures change (over relatively short time spans) to cause corresponding changes in relative humidity:

The effect of the first of these three processes (daily changes) is shown in Figure 4.11. Notice that during midafternoon, relative humidity reaches its lowest level, whereas the cooler evening hours are associated with higher relative humidity. In this example, the actual water-vapor content (mixing ratio) of the air remains unchanged; only the relative humidity varies. We will consider the other two processes in more detail in later chapters.

Figure 4.11
Typical daily variation in temperature and relative humidity during a spring day in Washington, DC

Dew-Point Temperature

The dew-point temperature, or simply the dew point, is the temperature at which water vapor begins to condense. The term dew point stems from the fact that during nighttime hours, 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 (Figure 4.12). Near Earth’s surface, when the air is cooled below its dew-point temperature it generates dew or fog—if the dew point is above freezing. By contrast, when the dew-point temperature is below freezing (0°C, 32°F) frost occurs.

Figure 4.12
Condensation and dew-point temperature

Condensation, or “dew,” occurs when a cold drinking glass chills the surrounding layer of air below the dew-point temperature.

You might have wondered . . . 

Why do my lips get chapped in the winter?

During the winter months, outside air is comparatively cool and dry. When this air is drawn into a home, it is heated, which causes the relative humidity to plunge. Unless your home is equipped with a humidifier, you are likely to experience chapped lips and dry skin at that time of year.

Dew point can also be defined as the temperature at which air reaches saturation and, hence, is directly related to the actual moisture content of a parcel of air. Recall that the saturation vapor pressure is temperature dependent and that for every 10°C (18°F) increase in temperature, the amount of water vapor needed for saturation doubles. Therefore, cold saturated air (0°C [32°F]) contains about half the water vapor of saturated air having a temperature of 10°C (50°F) and roughly one-fourth that of saturated air with a temperature of 20°C (68°F). Because the dew point is the temperature at which saturation occurs, we can conclude that high dew-point temperatures equate to moist air and, conversely, low dew-point temperatures indicate dry air (Table 4.2). 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 the dew-point temperature is 25°C (77°F), air contains about twice the water vapor as when the dew point is 15°C (59°F) and four times that of air with a dew point of 5°C (41°F).

Table 4.2
Dew-Point Thresholds

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°F (18°C), most people consider the air to feel humid; air with a dew point of 75°F (24°C) or higher is considered oppressive. Notice on the map in Figure 4.13 that much of the southeastern United States has dew-point temperatures that exceed 65°F (18°C). Also notice in Figure 4.13 that although the Southeast is dominated by humid conditions, most of the remainder of the country is experiencing comparatively drier air.

Figure 4.13
Surface map showing dew-point temperatures for a typical September day

Dew-point temperatures above 60°F dominate the southeastern United States, indicating that this region is blanketed with humid air.

Tutorial Video - Dewpoint (Click to watch the video)

How Is Humidity Measured?

Instruments called hygrometers are used to measure the moisture content of the air. In addition to being used in meteorology, hygrometers are used in greenhouses, humidors, museums, and numerous industrial settings that are sensitive to humidity, such as paint booths where protective coatings are applied to products. Because it is difficult to directly measure absolute humidity and the mixing ratio, most hygrometers measure either relative humidity or dew-point temperature. Once either of these is known, it is relatively easy to convert to any of the other humidity measurements as long as we know the temperature.


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 (Figure 4.14A). One thermometer, called the dry bulb, measures air temperature, and the other, called the wet bulb, has a thin cloth wick tied at the bottom. This cloth wick is saturated with water, and 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 (Figure 4.14B,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 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 C, you can easily determine the relative humidity and the dew-point temperature.

Figure 4.14
Sling psychrometer

A. A sling psychrometer consists of one dry-bulb thermometer and one wet-bulb thermometer.
B. The dry-bulb thermometer measures the current air temperature. The wet-bulb thermometer is covered with a cloth wick dipped in water.
C. As the instrument is spun, evaporation cooling causes the temperature of the wet-bulb thermometer to decrease. The amount of cooling that occurs is directly proportional to the dryness of the air. The temperature difference between the dry- and wet-bulb thermometers is used in conjunction with the tables in Appendix C to determine relative humidity and dew-point temperature.

Hair Hygrometers

One of the oldest instruments used for measuring relative humidity, called a hair hygrometer, operates on the principle that hair changes length in proportion to changes in relative humidity. Hair lengthens as relative humidity increases and shrinks as relative humidity drops. People with naturally curly hair experience this phenomenon: In humid weather their hair lengthens and hence becomes curlier. A hair hygrometer uses a bundle of hairs linked mechanically to an indicator that is calibrated between 0 and 100 percent. However, these instruments have become largely obsolete as more accurate tools have been developed.

Electric Hygrometers

Today, a variety of electric hygrometers are widely used to measure humidity. One type of electric hygrometer uses a chilled mirror and a mechanism that detects the temperature at which condensation begins to form on the mirror. Thus, a chilled mirror hygrometer measures the dew-point temperature of the air.

The Automated Weather Observing System (AWOS) operated by the National Weather Service (NWS) employs an electric hygrometer that 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 equates to higher relative humidity.

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

How is relative humidity different from absolute humidity and the mixing ratio?

Unlike mixing ratio and absolute humidity, relative humidity does not state the actual amount of water vapor present in the air. Rather, it is expressed as a percent and reveals how close the air is to being saturated.

Refer to the figure to the right and describe the relationship between the daily cycle of temperature and the cycle of relative humidity if the dew-point temperature is constant.

If the amount of water vapor in the air remains constant, a rise in temperature will cause the relative humidity to drop and a drop in temperature will cause the relative humidity to rise.

Which measure of humidity, relative humidity or dew point, best describes the actual quantity of water vapor in a mass of air?

Dew point. High dew points imply very moist air and low dew points correlate with dry air.