Section 4.3:
Humidity: Water Vapor in the Air

Learning Objective

Explain the relationship between air temperature and the amount of water vapor needed to saturate air.

Section Content

Humidity is the general term used to describe the amount of water vapor in the air (Figure 4.5). Water vapor constitutes only a small fraction of the atmosphere, varying from as little as 0.1 percent up to about 4 percent by volume. But the importance of water in the air is far greater than these small percentages indicate. In fact, water vapor is the primary source of energy (latent heat) for the formation of weather systems—thunderstorms, tornadoes, and hurricanes.

Figure 4.5
Meteorologists use several methods to express the water-vapor content of air

Mini-Lecture Video - Humidity: Water Vapor in the Air (Click to watch the video)

How Is Humidity Expressed?

Meteorologists employ several methods to express the water vapor content of the air, including (1) absolute humidity, (2) mixing ratio, (3) vapor pressure, (4) relative humidity, and (5) dew point. Two of these methods, absolute humidity and mixing ratio, are similar in that both are expressed as the quantity of water vapor contained in a specific amount of air.

Absolute Humidity

The mass of water vapor in a given volume of air (usually as grams per cubic meter) is known as absolute humidity:

As air moves from one place to another, changes in pressure and temperature cause changes in its volume. When volume changes, the absolute humidity also changes, even if no water vapor is added or removed. Consequently, it is difficult to monitor the water vapor content of a moving mass of air when using the absolute humidity index. Therefore, meteorologists generally prefer to use mixing ratio to express the water vapor content of air.

Mixing Ratio

The mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry air:

Because it is measured in units of mass (usually grams per kilogram), the mixing ratio is not affected by changes in pressure or temperature (Figure 4.6).

Figure 4.6
Comparison of absolute humidity and mixing ratio for a rising parcel of air

Notice that the mixing ratio is not affected by changes in pressure as the parcel of air rises and expands.

Vapor Pressure and Saturation

We can determine the moisture content of the air from the pressure exerted by water vapor. To understand how water vapor exerts pressure, imagine a closed flask containing pure water and overlain by dry air, as shown in Figure 4.7A. Almost immediately some of the water molecules begin to leave the water surface and evaporate into the dry air above. The addition of water vapor into the air can be detected by a small increase in pressure (Figure 4.7B). This increase in pressure is a result of the motion of the water vapor molecules that were added to the air through evaporation. This pressure, called vapor pressure, is defined as that part of the total atmospheric pressure attributable to its water vapor content.

Figure 4.7
The relationship between vapor pressure and saturation

A. Initial conditions—dry air at 20°C with no observable vapor pressure.
B. Evaporation generates measurable vapor pressure.
C. As more and more molecules escape from the water surface, the steadily increasing vapor pressure forces an increasing number of molecules to return to the liquid. Eventually, the number of water-vapor molecules returning to the surface will balance the number leaving—at which point the air is said to be saturated.
D. When the container is heated from 20°C to 30°C, the rate of evaporation increases, causing the vapor pressure to increase until a new balance is reached.

Initially, many more molecules will leave the water surface (evaporate) than will return (condense). However, as more and more molecules evaporate from the water surface, the steadily increasing vapor pressure in the air above forces more and more water molecules to return to the liquid. Eventually a balance is reached in which the number of water molecules returning to the surface equals the number leaving. At that point, the air is said to have reached an equilibrium called saturation (Figure 4.7C). When air is saturated, the pressure exerted by the motion of the water vapor molecules is called the saturation vapor pressure.

If the water in the closed container is heated, the equilibrium between evaporation and condensation will be disrupted, as illustrated in Figure 4.7D. The added energy increases the rate of evaporation, which causes the vapor pressure in the air above to increase, until a new equilibrium is reached. Thus, we can conclude that the saturation vapor pressure is temperature dependent, such that at higher temperatures it takes more water vapor to saturate air (Figure 4.8).

Figure 4.8
Graph illustrating the amount of water vapor required to saturate 1 kilogram of dry air at various temperatures

For example, the red arrows show that saturated air at 20°C contains 14 grams of water vapor per kilogram of dry air.

The amount of water vapor required to saturate 1 kilogram (2.2 pounds) of dry air at various temperatures is shown in Table 4.1. Note that for every 10°C (18°F) increase in temperature, the amount of water vapor needed for saturation almost doubles. Thus, roughly four times more water vapor is needed to saturate 30°C (86°F) air than 10°C (50°F) air.

Table 4.1
Saturation Mixing Ratio (at Sea-Level Pressure)

The atmosphere behaves in much the same manner as our closed container. In nature, gravity, rather than a lid, prevents water vapor (and other gases) from escaping into space. Also as with our container, water molecules are constantly evaporating from liquid surfaces (such as lakes or oceans), and other water vapor molecules are condensing (into cloud droplets or dew). However, in nature, a balance is not always achieved. More often than not, more water molecules are leaving the surface of a water puddle than are arriving. By contrast, during the formation of fog, more water molecules are condensing than are evaporating from the tiny fog droplets.

What determines whether the rate of evaporation exceeds the rate of condensation or vice versa? One major factor is the temperature of the water, which in turn determines how much motion (kinetic energy) the water molecules possess. At higher temperatures, water molecules have more energy and can more readily escape.

Vapor pressure is the other major factor that determines whether evaporation or condensation is the dominant process. Recall from our closed container example that vapor pressure influences the rate at which the water molecules leave (evaporate) and also the rate at which they return to the surface (condense). When the air is dry (low vapor pressure), the rate at which water molecules escape from a liquid surface is high. As the vapor pressure increases, the rate at which water vapor returns to the liquid phase increases as well.

You might have wondered . . . 

Why do snow piles seem to shrink a few days after a snowfall, even when the temperatures remain below freezing?

On clear, cold days following a snowfall, the air can be very dry. This fact, plus solar heating, causes the ice crystals to sublimate—turn from a solid to a gas. Thus, even without any appreciable melting, these accumulations of snow gradually get smaller.

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 do absolute humidity and mixing ratio differ? What do they have in common?

Mixing ratio and absolute humidity both indicate the actual amount of water vapor present in the air. However, mixing ratio is stated as the mass of water vapor per unit mass of dry air, while absolute humidity expresses the mass of water vapor per unit volume of air.

Define vapor pressure, and describe the relationship between vapor pressure and saturation. (Hint: See the figure to the right)

Vapor pressure is the part of the total atmospheric pressure attributable to its water vapor content. Vapor pressure reaches its maximum at saturation, when equilibrium is reached between evaporation and condensation.

After reviewing the table to the right, summarize the relationship between air temperature and the amount of water vapor needed to saturate air.

As temperature increases, the rate of evaporation—and the vapor pressure—increases until a new equilibrium is reached between evaporation and condensation (saturation). The greater the temperature, the greater the amount of water vapor needed for saturation.