7.2: Water Vapor in the Atmosphere

In the troposphere, air masses move continuously, both vertically and horizontally, and these movements control the Earth’s weather and climate and the winds that create ocean waves and ocean surface currents. The movements of air masses are caused primarily by changes in air mass density that occur as water vapor is added or removed.

Water Vapor and Air Density

Water vapor in the atmosphere behaves in much the same way as sugar in water. Sugar dissolves in water, but the maximum quantity of sugar that can be dissolved is limited. More sugar can be dissolved at higher temperatures. Similarly, water vapor can be “dissolved” in air in limited quantities, and the amount of water vapor that can be present in air at saturation is greater at higher temperatures. The maximum amount of water that can remain in the vapor phase in air is expressed by the water vapor saturation pressure. The variation of water vapor saturation pressure with temperature is shown in Figure 7-4. The saturation pressure of water vapor at average ocean surface temperatures (approximately 20°C) is many times greater than the saturation pressure of water vapor in air near the top of the troposphere (approximately –40°C to –60°C).

Figure 7-4. Variation of the water vapor saturation pressure with temperature in air. The water vapor saturation pressure is lower at lower temperatures. This is the reason that water condenses into rain as warm, moist air rises and cools.

When water evaporates from the sea surface, water molecules displace (move aside) oxygen and nitrogen molecules in the atmosphere. The displaced molecules are mixed and distributed in the surrounding air mass by random motions of the gas molecules. This is important because the atmospheric pressure and, therefore, density of the air mass would be increased if this were not the case. In contrast, water molecules (H₂O, molecular mass 18) are lighter than either the nitrogen (N₂, molecular mass 28) or the oxygen (O₂, molecular mass 32) molecules that they displace. Hence, light molecules displace heavier molecules, and the density of air is, therefore, reduced when water vapor is added (provided there is no change in pressure). As a result, moist air is less dense than dry air at the same temperature and pressure. Therefore, an air mass to which water vapor has been added tends to rise until it reaches its equilibrium density level in the density-stratified atmosphere (CC1). Conversely, if water vapor is removed, air increases in density and tends to sink.

Water vapor is continuously added to the atmosphere by evaporation of water from the ocean surface and removed by condensation and precipitation. The rate of evaporation varies with location, as explained later in this chapter. Where evaporation of ocean surface water is particularly rapid, the density of the air mass at the ocean surface is reduced, and the air tends to rise. In many parts of the oceans where the surface water is warmer than the overlying air, conduction and radiation of heat from the ocean surface also reduce the density of the air immediately above it.

Water Vapor, Convection, and Condensation 

If a gas expands without any external source of heat, its temperature decreases in a process called adiabatic expansion. As a result, when air rises in the atmosphere, the pressure decreases, and the air is cooled by adiabatic expansion. Because water vapor saturation pressure is highly temperature-dependent, cooling the air also results in a decrease in the water vapor saturation pressure. When the air has risen sufficiently that the actual vapor pressure becomes greater than the saturation vapor pressure, the air is supersaturated. In this condition, excess water vapor condenses into liquid water droplets (if above freezing). Because water molecules are typically too far apart to condense spontaneously, condensation in the atmosphere almost always occurs on cloud condensation nuclei (CCN). 

In many cases, the water will condense to form large numbers of extremely small water droplets—droplets that are too small to fall out of the sky (CC4) or to combine with each other to form larger droplets. These water droplets form the clouds of the atmosphere. When cloud droplets are in the right environment for significant growth, precipitation can eventually form.

Water vapor contributes to vertical movements of and within air parcels, not only because of its low molecular weight, but also because of water’s latent heat of vaporization (Chap. 5). When warm, moist air rises through the atmosphere and cools, and water vapor condenses, the water’s latent heat of vaporization is released. The heat warms the air mass and thus reduces its density, causing the air parcels to rise. Release of latent heat is the major driving force for the spectacular rising plumes of moist air that form cumulonimbus (thunderstorm) clouds, which can reach the upper parts of the troposphere. Latent heat released to the atmosphere is also the main energy source for hurricanes and other storms.

Atmospheric convection cells are created by evaporation or warming at the sea surface that decreases the density of the air mass and causes it to rise. The rising air mass cools by adiabatic expansion, and eventually water vapor is lost by condensation. As the water vapor condenses, temperature is increased by the release of latent heat and the air tends to rise farther. Adiabatic expansion and radiative heat loss then cool the air parcel and increase its density so that it eventually sinks (CC3). The location, size, and intensity of convection cells determine the location and intensity of rainfall and snowfall, as well as the distribution of atmospheric heat energy. Convection cells exist at a range of spatial scales. We will discuss how global-scale cells form and how they influence climate and weather at different latitudes in the next several sections of this chapter.