The equilibrium pressure of water vapor above a flat surface of liquid water in a chamber such as shown in Figure 5.1 is called the saturation vapor pressure. It is independent of the shape of the volume in the cylinder (since it only depends on the number density ns of the vapor). The saturation vapor pressure (usually denoted es) is however a very strong function of temperature T. This is intuitively reasonable since an increase in temperature will increase the proportion of liquid molecules having velocities above the threshold to depart from the surface. More departures will require more arrival rates to maintain equilibrium. This in turn will require a larger number density which is proportional to the vapor pressure. Note that the flux of molecules moving down perpendicularly is 4nsv (see Chapter 2).
Figure 5.3 shows a graph of the saturation vapor pressure of water over a flat liquid surface versus the temperature in degrees Celsius. Many aspects of weather and climate depend on this very rapid increase with temperature. As a rough but es(hPa) 80 70 60 50 40 30 20 10
Figure 5.3 Saturation vapor pressure for water over a flat liquid surface versus temperature in degrees Celsius.
useful rule of thumb, the saturation vapor pressure doubles for every 10 °C increase in temperature (at least in the range of interest for atmospheric science). Even so, at moderate temperatures the saturation vapor pressure is very small compared to atmospheric pressure near the surface (usually 5 to 30 hPa compared to 1000 hPa).
Does the presence of dry air affect the saturation vapor pressure of water? Perhaps the added pressure of the air on the liquid surface squeezes more water molecules into the vapor phase. But on the contrary, some air dissolves in the liquid and thereby might hinder the flux of molecules out of the liquid surface. Both effects are present but together their impact is less than 1% of the saturation vapor pressure.
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