Water Vapor and Other Gases
and a sink at night (when condensation and dew formation occur), and is
therefore responsible for the shape of the vapor pressure profiles. Because
the surface acts as a source or sink, and water vapor is transported in the
atmosphere, there also exist high frequency random fluctuations in vapor
pressure like those for temperature shown in Fig. 2.4.
While these patterns are easily demonstrated, the magnitude of spatial
and temporal vapor pressure variation is much smaller than for temperature, and is usually small enough that it can be ignored in comparison
with other sources of uncertainty in the measurements. If only the average vapor pressure for a day is known, the best estimate of hourly vapor
pressures is that they equal the average for the day. Variation of vapor
pressure with height can be described by an equation similar to Eq. (2. l),
so a log plot of two or more measured vapor pressures with height would
allow extrapolation or interpolation to other heights, as was done with
temperature. However, the changes in vapor pressure with height are relatively small, so vapor pressures in an organism microenvironment are
similar to the the vapor pressure at measurement height.
Not all measures of atmospheric moisture are as well behaved as vapor pressure or mole fraction, however. Figure 3.3 shows the diurnal
variation in relative humidity and vapor deficit for the temperatures in
Fig. 2.2, assuming the vapor pressure is constant throughout the day at
1 .OO kPa (Td = 7' C). Note that the humidity is near one and the vapor
deficit is near zero early in the morning. In the early afternoon the humidity is around 0.3 and the vapor deficit is 2 kPa. All of this variation is
0
6
12
18
24
Time of Day
FIGURE 3-3. Diurnal variation in relative humidity and atmospheric vapor deficit
for the temperature variation in Fig. 2.2. Vapor pressure is assumed to be constant
throughout the day at 1 .OO kPa.
and a sink at night (when condensation and dew formation occur), and is
therefore responsible for the shape of the vapor pressure profiles. Because
the surface acts as a source or sink, and water vapor is transported in the
atmosphere, there also exist high frequency random fluctuations in vapor
pressure like those for temperature shown in Fig. 2.4.
While these patterns are easily demonstrated, the magnitude of spatial
and temporal vapor pressure variation is much smaller than for temperature, and is usually small enough that it can be ignored in comparison
with other sources of uncertainty in the measurements. If only the average vapor pressure for a day is known, the best estimate of hourly vapor
pressures is that they equal the average for the day. Variation of vapor
pressure with height can be described by an equation similar to Eq. (2. l),
so a log plot of two or more measured vapor pressures with height would
allow extrapolation or interpolation to other heights, as was done with
temperature. However, the changes in vapor pressure with height are relatively small, so vapor pressures in an organism microenvironment are
similar to the the vapor pressure at measurement height.
Not all measures of atmospheric moisture are as well behaved as vapor pressure or mole fraction, however. Figure 3.3 shows the diurnal
variation in relative humidity and vapor deficit for the temperatures in
Fig. 2.2, assuming the vapor pressure is constant throughout the day at
1 .OO kPa (Td = 7' C). Note that the humidity is near one and the vapor
deficit is near zero early in the morning. In the early afternoon the humidity is around 0.3 and the vapor deficit is 2 kPa. All of this variation is
0
6
12
18
24
Time of Day
FIGURE 3-3. Diurnal variation in relative humidity and atmospheric vapor deficit
for the temperature variation in Fig. 2.2. Vapor pressure is assumed to be constant
throughout the day at 1 .OO kPa.