Spatial and Temporal Variation of Atmospheric Water Vapor

3.4 Spatial and Temporal Variation of Atmospheric Water Vapor

The spatial and temporal patterns of vapor pressure in the atmosphere resemble those given in Ch. 2 for temperature. During the

vapor pressures are highest near a soil or plant surface and decrease with height. At night, vapor pressures tend to be lowest near the surface and increase with height. Vapor pressures tend to be a bit higher in the day than at night

and typically reach a minimum at the time temperature is at the minimum.

As with temperature, the surface acts as a source of water vapor in the day

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 temper- ature, and is usually small enough that it can be ignored in comparison with other sources of uncertainty in the measurements. If only the aver- age 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.

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 rel- atively small, so vapor pressures in an organism

are similar to the the vapor pressure at measurement height. Not all measures of atmospheric moisture are as well behaved as va- por 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 = C). Note that the humidity is near one and the vapor deficit is near zero early in the morning. In the early afternoon the hu- midity is around 0.3 and the vapor deficit is 2

All of this variation is

Time of Day

F IG U RE 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 .OO

Estimating the Vapor Concentration in Air

brought about by the change in temperature, with no change in the vapor concentration in the air.

It is perhaps unfortunate that one of the most common measurements of atmospheric moisture is relative humidity. The measurement itself is essentially useless as an environmental variable except as a means, along

with air temperature, of obtaining the vapor pressure, mole fraction, or dew point temperature. Some people compute and report averages of

humidity over time periods of a day or longer. It should be clear from Fig. 3.3 that an average humidity is worse than meaningless. In addition to failing to communicate any useful information by itself, averaging individual humidity measurements destroys any possibility of obtaining useful information from the original data because the average humidity depends on the pattern of temperature variation (which is lost in the averaging process). It is best to immediately convert humidity data to vapor pressure or dew point. Then record, average, and process these data.

Averaging the vapor deficit is a slightly different matter. We show later that the vapor deficit gives an estimate

driving force for evaporation, and is

in relating transpiration and biomass production in plant communities. The average vapor deficit for the atmosphere is therefore

a useful number, but it can be estimated reliably from average vapor pressure and temperature.