Estimating the Vapor Concentration in Air
49
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 ofthe driving force for evaporation,
and is useful 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.
3.5 Estimating the Vapor Concentration in Air
Reliable measurements of atmospheric moisture are difficult to obtain,
but estimates of the vapor pressure, which are quite reliable, are relatively easy to make. In the absence of airmass changes and advection,
the vapor pressure in the air is relatively constant throughout the day and
from day to day. It also varies little between indoors and outdoors. Figure
3.3 shows the humidity going to nearly 1.0 at the time of minimum temperature. This behavior is typical of all except arid, summer conditions.
Therefore, the minimum daily temperature can often be taken as the dew
point temperature.
Example 3.8. Summer minimum temperatures at locations in the Midwestern U.S. can be around 20" C, while in the arid Southwest they are
10" C or below. Compare the vapor pressure of the two locations.
Solution. Assuming the minimum temperature equals the dew point temperature, the vapor pressure can be looked up either in Fig. 3.2 or Table
A.3. At 20" C it is 2.3 kPa and at 10" C it is 1.2 H a . That difference
makes an enormous difference in human comfort when temperatures are
in the high 30s (C).
49
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 ofthe driving force for evaporation,
and is useful 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.
3.5 Estimating the Vapor Concentration in Air
Reliable measurements of atmospheric moisture are difficult to obtain,
but estimates of the vapor pressure, which are quite reliable, are relatively easy to make. In the absence of airmass changes and advection,
the vapor pressure in the air is relatively constant throughout the day and
from day to day. It also varies little between indoors and outdoors. Figure
3.3 shows the humidity going to nearly 1.0 at the time of minimum temperature. This behavior is typical of all except arid, summer conditions.
Therefore, the minimum daily temperature can often be taken as the dew
point temperature.
Example 3.8. Summer minimum temperatures at locations in the Midwestern U.S. can be around 20" C, while in the arid Southwest they are
10" C or below. Compare the vapor pressure of the two locations.
Solution. Assuming the minimum temperature equals the dew point temperature, the vapor pressure can be looked up either in Fig. 3.2 or Table
A.3. At 20" C it is 2.3 kPa and at 10" C it is 1.2 H a . That difference
makes an enormous difference in human comfort when temperatures are
in the high 30s (C).