Water Vapor and Other Gases
Example 3.9. On a particular foggy, cold day the outdoor temperature
is -20" C. Compare the humidity outdoors with the humidity in a heated
building where air temperature is 22" C.
Solution. If there is fog, the humidity outdoors must be 1.0. In order to
find the humidity indoors the vapor pressure of the air and the saturation
vapor pressure at the indoor air temperature are needed. Assume that the
indoor and outdoor vapor pressures are equal. Neither our figure nor our
table give values at -20" C, so we use Eq. (3.8):
17.502 x (-20)
e, = 0.611 exp
= 0.125 kPa.
240.97 - 20
The saturation vapor pressure at 22" C is 2.64 kPa (Table A.3), so the
indoor humidity is h, = 0.12512.64 = 0.05. Note that the same amount
of vapor in the air gives very different humidities in the two environments.
Vapor pressure or humidity normally is measured in weather stations.
Tyl?ically such measurements are taken 1.5 to 2 m above the ground in an
open area. The humidity of microenvironments in plant canopies or near
leaves can be quite different. For example, a tiny mite living on a corn leaf
may be experiencing a relative humidity of 70 percent when the humidity
measurement at an adjacent weather station indicates 30 percent. This
can occur because the mite is small enough to be reside between the leaf
surface and the top of the boundary layer surrounding the leaf, so that
moisture from the transpiration stream leaving the stomata humidifies
the mite environment. This humidification of leaf boundary layers is also
important to transpiration because the conductance of the leaf surface
(stomata1 conductance) is influenced by the humidity in this leaf boundary
layer. Humidity can vary greatly among various microenvironments, and
we study such effects in following chapters.
References
Buck, A. L. (1981) New equations for computing vapor pressure and
enhancement factor. J. Appl. Meteorol. 20: 1527-1532.
Geiger, R. (1965) The Climate Near the Ground. Cambridge, Mass.:
Harvard University Press.
List, R. J. (1971) Smithsonian Meteorological Tables, 6th. ed. Washington, D. C.: Smithsonian Institution Press.
Problems
3.1. A psychrometer gives air temperature of 34" C and wet bulb temperature of 22" C. Find the vapor pressure, the vapor mole fraction, the
dew point temperature, the relative humidity, and the vapor deficit.
The altitude of the site is 1200 m.
Example 3.9. On a particular foggy, cold day the outdoor temperature
is -20" C. Compare the humidity outdoors with the humidity in a heated
building where air temperature is 22" C.
Solution. If there is fog, the humidity outdoors must be 1.0. In order to
find the humidity indoors the vapor pressure of the air and the saturation
vapor pressure at the indoor air temperature are needed. Assume that the
indoor and outdoor vapor pressures are equal. Neither our figure nor our
table give values at -20" C, so we use Eq. (3.8):
17.502 x (-20)
e, = 0.611 exp
= 0.125 kPa.
240.97 - 20
The saturation vapor pressure at 22" C is 2.64 kPa (Table A.3), so the
indoor humidity is h, = 0.12512.64 = 0.05. Note that the same amount
of vapor in the air gives very different humidities in the two environments.
Vapor pressure or humidity normally is measured in weather stations.
Tyl?ically such measurements are taken 1.5 to 2 m above the ground in an
open area. The humidity of microenvironments in plant canopies or near
leaves can be quite different. For example, a tiny mite living on a corn leaf
may be experiencing a relative humidity of 70 percent when the humidity
measurement at an adjacent weather station indicates 30 percent. This
can occur because the mite is small enough to be reside between the leaf
surface and the top of the boundary layer surrounding the leaf, so that
moisture from the transpiration stream leaving the stomata humidifies
the mite environment. This humidification of leaf boundary layers is also
important to transpiration because the conductance of the leaf surface
(stomata1 conductance) is influenced by the humidity in this leaf boundary
layer. Humidity can vary greatly among various microenvironments, and
we study such effects in following chapters.
References
Buck, A. L. (1981) New equations for computing vapor pressure and
enhancement factor. J. Appl. Meteorol. 20: 1527-1532.
Geiger, R. (1965) The Climate Near the Ground. Cambridge, Mass.:
Harvard University Press.
List, R. J. (1971) Smithsonian Meteorological Tables, 6th. ed. Washington, D. C.: Smithsonian Institution Press.
Problems
3.1. A psychrometer gives air temperature of 34" C and wet bulb temperature of 22" C. Find the vapor pressure, the vapor mole fraction, the
dew point temperature, the relative humidity, and the vapor deficit.
The altitude of the site is 1200 m.