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Chapter XIII. Water in the Local Air
WATER I N THE LOCAL AIR
vapor concentration. The profile of humidity decreases upward in an
exponential decay curve log eh = log e , - hi6.5, in which e represents
vapor pressure at sea level (subscript 0) and at any height h (in
kilometers) (Hann, 1897, p . 279). The removal mechanisms operate
throughout the troposphere but with decreasing effect a t the higher
altitudes where they find less vapor to condense on the average.
Atmospheric vapor concentration is greatest in the local air that bathes
the ecosystems on the earth's surface.
Table I shows values of vapor pressure (in millibars) at typical wet
surfaces in nature, and the sea-level specific humidity of moist air in
contact with these surfaces (as grams of vapor per kilogram of air). The
dryness of cold high-latitude surfaces and of the air above them form
one extreme. The other extreme is found in the high humidities of
equatorial oceans or rainforest and the air in contact with them, in
which vapor concentration is more than two orders of magnitude
greater than in the high latitudes, as the surfaces approach the 32-34°C
temperature limit discussed in Chapter X.
Diurnal Regimes The diurnal regime of vapor pressure in the air
near the evaporating surfaces depicts the response of a stor'ige
function in the local air, which buffers moisture input from evapotranspiration during dayIight hours to a varying outflow, the upward
mixing of vapor. Over many land surfaces nocturnal inversions that
V a p o r Pressure a n d Specific H u m i d i t y a t Selected Surfaces"
temperature pressure Specific humidity
Snow surface of ice cap
Mid-latitude snow field i n winter
Mid-latitude water or vegetation
Low-latitude ocean or forest
Under intense radiation
' Source: Smithsonian Tables (1966).
WATER VAPOR I N THE LOCAL A I R
persist into the forenoon hours limit upward mixing and a maximum
in the daily vapor pressure cycle occurs in the late morning.
This often is followed by a minimum at the time when upward
mixing of vapor out of the local air is most vigorous. A second
maximum may follow, when turbulent mixing diminishes in the
evening. The primary minimum, at night, indicates the lack of input
from the surface. Sometimes, in fact, a net downward flow of vapor
takes place out of the air to the surface, and the vapor pressure in the
local air declines still more.
The vapor concentration in the local air over water surfaces, which
generate a fairly constant upward flow of vapor, exhibits a different
diurnal regime. This regime is primarily influenced by the variations
in upward mixing of vapor out of the local air into the free atmosphere. The maximum vapor pressure therefore comes at the time of
weakest vertical mixing, near sunrise, and the lowest at the time of
Annual Regime The dependence of atmospheric vapor pressure on
the rate of evaporation from the underlying surface influences its
variation through the year. Over Lake Ontario, for example, it varies
from 4.5 in late winter to 20 mb in late summer. The effect of the
underlying surface is further demonstrated by the fact that during the
months of active evaporation from the lake, the atmospheric vapor
pressure over i t is substantiallv greater than over the land adjacent
(Richards and Fortin, 1962). The ratio of l a k e - b l a n d values shown
over the annual cycle in Fig. XIII-1 is largest-about
Fig. XIII-I. Annual regime of the ratio between vapor pressure over Lake Ontario to
that over adjacent land surfaces (Richards and Fortin, 1962).
W A T E R IN T H E LOCAL A I R
ter, when evaporation is weak from the frozen land but active from the
lake. The ratio during May and June, on the other hand, indicates the
coldness of the lake surface relative to the surrounding lands; vapor
pressure is lower over the lake than over the land.
Annual regimes of vapor pressure at four places in Australia are
given in Table 11. Between winter and summer at Sydney, vapor
pressure doubles, a response more to the change in evaporation from
land than to the smaller change in sea-surface temperature and
evaporation. The author’s experience with Sydney humidity in February and March, however, confirms the high vapor pressures then,
which are associated with the continued warmth of the near-shore
waters. Days of easterly (on-shore) winds are especially sultry.
The change between winter and summer at Darwin is large, 13 mb.
The whole annual cycle is found at a higher level than at the other
stations; this indicates the warmth of the Timor Sea. The large change
through the year indicates the effect of a small change in a high seasurface temperature as well as the occurrence of dry air in winter (June
and July) moving from the interior of the continent. In the Northern
Hemisphere continents, such movement of dry air also occurs in
winter, but it is much drier than in Australia.
The World Pattern
The air over the cold snow surfaces of the northern continents in
winter is particularly dry. Over great areas, vapor pressure is less than
2 mb. These are source regions of very dry air, which moves south and
dominates many winter days in the midwestern United States.
The contrasting surfaces of moist warm areas of the equatorial
latitudes average higher than 25 m b in vapor pressure. Latitudinal
means of vapor pressure over the land are shown in Table 111, which
V a p o r Pressure in Australiaa
18 21 25 28
11 11 13 15
10 10 11 13
Units: mb. Source: Commonwealth of Australia Yearbook (1965).
WATER V A P O R I N THE LOCAL AIR
V a p o r Pressure
the Air near the Surface o f
'' Units: mb. Source: Kessler (1968, p . 14).
shows that the zone of high humidity extends from 20s to 1ON
Figure XIII-2, the world pattern in July (Landsberg, 1964), displays
large areas where vapor pressure exceeds 30 mb. The Caribbean Sea,
the Gulf of Mexico, and their coastal lands are well known sources of
Average vapor pressure (mb) in July (Landsberg, 1964)
WATER I N THE LOCAL AIR
humid air streams. Days of sweltering humidity in central North
America occur during southerly airflow, which on its traverse across
the evaporating fields and forests of the lower Mississippi Valley gains
even more moisture.
It is significant that, when we look for the sources of the rain and
snow that fall on central and eastern North America, we see over the
western Pacific Ocean a vapor pressure that lies between 12 and 15 mb
during most of the year. Over the Arctic Ocean we see a vapor
pressure lower than 10 mb. In contrast, over the Gulf of Mexico we see
a vapor pressure of 20 mb in winter and 30 nib in summer. Which
body of water is most likely to provide moisture for the central plains
of the continent?
Humidity As a Component of the Environment
Vapor content is affected by the source of the airstream, but this
quality changes as the air moves over drier or moister land. Characteristics of the boundary layer and even more of the local air reflect
conditions at the directly underlying surface. Movement of the local
air is braked over rough ecosystem canopies and its warmth, C 0 2 , and
water-vapor content are modified by the fluxes of sensible heat, CO.,,
and vapor from the ecosystems that make up its porous lower
Over an ecosystem surface having "infinite" extent, as was hyyothesized in the definition of potential evapotranspiration in an earlier
chapter, the partnership between ecosystem and local air tends to be
dominated by the ecosystem. The rhythms of warmth, moisture, and
so on, in the local air are forced by the rhythms of the corresponding
fluxes at the underlying surface.
In reality, this hypothetical situation is rare. Most terrestrial ecosystems are limited in size, often covering an area of only a few hectares,
100 m or so across. The air that fills their foliage volumes is still
dominated by exchanges of water and energy at the leaf surfaces, as
evidenced by the vertical profiles of humidity or temperature within a
forest stand; but above these systems the main body of the local air
moves on across the countryside, floating above a mosaic of many
contrasting ecosystems and carrying, for example, the moisture it
acquires from one system over a neighboring system.
For this reason, the expressions for evapotranspiration discussed
previously include terms for atmospheric humidity. The Bowen ratio
can be approximated by measuring the differences in temperature and
moisture between an ecosystem and the overlying air. The gradient of
WATER V A P O R I N THE LOCAL AIR
moisture is important in the evaporation process and depends on the
concentration of vapor within the storage zone represented by the
One effect of high specific humidity is, by reducing transpiration, to
slow the flow of nutrients brought into a plant in the stream of water
moving from soil to leaves. This decrease in nutrient intake might well
account for the stunted growth of trees in perennially cloudy, foggy
zones of some mountains (Odum, 1971, p. 376).
The local air, not being precisely defined as to thickness, forms a
reservoir of indefinite size for water vapor. We can obtain a rough
idea of its storage capacity from the following: Assume it to be 1 km
deep; it then contains 1300 kg of air in a column of 1-mz crosssectional area. Over an ice cap, at a specific humidity of 0.1 g kg-' (see
Table I), this column contains about 130 g of vapor m-' of surface.
Over an equatorial forest, at a specific humidity of 20 g kg-' (Table I
again), it contains 26 kg of vapor. Obviously, the storage of water in
this zone can vary tremendously. Suppose we take a typical midlatitude instance of 10 kg of vapor stored m-' of area. This value, 10 kg
. of the same general size as the mass of water delivered to the
surface in a day of moderate rain (10 kg m-' day-'), or the amount
evaporated from a corn field in two or three days of hot summer
Conditions of high moisture concentration slow down the further
transfer of water from the ecosystems into the air, as quantitatively
shown in the various evaporation formulas. Dry air, on the other
hand, accepts vapor from below avidly, and brings about high
evaporation rates best exemplified where desert air invades an oasis.
The cool air we feel as we drive from the desert onto a road between
irrigated alfalfa fields is a consequence of the fact that available energy
at the alfalfa system is channeled almost entirely into evaporation,
leaving little or none to warm the air. The moisture stored or not
stored in the local air thus is an important factor in the environment of
an ecosystem, affecting its rate of evaporation and indirectly its
openness to assimilate CO, from the air.
Gentilli (1955) investigated the generally accepted correspondence of
dew-point temperature (a common measure of vapor pressure or
specific humidity) with the minimum air temperature reached during
the preceding night (a common forecast variable). The correspondence
in the Plains States and eastward is good, especially in seasons when
nights are warmer than about -10°C. It demonstrates the important
role played by water in the atmosphere. Water vapor effectively
transfers atmospheric energy to the ground by radiation, and main-
W A T E R I N THE LOCAL A I R
tains ecosystems in an equable environment during the stress period
of the night. This energetic bond from vapor in the local air to
moisture at the surface means that vapor concentration influences
snow melting, freeze-thaw cycles, evaporation, and dew formation.
This income of 30-50 W m-’ in long-wave flux density is particularly
important in the small energy budgets of night hours, when the
warmth it brings accelerates plant respiration and reduces net photosynthetic productivity of ecosystems, and adds to human heat stress in
Humidity in the Environment of M a n Moisture in the local air also
affects man when i t reaches high values. The isarithm of 20 mb in Fig.
XIII-2 is of interest because this value is generally accepted a s an index
of sultriness in the human environment. In Asia this line takes in most
of China and Japan in July; it covers the eastern Mediterranean and its
southern shores. In North America it takes in the eastern part of the
continent to a latitude of 40N. The zonal averages of Table I11 show
that the 20-mb area stretches from 15s to 30N latitude and encompasses a major fraction of the world.
Air conditioning, the American term for artificial cooling and drying
of summer air, has become one of the major consumers of electric
power in this country. A rough indication of the desire for it is given
by a combined index* of air temperature and dew point. Dew point i s
uniquely related to vapor pressure and is weighted about a third as
heavily as air temperature in the formu1ation.t Table IV shows the
frequency distribution of the units of this index at Baltimore in
midsummer, when evapotranspiration in eastern North America i s
most active. At index values above 75 units about half the people are
uncomfortable. These values occur 0.82 of the time in the afternoon
and even at night Q.09 of the time. At index values above 80 units
nearly everyone is uncomfortable; such values occur 0.50 of the
afternoon hours at Baltimore, in the open air. In the city, and
especially in buildings, the frequency of occurrence is larger.
The physiological effects of moisture in the atmosphere are not well
understood, beyond the obvious fact that high vapor pressure suppresses evaporative cooling of the heat-stressed human body. One
panel (Sargent et al., l967) also notes that ”there is need to know the
* At first called the ”discomfort index” (Thorn, 1956), this was later euphemized to
t A similar index, called “effective temperature” (Landsberg, 1969, p. 54) is ‘tffectetl
about the same by an increase of 6 m b in vapor pressure alone (= 4 g kg ‘ increase in
specific humidity) as by a 6°C rise in air temperature alone.
V I S I B L E FORMS OF WATER I N THE L O C A L AIR
Frequency Distribution of the Discomfort index (or
Temperature-Humidity Index) at Baltimore in July"
'' Units: percent of occurrences. Source: Thorn
chronic effects of exposure to very low humidity, for man now spends
so much of his life in artificial atmospheres that may be exceedingly
dry." As midwesterners know, heating polar air at a vapor pressure of
only 2 4 mb without humidifying it results in very low relative
humidity in the home and severe respiratory problems.
VISIBLE FORMS OF WATER IN THE LOCAL AIR
Most atmospheric water is in the vapor state, but sometimes a
myriad of tiny droplets or ice crystals also are present. Although these
total only a small mass of water substance, they are conspicuous in the
landscape and form an important part of the environment of ecosystems at the surface.
Amounts and Significance
A typical figure for the liquid-water content of a cloud is 0.5 g m-'3.
This means only 0.5 g of water per 1300 g of air, or, in terms of specific
humidity, only 0.4 g kg-l. Compare this figure with ordinary vaporcontent values of 10 g kg-' in the middle latitudes (Table I), or more
than 20 g kg-l in the low latitudes. Yet clouds and fog are prominent
features of the environment of ecosystems.
WATER I N THE LOCAL AIR
The significance of the small amounts of condensed water in clouds
or fogs stems from the finely divided state. Each tiny crystal or droplet
of the size of a raindrop) refracts and reflects light. A cloud of
droplets reduces vision through the atmosphere to almost zero, cuts
off the direct beam of the sun, and becomes a source of diffused shortwave radiation and of long-wave radiation to the ecosystems below. A
cloud deck passing across the sky causes a rapid change in the flow of
radiant energy to an ecosystem. The total input of energy changes,
usually decreasing, and its spectral composition shifts to favor both
the chemically and biologically effective blue wavelengths and far
infrared wavelengths that have chiefly a heating effect. At the same
time, the change from a direct beam to a diffused source for the
shortwave radiation brings about deeper penetration of light into the
ecosystem, benefiting photosynthesis. Water droplets and ice particles
in the local air thus have important effects on the functioning of
ecosystems, affecting their water balances.
One mode of formation of visible water particles in the local air
results from condensation in place of vapor already present in the air.
On nights of weak vertical mixing, radiation cooling is concentrated in
a shallow layer of air, and if initial humidity were high, some of the
vapor in this layer would condense as radiation fog. Radiative cooling
of the top surface of the fog now goes on more rapidly after droplets
have formed, and supports further condensation of vapor, strengthening the development of the fog.
Radiation fog has many hours in which to develop during winter
nights. It continues well into the next morning, perhaps even lasting
through the day if it is thick enough to prevent solar heating of the
Cold air that has drained into topographic depressions is often the
medium in which vapor easily condenses into fog. Inversions above
the cold air reduce upward mixing of the droplets and confine vapor,
cold air, and fog droplets with their associated pollutants within the
basins (see Plate 24). Ecosystems in such sites are subjected to
prolonged attacks by such pollutants as sulfur dioxide, often in the
form of droplets of dilute sulfuric acid. They are experiencing ”one of
the most common causes of the accumulation of pollutants to obnoxious concentrations for long periods” (Scorer, 1968, p. 31) that human
In periods of weak general movement in the atmosphere, such fog
V I S I B L E F O R M S OF W A T E R I N T H E L O C A L A I R
Plate 24. Patches of radiation fog in Morioka, northern Japan, in winter. Some
downslope movement also is taking place (December 1966).
tends to perpetuate itself because it screens the wet, cold soil from
solar heating. The author recalls a winter of frequent radiation fog at
airports on the Colorado Plateau, a region in which fog is seldom
forecast. Early winter rains saturated the soil and were followed by a
long period of stagnant air circulation, in which water repeatedly
circulated between the wet soil and the foggy local air. Closed airports
disrupted the short-hop air operations of the day (1943-1944), trapping
almost all of one airline’s planes at Albuquerque. The lifting of the fog
was forecast correctly after the high level of soil moisture was taken
The tectonic valley of the Rhine between the Vosges and the Black
Forest often fills with cold, damp air in periods of anticyclonic
stagnation. One such period in 1972 and 1973 lasted for 32 days
continuously (Weischet, 1973). The sun can barely be discerned,
visibility at ground level is low, and heavy frost forms on trees.
Meanwhile, above the fog clear skies reign. Skiers go up into the Black
Forest, only 0.5 km higher in altitude, and enjoy solar warmth, an air
temperature around 5°C higher, and a distant view of the Alps. Indeed
an abrupt transition to be caused by a thimbleful of water! The
contrast increases as pollutants continue to accumulate in the valley
WATER I N THE LOCAL AIR
Plate 25. Visible-channel scanning by DMSP satellite 7529 R showing radiation fog
filling the lowlands of Puget Sound, the Willamette Valley, and the Columbia Basin on
18 December 1975. Two thousand holiday travelers were immobilized for several days
at the Seattle-Tacoma airport alone, and a like number waited at other airports to get
into Seattle. Meanwhile, AMTRAK trains arrived on schedule.
The humid conditions here can be expected to worsen still more
after new power-plant cooling towers are built and further humidify
the hapless valley’s air. The wet plumes from these gigantic evaporating devices* represent an impact on the local air that needs study in
many parts of the industrial world.
The valleys of the western US under winter anticyclones often fill
with long-lived bodies of radiation fog. Plate 25 is a satellite photograph taken at a time of anticyclonic dominance in December 1975.
The upper, visible surfaces of the fog bodies are uniformly white but
the ecosystems in the valleys-and
people, to+--live in a gray,
windless, clammy world of weak light and slowed biological activity.
The amount of liquid water that shows so clearly in these photographs from space is very small. If we take the fog bodies as being 200
m deep and having a liquid-water content of 0.5 g mP3, we compute a
* The source strength of such a system approaches
lo5 tons of
vapor per day