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C H A P T E R 6 • M O I S T U R E , C O N D E N S AT I O N , A N D P R E C I P I TAT I O N
Moist air
Condensation
Condensation
Precipitation
Precipitation
Soil
moisture
Evaporation from rivers,
soils, vegetation, lakes,
and falling precipitation
Evaporation
from ocean
Seepage
Ground water
(fresh)
Interface
● FIGURE
Salt water
6.4
Environmental Systems: The Hydrologic System The hydrologic system is concerned with the circulation
of water from one part of the Earth system to another. The subsystem of the hydrologic system illustrated in this
diagram is referred to as the hydrologic cycle. Largely through condensation, precipitation, and evaporation, water
is cycled endlessly between the atmosphere, the soil, subsurface storage, lakes and streams, plants and animals,
glacial ice, and the principal reservoir—the oceans.
Is this a closed system or an open system? Why?
The hydrologic cycle is one of the most important subsystems of the larger Earth system. It is linked to numerous other
subsystems that rely on water as an agent of movement. For
example, it plays a major role in the redistribution of energy over
Earth’s surface. ( ● Fig. 6.4) provides a schematic illustration of the
circulation of water in the hydrologic cycle.
Water in the Atmosphere
The Water Budget and Its Relation
to the Heat Budget
We are most familiar with and most often take notice of water in its liquid form, as it pours from a tap or as it exists in
fine droplets within clouds or fog. Water also exists as a tasteless, odorless, transparent gas known as water vapor, which
is mixed with the other gases of the atmosphere in varying
proportions. Water vapor is found within approximately the
first 5500 meters (18,000 ft) of the troposphere and makes
up a small but highly variable percentage of the atmosphere
by volume. Atmospheric water is the source of all condensation and precipitation. Through these processes and through
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evaporation, water plays a significant role as Earth’s temperature regulator and modifier. In addition, as we noted in
Chapter 4, water vapor in the atmosphere absorbs and reflects a significant portion of both incoming solar energy and
outgoing Earth radiation. By preventing great losses of heat
from Earth’s surface, water vapor helps maintain the moderate
range of temperature found on this planet.
As we stated previously, Earth’s hydrosphere is a closed system; that is, water is neither received from outside the Earth
system nor given off from it. Thus, an increase in water within
one subsystem must be accounted for by a loss in another. Put
another way, we say that the Earth system operates with a water
budget, in which the total quantity of water remains the same
and in which the deficits must balance the gains throughout
the entire system.
We know that the atmosphere gives up a great deal of water,
most obviously by condensation into clouds, fog, and dew and
through several forms of precipitation (rain, snow, hail, sleet). If
the quantity of water in the atmosphere remains at the same level
through time, the atmosphere must be absorbing from other parts
of the system an amount of water equal to that which it is giving
up. During 1 minute, over 1 billion tons of water are given up by
the atmosphere through some form of precipitation or condensation, while another billion tons are evaporated and absorbed as
water vapor by the atmosphere.
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W AT E R I N T H E AT M O S P H E R E
50
40
Grams per cubic meter
If we look again at our discussion of the heat energy budget
in Chapter 4, we can see that a part of that budget is the latent
heat of condensation. Of course, this energy is originally derived
from the sun. The sun’s energy is used in evaporation and is then
stored in the molecules of water vapor, to be released only during
condensation. Although the heat transfers involved in evaporation
and condensation within the total heat energy budget are proportionately small, the actual energy is significant. Imagine the
amount of energy released every minute when a billion tons of
water condense out of the atmosphere. It is this vast storehouse
of energy, the latent heat of condensation, that provides the major source of power for Earth’s storms: hurricanes, tornadoes, and
thunderstorms.
There are limits to the amount of water vapor that can be
held by any parcel of air. A very important determinant of the
amount of water vapor that can be held by the air is temperature.
The warmer air is, the greater the quantity of water vapor it can
hold. Therefore, we can make a generalization that air in the polar regions can hold far less water vapor (approximately 0.2% by
volume) than the hot air of the tropics and equatorial regions of
Earth, where the air can contain as much as 5% by volume.
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Saturation and Dew Point
When air of a given temperature holds all the water vapor that it
possibly can, it is said to be in a state of saturation and to have
reached its capacity. If a constant temperature is maintained in
a quantity of air, there will come a point, as more water vapor
is added, when the air will be saturated and unable to hold any
more water vapor. For example, when you take a shower, the air
in the room becomes increasingly humid until a point is reached
at which the air cannot contain more water. Then, excess water
vapor condenses onto the colder mirrors and walls.
We know that the capacity of air to hold water vapor varies
with temperature. In fact, as we can see in ( ● Fig. 6.5), this capacity of air to contain moisture increases with rising temperatures.
Some examples will help illustrate the relationship between temperature and water vapor capacity. If we assume that a parcel of air
at 30°C is saturated, then it will contain 30 grams of water vapor
in each cubic meter of air (30 g/m3). Now suppose the temperature of the air increases to 40°C without increasing the water vapor content. The parcel is no longer saturated because air at 40°C
can hold more than 30 grams per cubic meter of water vapor
(actually, 50 g/m3). Conversely, if we decrease the temperature of
saturated air from 30°C (which contains 30 g/m3 of water vapor)
to 20°C (which has a water vapor capacity of only 17 g/m3), some
13 grams of the water vapor will condense out of the air because
of the reduced capacity.
It is also evident that if an unsaturated parcel of air is cooled,
it will eventually reach a temperature at which the air will become
saturated. This critical temperature is known as the dew point—
the temperature at which condensation takes place. For example,
we know that if a parcel of air at 30°C contains 20 grams per
cubic meter of water vapor, it is not saturated because it can hold
30 grams per cubic meter. However, if we cool that parcel of air
to 21°C, it would become saturated because the capacity of air at
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● FIGURE
0
Temperature (°C)
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6.5
The graph shows the maximum amount of water vapor that can be contained in a cubic meter of air over a wide range of temperatures.
Compare the change in capacity when the air temperature is raised
from 0°C to 10°C with a change from 20°C to 30°C. What does this
indicate about the relationship between temperature and capacity?
21°C is 20 grams per cubic meter. Thus, that parcel of air at 30°C
has a dew point of 21°C. It is the cooling of air to below its dew
point temperature that brings about the condensation that must precede any precipitation.
Because the capacity of air to hold water vapor increases with
rising temperatures, air in the equatorial regions has a higher dew
point than air in the polar regions. Thus, because the atmosphere
can hold more water in the equatorial regions, there is greater
potential for large quantities of precipitation than in the polar regions. Likewise, in the middle latitudes, summer months, because
of their higher temperatures, have more potential for large-scale
precipitation than do winter months.
Humidity
The amount of water vapor in the air at any one time and place
is called humidity. There are three common ways to express the
humidity content of the air. Each method provides information
that contributes to our discussion of weather and climate.
Absolute and Specific Humidity Absolute humidity
is the measure of the mass of water vapor that exists within a given
volume of air. It is expressed either in the metric system as the
number of grams per cubic meter (g/m3) or in the English system
as grains per cubic foot (gr/ft3). Specific humidity is the mass of
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G EO G R A P H Y ’ S S PAT I A L SC I E N C E P E R S P EC T I V E
The Wettest and Driest Places on Earth
T
Though some frontal activity brings
about winter rainfall, most of it comes in
the form of orographic precipitation. The
trade winds flow mainly out of the east,
but any direction of the winds will waft
the moisture-laden ocean air up the sides
of this mountain. The precipitation falls
mainly in the form of light rain, and drizzle.
The area around this mountain peak is
perennially covered in fog and mist. The
rainfall is rarely heavy, but it is consistent.
The small amounts of rain each day accumulate through the year to reach these
huge totals.
The driest spot on Earth is located in
northern Chile, South America. Two locations can claim some fame in this discussion. Arica, Chile, claims the lowest average annual precipitation at 0.5 millimeters
(considered a trace) of precipitation. However the location shown in the image below is located between Arica and the city
of Iquique, Chile. This valley nested in the
Andes mountain range has not received
rain according to anyone’s living memory.
No data has been collected, because as
far as anyone knows, it has never rained
there. No doubt, if it ever rains at this location, it will be recorded and likely to be
celebrated as well! This location contains
no vegetation and is likely not to contain
many living microbes either.
Northern Chile is dominated by two
mechanisms that keep it rain free. The
first is the dominance of a subtropical high
pressure system. This semipermanent anticyclone with its stabilizing force remains
in close proximity to this location for long
periods of time. Second, this valley is on
the leeward (rain shadow) side of the
Andes Mountains. Thus, with both of these
processes working together, this location
simply does not receive precipitation.
Although both the wettest and driest
locations are unique in their own ways, by
applying our knowledge of pressure systems, winds, and precipitation processes
(Chapters 5 and 6) it is not difficult to understand how these geographical distinctions can exist.
M. Trapasso
M. Trapasso
hough there may be disagreement
as to which exact locations on Earth
hold the distinction of being the wettest and driest, there are two geographical
locations that lay claim to these titles.
Mount Waialeale (Hawaiian: for rippling
waters) stands at 1569 meters (5148 ft)
above sea level and is the second-highest
peak on the island of Kauai in the Hawaiian
Island Chain. This location averages 11.68
meters (460 in.) of rain per year, with a record of 17.34 meters (683 in.) in 1982.
There are several reasons why this peak
consistently accumulates huge amounts
of rainfall: (1) Being northernmost of the
inhabited Hawaiian Islands allows some
exposure to winter time frontal systems;
(2) its shape is nearly conical and thus all
sides are exposed to the moisture-laden
winds; and (3) its very steep cliffs cause
the humid air to rise dramatically—over
1000 meters (3000 ft) in less than 1/2
mile distance. This allows the rainfall to concentrate more on one spot instead of being
spread out over a gentler mountain slope.
The wettest place on Earth is Mt. Waialeale on the Hawaiian
island of Kauai.
water vapor (given in grams) per mass of air (given in kilograms).
Obviously, both are measures of the actual amount of water vapor
in the air. Because most water vapor gets into the air through the
evaporation of water from Earth’s surface, it stands to reason that absolute and specific humidity will decrease with height from Earth.
We have also learned that air is compressed as it sinks and
expands as it rises.Thus, a given parcel of air changes its volume as
it moves vertically, but there may be no change in the amount of
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The driest spot on Earth is a valley in northern Chile.
water vapor in that quantity of air. We can see, then, that absolute
humidity, although it measures the amount of water vapor, can
vary as a result of the vertical movement changing the volume of
the air parcel. Specific humidity, on the other hand, changes only
as the quantity of the water vapor changes. For this reason, when
assessing the changes of water vapor content in large masses of air,
which often have vertical movement, specific humidity is the preferred measurement among geographers and meteorologists.
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Relative Humidity Probably the best-known means of
describing the content of water vapor in the atmosphere—the
one commonly given on television and radio weather reports—is
relative humidity. It is simply the ratio between the amount
of water vapor in air of a given temperature and the maximum
amount of vapor that the air could hold at that temperature; it
is reported as a percentage that expresses how close the air is to
saturation. This method of describing humidity has its strengths
and weaknesses. Its strength lies in the ease with which it is communicated. Most people can understand the concept of “percent”
and are unlikely to understand “grams per cubic meter.” However,
relative humidity percentages can vary widely through the day,
even when the water vapor content of the air remains the same.
The unsteady nature of relative humidity is its weakness.
If the temperature and absolute humidity of an air parcel are
known, its relative humidity can be determined by using Figure 6.5.
For instance, if we know that a parcel of air has a temperature of
30°C and an absolute humidity of 20 grams per cubic meter, we
can look at the graph and determine that if it were saturated its
absolute humidity would be 30 grams per cubic meter. To determine relative humidity, all we do is divide 20 grams (actual
content) by 30 grams (content at capacity) and multiply by 100
(to get an answer in percentage):
(20 grams ÷ 30 grams) × 100 = 67%
● FIGURE
The relative humidity in this case is 67%. In other words, the
air is holding only two thirds of the water vapor it could contain
at 30°C; it is at only 67% of its capacity.
Two important factors are involved in the horizontal distribution and variation of relative humidity. One of these is the
availability of moisture. For example, the air above bodies of water
is apt to contain more moisture than similar air over land surfaces
because there is simply more water available for evaporation. Conversely, the air overlying a region like the central Sahara Desert is
usually very dry because it is far from the oceans and little water
is available to be evaporated. The second factor in the horizontal
variation of relative humidity is temperature. In regions of higher
temperature, relative humidity for the same amount of water vapor will be lower than it would be in a cooler region.
At any one point in the atmosphere, relative humidity varies if the amount of water vapor increases as a result of evaporation or if the temperature increases or decreases. Thus, although
the quantity of water vapor may not change through a day, the
relative humidity will vary with the daily temperature cycle. As
air temperature increases from around sunrise to its maximum
in midafternoon, the relative humidity decreases as the air becomes capable of holding greater and greater quantities of water vapor. Then, as the air becomes cooler, decreasing toward
its minimum temperature around sunrise, the relative humidity
increases ( ● Fig. 6.6).
6.6
These graphs illustrate the relationship between air temperature (top) and relative humidity (bottom) through a
week in May in Bowling Green, Kentucky.
What do you notice about the relationship between these two lines?
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Relative humidity affects our comfort through its relationship to the rate of evaporation. Perspiration evaporates into the
air, leaving behind a salty residue, which you can taste if you lick
your lips after perspiring a great deal. Evaporation is a cooling
process because the heat used to change the perspiration to water
vapor (which becomes locked in the water vapor as latent heat) is
subtracted from your skin. This is the reason that, on a hot August
day when the temperature approaches 35°C (95°F), you will be
far more uncomfortable in Atlanta, Georgia, where the relative
humidity is 90%, than in Tucson, Arizona, where it may be only
15% at the same temperature. At 15%, your perspiration will be
evaporated at a faster rate at the lower relative humidity, and you
will benefit from the resultant cooling effects. When the relative
humidity is 90%, the air is nearly saturated, far less evaporation
can take place, and less heat is drawn from your skin.
Sources of Atmospheric
Moisture
In our earlier discussion of the hydrologic cycle, we saw that the
atmosphere receives water vapor through the process of evaporation. Water evaporates into the atmosphere from many different
places, most important of which are the surfaces of Earth’s bodies of water. Water also evaporates from wet ground surfaces and
soils, from droplets of moisture on vegetation, from city pavements, building roofs, cars, and other surfaces, and even from
falling precipitation.
Vegetation provides another source of water vapor. Plants
give up water in a complex process known as transpiration,
which can be a significant source of atmospheric moisture. A
mature oak tree, for instance, can give off 400 liters (105 gal)
of water per day, and a cornfield may add 11,000–15,000 liters
(2900–4000 gal) of water to the atmosphere per day for each acre
under cultivation. In some parts of the world—notably tropical
rainforests of heavy, lush vegetation—transpiration accounts for
a significant amount of atmospheric humidity. Together, evaporation and transpiration, or evapotranspiration, accounts for
virtually all the water vapor in the air.
Rate of Evaporation
The rate of evaporation is affected by several factors. First, it is affected by the amount and temperature of accessible water. Thus,
as Table 6.1 shows, the rate of evapotranspiration tends to be
greater over the oceans than over the continents. The only place
this generalization is not true is in equatorial regions between 0°
and 10°N and S, where the vegetation is so lush on the land that
transpiration provides a large amount of water for the air.
Second is the degree to which the air is saturated with water
vapor. The drier the air and the lower the relative humidity, the
greater the rate of evaporation can be. Some of us have had direct experience with this principle. Compare the length of time it
takes your bathing suit to dry on a hot, humid day with how long
it takes on a day when the air is dry.
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Third is the wind, which affects the rate of evaporation. If
there is no wind, the air that overlies a water surface will approach saturation as more and more molecules of liquid water
change to water vapor. Once saturation is reached, evaporation
will cease. However, if there is a wind, it will blow the saturated or nearly saturated air away from the evaporating surface,
replacing it with air of lower humidity. This allows evaporation
to continue as long as the wind keeps blowing saturated air
away and bringing in drier air. Anyone who has gone swimming on a windy day has experienced the chilling effects of
rapid evaporation.
Temperature affects the rate of evaporation by affecting the
first and second factors above. As air temperature increases, so
does the temperature of the water at the evaporation source. Such
increases in temperature ensure that more energy is available to
the water molecules for their escape from a liquid state to a gaseous one. Consequently, more molecules can make the transition.
Also as the temperature of the air increases, its capacity to contain
moisture also increases. As the air gets warmer, it is energized, the
molecules of air separate farther apart, and air density decreases.
With more energy and wider spacing between the air molecules,
additional water molecules can enter the atmosphere, thus increasing evaporation.
Potential Evapotranspiration
So far, we have discussed actual evaporation and transpiration
(evapotranspiration). However, geographers and meteorologists are also concerned with potential evapotranspiration
( ● Fig. 6.7). This term refers to the idealized conditions in an area
under which there would be sufficient moisture for all possible
evapotranspiration to occur. Various formulas have been derived
for estimating the potential evapotranspiration at a location because it is difficult to measure directly. These formulas commonly
use temperature, latitude, vegetation, and soil character (permeability, water-retention ability) as factors that could affect the potential evapotranspiration.
In places where precipitation exceeds potential evapotranspiration, there is a surplus of water for storage in the
ground and in bodies of water, and water can even be exported
to other places if transportation (like canals) is feasible. When
potential evapotranspiration exceeds precipitation, as it does
during the dry summer months in California, then there is no
water available for storage; in fact, the water stored during previous rainy months evaporates quickly into the warm, dry air
( ● Fig. 6.8). Soil becomes dry and vegetation turns brown as
the available water is absorbed into the atmosphere. For this
reason, fires become a potential hazard during the late summer
months in California.
Knowledge of potential evapotranspiration is used by irrigation engineers to learn how much water will be lost through
evaporation.With that, they can determine whether the water that
is left is enough to justify a drainage canal. Farmers, by assessing
the daily or weekly relationship between potential evapotranspiration and precipitation, can determine when and how much to
irrigate their crops.
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