Geoscience Reference
In-Depth Information
water in the glass than it can hold, the water will spill over the
top. This analogy is useful because the atmosphere works in
somewhat the same way as water moves from vapor to liquid.
Sometimes the air can hold a lot of vapor, similar to a large glass,
and other times the amount of vapor the air can contain is less.
Maximum humidity
refers to the maximum amount of water
vapor that a definable body of air can hold. Although the amount of
liquid a glass can hold depends on the size of the glass, the maxi-
mum humidity of an air parcel is dependent on the air temperature.
An important rule to remember is that
warmer air can hold much
more water vapor than colder air
. This relationship exists because
air expands in the process of warming, which leaves room for more
water vapor, and contracts when cooling, leaving less room. This
is a very simple but important rule in physical geography that ex-
plains the processes of evaporation and precipitation. Figure 7.5
presents a simplified example of this concept.
The relationship between air temperature and maximum
humidity is described by a
saturation curve
, such as the one pre-
sented in Figure 7.6. In the case of the atmosphere, the term
saturation
refers to the point where the air cannot hold any more
water vapor based on the temperature at that time. To help make
sense of this concept, simply think of what happens when a
sponge absorbs water. The sponge absorbs water until it is satu-
rated and unable to hold any more. The atmosphere works on
much the same principle. The line on the saturation curve merely
illustrates the respective intersections of maximum humidity and
air temperature. Notice that maximum humidity is measured as
grams of water per kilogram of air, or g/kg in abbreviated form.
You can see, for example, that at an air temperature of
−
10°C (14°F) the maximum humidity is approximately 2 g/kg.
60
50
40
30
20
10
-40
Temperature (°C)
-30
-20
-10
0
10
20
30
40
50
Temperature (°F)
-40
-20
0
20
40
60
80
100
120
Figure 7.6 The saturation curve.
Maximum humidity in-
creases as temperature increases. In other words, warmer air
can hold more water vapor than cooler air.
Notice that the maximum humidity increases exponentially with
increasing temperature. At 20°C (68°F) the maximum humid-
ity is about 15 g/kg, and at 30°C (86°F) the maximum humidity
nearly doubles again to about 26 g/kg. If you are having difficulty
relating this concept to the water glass, try to imagine what would
happen if the glass could expand and contract with warmer and
colder temperatures, respectively. In this scenario, the glass can
hold a larger amount of
total
water when it is warm, and less
as it contracts with progressively decreasing temperature. The
atmosphere operates on somewhat the same principle in that it
expands when warm and contracts when cold.
Whereas the term
maximum humidity
refers to the maxi-
mum amount of water vapor that a mass of air can hold, the
term
speciic humidity
refers to how much water vapor is
actually
in the air. Think back to the glass analogy. Recall that
the glass can hold a maximum amount of water, which, in fact,
rarely occurs in the course of everyday use. Instead, you usually
fill the glass to some point that is short of the container lip. The
volume of this
specific
amount of liquid can be measured; for
example, the glass may contain 1 cup of water, even though the
glass can actually hold 2 cups.
Finally, we have the concept of
relative humidity
, which is
the ratio of specific to maximum humidity—in other words, how
close the air is to being saturated. In the case of the glass, you
could say that the glass is half full if it contains 1 cup of water,
Colder air
Warmer air
= nitrogen
= oxygen
= water
Figure 7.5 Maximum humidity and air temperature.
A
given parcel of warmer air can hold more water vapor than
a similar parcel of colder air. The reason is that molecules at
higher temperature are farther apart, so more water molecules
can fit in a given amount of air.
Maximum humidity
The maximum amount of water vapor
that a definable body of air can hold at a given temperature.
Specific humidity
The measurable amount of water vapor
that is in a definable body of air.
Relative humidity
The ratio between the specific and maxi-
mum humidity of a definable body of air.
