Geoscience Reference
In-Depth Information
17.3 g/kg. That is, the air mass has the
potential
to contain 17.3 g/kg
of water vapor. If the specific humidity is also 17.3 g/kg, that means
the parcel is saturated, the relative humidity is 100%, and the dew-
point temperature is 20°C (68°F). On the other hand, if the specific
humidity is only 9.4 g/kg (Figure 7.12b), then the relative humidity
is about 54% and the dew-point temperature is approximately 10°C
(50°F). If the air temperature were to increase without a change in
specific humidity, the relative humidity would decrease further.
How could we get the relative humidity of the drier air
mass to increase? This kind of increase can happen in one of
two ways. One way would be to somehow increase the amount
of water vapor in the air mass, perhaps through the process of
evaporation at a nearby ocean source. For example, if another
4 g/kg of vapor was added to the air mass through this process
(Figure 7.12c), then the specific humidity would be 13.4 g/kg
and the relative humidity would increase to 78%. At the same
time, the dew-point temperature would increase to about 16°C
(61°F). The second way would be to lower the temperature
(Figure 7.12d), let's say to 15°C (59°F). If this cooling pro-
cess occurred at the same time that specific humidity remained
9.4 g/kg, then the relative humidity would increase to about
74% because the air could hold less moisture; in other words,
the maximum humidity would be less.
An important reason why understanding atmospheric hu-
midity is relevant to your life is that it allows meteorologists to
predict when and how precipitation will occur. You may have
encountered such a discussion on your local weather station,
when the weatherperson talks about how much moisture is in
the air at a given time, especially when thunderstorms are possi-
ble. When these scenarios occur, the meteorologist may say that
the dew-point temperatures in your region are “currently high.”
This is simply another way of saying that the air is laden with
water vapor and it would take much cooling to set off a storm.
At another level, understanding atmospheric humidity is
critical for agriculture because it is closely associated with crop
production. Farmers are keenly aware of atmospheric mois-
ture conditions because their crops depend on the appropriate
amount of water, which, in turn, is relevant to you because the
price of food is directly related to supply. If an extended period
of drought occurs in a major agricultural region, production
yields decrease dramatically and the cost of food increases.
Atmospheric humidity conditions also have ramifications
for human comfort and health, especially during the warm, hu-
mid days of summer when dew-point temperatures are elevated.
When the heat index is high, most people are very uncomfort-
able working outside for any length of time. The reason for this
discomfort is that our bodies do not regulate internal tempera-
ture effectively when the air is warm and muggy. The primary
way that we regulate internal temperature is through the produc-
tion of sweat. When the air is dry, sweat evaporates and body
heat is removed in the same manner that the wet bulb cools
on a sling psychrometer. On warm/humid days, however, sweat
evaporates less efficiently and our core temperature increases.
If the sweat-producing activity continues, body temperatures
can rise to the point that heat stroke occurs and the temperature
regulation system shuts down completely.
Figure 7.11 Formation of dew.
Water condenses on grass
when nighttime temperatures cool to the dew-point temperature.
dew-point temperature, therefore, is an important designation
and is defined as the temperature at which a mass of air be-
comes saturated. The term
dew point
originates from the for-
mation of dew on grass (Figure 7.11). This occurs when air at
ground level cools to the dew-point temperature at night and
water condenses on blades of grass. As the year progresses, dew
typically forms during the early fall and later part of the spring,
when nighttime temperatures cool sufficiently for condensation
to occur at ground level, but not so much that water crystallizes
as it does in winter. Dew is less likely in the summer months
because nighttime temperatures remain relatively high and the
dew-point temperature is not reached at ground level.
Higher in the atmosphere, the dew-point temperature is im-
portant because it marks the temperature at which water begins
to condense into liquid form as clouds, fog, and ultimately vari-
ous forms of precipitation. This condensation occurs because
the air mass can no longer hold any more water vapor, either
because the air cools to the dew-point temperature or because
more water vapor is added to the air mass through evaporation
of surface water. Whereas the maximum humidity of an air
mass is a function of its temperature, the dew-point tempera-
ture depends on the specific humidity, which is another way of
referring to how much water vapor is actually in the air.
You can observe the trend in dew-point temperature by
reexamining the saturation curve in Figure 7.6. This time, first
consider the humidity values on the vertical (
y
) axis and then
temperature data on the horizontal (
x
) axis. In this fashion, you
are simply reversing the association you made earlier when you
determined the maximum humidity at certain temperatures. For
example, if the specific humidity of the air is 2 g/kg, then the dew-
point temperature is
−
10°C (14°F). If the specific humidity in-
creases to 15 g/kg or 26 g/kg, then the dew-point temperature also
rises, in this case, to 20°C (68°F) and 30°C (86°F), respectively.
In an effort to integrate all of these new concepts into a co-
herent mathematical model that explains how precipitation oc-
curs, let's look at some examples in Figure 7.12. First consider an
air mass with a temperature of 20°C (68°F), which you can see
in Figure 7.12a. At this temperature, the maximum humidity is
