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Increasing altitude Parcel Temperature of
surrounding air
of air

Temperature
of a rising
parcel of air

Increasing temperature

FIGURE 23.3 In a state of atmospheric stability, the parcel of air
will always be cooler, and therefore denser, than the surrounding air
at any altitude. It will, therefore, return to the original level when the
upward force is removed.

FIGURE 23.1 Weather is a description of the changeable
aspects of the atmosphere, the temperature, rainfall, pressure, and
What happens to a parcel of air that is pushed upward
so forth, at a particular time. These changes usually affect your daily depends on the difference in density between the parcel and
life one way or another, but some seem more inconvenient than
the surrounding air. Air temperature will tell you about air den-
others.
sity since the density of air is determined by its temperature.
Instruments attached to a weather balloon can measure the
change of temperature with altitude. By comparing this change
The three major causes of uplifted air sometimes result in
with the rate of cooling by expansion, the state of atmospheric
clouds, but just as often they do not. Whether clouds form or
stability can be determined. There are many different states of
not depends on the condition of the atmosphere at the time. As
atmospheric stability, and the following is a simplified descrip-
a parcel of warm air is moved upward, it tends to stay together,
tion of just a few of the possible states, first considering dry
mixing very little with the surrounding air. As it is forced upward,
air only.
it expands and becomes cooler because it is expand ing. Similarly,
The atmosphere is in a state of stability when a lifted parcel
the temperature of a gas increases when it is compressed. So
of air is cooler than the surrounding air (Figure 23.3). Being
rising air is cooled and descending air is warmed.
cooler, the parcel of air will be denser than the surroundings. If
it is moved up to a higher level and released, it will move back
to its former level. A lifted parcel of air always returns to its
original level when the atmosphere is stable. Any clouds that
do develop in a stable atmosphere are usually arranged in the
horizontal layers of stratus-type clouds.
The atmosphere is in a state of instability when a lifted par-
cel of air is warmer than the surrounding air (Figure 23.4). Being
warmer, the parcel of air will be less dense than the surround-
ings. If it is moved up to a higher level, it will continue mov-
ing after the uplifting force is removed. Cumulus clouds usually
develop in an unstable atmosphere, and the rising parcels of air,
called thermals, can result in a very bumpy airplane ride.
So far, only dry air has been considered. As air moves up-
ward and cools from expansion, sooner or later the dew point
is reached and the air becomes saturated. As some of the water
vapor in the rising parcel condenses to droplets, the latent heat of
vaporization is released (see page 100). The release of latent heat
warms the air in the parcel and decreases the density even more,
FIGURE 23.2 The main events of the hydrologic cycle are
accelerating the ascent. This leads to further condensation and
(1) the evaporation of water from the ocean, (2) the transport of
water vapor through the atmosphere, (3) the condensation and the formation of towering cumulus clouds, often leading to rain.
precipitation of water on the land, and (4) the return of water to the The state of the atmosphere and the moisture content of
ocean by rivers and streams. an uplifted parcel of air are not the only variables affecting

23-3 CHAPTER 23 Weather and Climate 567