Geoscience Reference
In-Depth Information
warmed, its density will decrease, its surface pressure will
fall and the air will tend to rise. Typically, after the bubble
of air has risen a distance equal to about once or twice its
own diameter it sinks back. New and larger bubbles form
in its wake, however, and each rises a little higher.
What controls this movement? The answer, simply,
is density or relative temperature. If the bubble of air is
warmer than its surroundings it will continue to rise; if it
is cooler, it will sink. We know already that the general
temperature of the air declines upwards - that is the
environmental lapse rate. We might imagine, therefore,
that once the bubble starts to rise it will continue to do
so indefinitely, for the air around it is becoming
progressively cooler with height. That does not happen,
however, and the reason is that as the air bubble rises it
will also cool. The critical factors that determine the
height to which the bubble rises are the relative rates of
cooling of the bubble and of the surrounding air.
The next question, then, is why does the bubble get
cooler? As we move away from the surface, air pressure
will decrease. As the air bubble rises, it encounters air of
lower density. The pressure confining the bubble is
reduced and it expands. As it does so heat is extracted from
the bubble and it becomes cooler. This is in accord with
the gas laws, which state that:
Height at which
parcel has same
temperature as
environment so
stops rising
Temperature inversion
Rate at which
temperature of
parcel changes
Dry adiabatic rate
(9.8°C/ 1000 m)
Environmental
lapse rate
(variable)
Parcel warmer than
environment so will
continue rising
Temperature
Figure 4.1 Thermal buoyancy of an air parcel. The parcel will
continue to rise as long as it is warmer than the surrounding
air.
bubble will rise only if it is warmed sufficiently to
overcome the restraining effect of the environmental
lapse rate. If the bubble cannot rise it is said to be stable.
If the temperature rises sufficiently, however, or if the
environmental lapse rate is great enough, the air bubble
can ascend a considerable distance. It is in these
circumstances that part of the troposphere is said to be
unstable.
PV / T = K (constant)
In other words, the pressure ( P ), volume ( V ) and
temperature ( T ) of a gas are interdependent. A change in
any one of these properties tends to cause changes in the
others.
The rate at which the air cools with height as a result
of this expansion is constant, at 9·8 C for each 1,000 m of
ascent. It is known as the dry adiabatic lapse rate (DALR).
Adiabatic means that there is no heat exchange between
the bubble and its surroundings, and so long as the bubble
of air rises rapidly, this condition applies. It is called dry,
not because the air does not contain any moisture, but
because no condensation has taken place.
We have, therefore, a framework for determining how
far a bubble will rise. So long as no condensation occurs ,
the warmer, rising air of the bubble will cool at the
dry adiabatic lapse rate. The surrounding air changes
temperature at the environmental lapse rate. The bubble
rises until its temperature (and therefore its density) is
equal to that of the surrounding air. This is shown
diagrammatically in Figure 4.1 .
As the dry adiabatic lapse rate is a constant, the two
variables in this relationship are the environmental lapse
rate and the initial temperature of the air bubble. The
CONDENSATION
'So long as no condensation occurs . . .' That was the
proviso we established when considering the dry adiabatic
lapse rate. But all air, even in the driest desert, does contain
some moisture.
The ability of the air to hold moisture is dependent
upon its temperature. Water molecules are continuously
changing from one state to another. If more molecules are
leaving a liquid surface than arriving, net evaporation is
experienced; if more arrive than leave there is net
condensation. As the temperature of the air increases its
moisture-holding capacity will rise; or, to put it another
way, more moisture must be added to reach saturation at
the higher temperature. If air is cooled, the evaporation
rate will decrease more rapidly than does the condensa-
tion rate and eventually the air will be saturated. This
temperature is called the dew point - the temperature at
 
 
 
Search WWH ::




Custom Search