Geoscience Reference
In-Depth Information
causes its volume to increase and temperature to
decrease (see Chapter 2B). The rate at which
temperature decreases in a rising, expanding air
parcel is called the
adiabatic lapse rate
. If the
upward movement of air does not produce
condensation, then the energy expended by
expansion will cause the temperature of the mass
to fall at the constant
dry adiabatic lapse rate
or
DALR (9.8
the lapse rate approaches the dry adiabatic value
at an elevation of 100m or so.
The changing properties of rising air parcels
can be determined by plotting
path curves
on
suitably constructed graphs such as the skew
T
-log
p
chart and the
tephigram
, or
T-
φ
refers to entropy. A tephigram (
Figure 5.1
)
displays five sets of lines representing properties
of the atmosphere:
φ
-gram, where
C/km). However, prolonged cooling
of air invariably produces condensation, and
when this happens latent heat is liberated,
counteracting the dry adiabatic temperature
decrease to a certain extent. Therefore, rising and
saturated (or precipitating) air cools at a slower
rate (the
saturated adiabatic lapse rate
or SALR)
than air that is unsaturated. Another difference
between the dry and saturated adiabatic rates is
that whereas the DALR is constant the SALR varies
with temperature. This is because air at higher
temperatures is able to hold more moisture and
therefore on condensation releases a greater
quantity of latent heat. At high temperatures, the
saturated adiabatic lapse rate may be as low as
4
°
1
Isotherms - i.e., lines of constant temperature
(parallel lines from bottom left to top right).
2
Dry adiabats (parallel lines from bottom right
to top left).
3
Isobars - i.e., lines of constant pressure and
corresponding height contours (slightly curved
nearly horizontal lines).
4
Saturated adiabats (curved lines sloping up
from right to left).
5
Saturation mixing ratio lines (at a slight angle
to the isotherms).
Air temperature, dew-point temperature
and wind velocity are determined from atmos-
pheric soundings made by rawinsondes (radar
wind soundings). Helium-filled balloons with a
suspended instrument package and a radar
reflector for tracking them are released at upper-
air stations around the world once or twice daily,
The instruments in the package are an aneroid
barometer to determine altitude, a temperature
sensor and a dew-point sensor. Radar is used to
track the balloon as it rises and to calculate the
wind speed and direction. The data are reported
at standard levels (1000, 850, 700, 500, 300, 200,
100, 50, 20 and 10mb) and at intermediate levels
where significant departures occur from a linear
interpolation between standard levels.
Air temperature and dew-point temperature
are the variables that are commonly plotted on an
adiabatic chart. The dry adiabats are also lines of
constant potential temperature,
C/km, but this rate increases with decreasing
temperatures, approaching 9
°
C. The
DALR is reversible (i.e., subsiding air warms
at 9.8
°
C/km at -40
°
C/km); whereas in any descending cloud
air saturation cannot persist because droplets
evaporate.
Three different lapse rates must be distin-
guished: two dynamic and one static. The static,
environmental lapse rate
(ELR) is the actual
temperature decrease with height on any occasion,
such as an observer ascending in a balloon or
climbing a mountain would record (see Chapter
2C.1). This is not an adiabatic rate, therefore,
and may assume any value depending on the
local vertical profile of air temperature. In
contrast, the dynamic
adiabatic dry and saturated
lapse rates
(or cooling rates) apply to rising parcels
of air moving through their environment. Above
a heated surface, the vertical temperature gradient
sometimes exceeds the dry adiabatic lapse rate
(i.e., it is superadiabatic). This is common in arid
areas in summer. Over most ordinary dry surfaces,
°
(or isentropes).
Potential temperature is the temperature of an air
parcel brought dry adiabatically to a pressure of
1000mb. Mathematically,
