Geoscience Reference
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1000
800
Subadiabatic
600
Isothermal
Adiabatic
400
Inversion
Superadiabatic
200
Figure 3.4 Classification
of atmospheric stability
conditions based on the
vertical gradient of virtual
temperature.
0
290
295
300
305
310
Virtual temperature (K)
Important points in this chapter
Hydrostatic atmosphere : in the absence of external influences the atmosphere
would have hydrostatic vertical gradients of pressure, density, and tempera-
ture. Deviations from the temperature gradient in hydrostatic conditions
control the thermal stability of the atmosphere and it is convenient to
calculate potential temperature or, if water vapor content varies, virtual
potential temperature profiles that are directly related to thermal stability.
Hydrostatic pressure law : the rate of change of pressure with height is given
by the product of local air density and the acceleration due to gravity.
Dry adiabatic lapse rate : if moist air moving vertically cools adiabatically
but remains unsaturated, it cools at a rate
Γ
=
g / c p , i.e., 0.00968 K m −1 or
9.68 K km −1 .
Moist adiabatic lapse rate : if moist air moving vertically cools adiabatically
in a saturated atmosphere, it cools at a rate
Γ m which is less than
Γ
.
Environmental lapse rate : the actual rate at which air temperature falls away
from the ground is determined by the history of heat inputs/outputs to it, but
it may be approximately constant and in the 'US Standard Atmosphere' is
6.5 K km −1 .
Vertical pressure and temperature gradients : are linked by the ideal gas
law hence, if
Γ local is the local environmental lapse rate, the temperatures
and pressures at two levels ( T 1 and T 2 and P 1 and P 2 , respectively) are
related by:
 
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