Geoscience Reference
In-Depth Information
3
Vertical Gradients
in the Atmosphere
Introduction
Air nearer to the ground must support the column of air above it and so has to
exert an upward force per unit surface area (i.e., an upward pressure) that is equal
and opposite to the downward gravitational force exerted by all the overlying air.
Because the mass of overlying air reduces with distance from the ground, air pres-
sure reduces with height. Air temperature is related to pressure and density
through the ideal gas law, Equation (2.5), so air temperature necessarily also
changes with height. In the absence of any disturbing influences such as heat
inputs from the Sun or surface, the atmosphere would therefore settle into a stable
condition with
hydrostatic
vertical gradients of pressure, density, and temperature,
all of which can be calculated.
Heat flow occurs when there is non-uniformity in the spatial distribution of
heat in a medium, with movement away from regions with higher temperature
toward regions with lower temperature. But the temperature gradient that is
established in a hydrostatic atmosphere is
not
associated with vertical heat flow.
Rather it is
deviations
from this temperature gradient that are associated with the
vertical movement of heat. Consequently, it is convenient to define a variable
that is directly related to vertical heat flow,
potential temperature
, which is a
combination of both local temperature and pressure, and to calculate the vertical
profile of this potential temperature to diagnose the thermal stability of the
atmosphere.
Because the gram molecular weight of water vapor is less than the average
gram molecular weight of the other gases (mainly nitrogen and oxygen) that
make up air, the density of air with more water vapor is less than that of air with
less water vapor. The concentration of atmospheric water vapor can change with
height, and height dependent variations in density associated with changes in
fractional vapor content complicate the relationship between the density,
