Geoscience Reference
In-Depth Information
C THE LAYERING OF THE ATMOSPHERE
1 Troposphere
The atmosphere can be divided conveniently into a
number of rather well-marked horizontal layers, mainly
on the basis of temperature (Figure 2.15). The evidence
for this structure comes from regular rawinsonde (radar
wind-sounding) balloons, radio wave investigations,
and, more recently, from rocket flights and satellite
sounding systems. There are three relatively warm
layers (near the surface; between 50 and 60 km; and
above about 120 km) separated by two relatively cold
layers (between 10 and 30 km; and 80 and 100 km).
Mean January and July temperature sections illustrate
the considerable latitudinal variations and seasonal
trends that complicate the scheme (see Figure 2.16).
The lowest layer of the atmosphere is called the
troposphere
. It is the zone where weather phenomena
and atmospheric turbulence are most marked, and it
contains 75 per cent of the total molecular or gaseous
mass of the atmosphere and virtually all the water
vapour and aerosols. Throughout this layer, there is a
general decrease of temperature with height at a mean
rate of about 6.5°C/km. The decrease occurs because
air is compressible and its density decreases with
height, allowing rising air to expand and thereby cool.
In addition, turbulent heat transfer from the surface
mainly heats the atmosphere, not direct absorption of
radiation. The troposphere is capped in most places by
Figure 2.15
The generalized vertical
distribution of temperature and pressure
up to about 110 km. Note particularly the
tropopause and the zone of maximum
ozone concentration with the warm
layer above. The typical altitudes of polar
stratospheric and noctilucent clouds are
indicated.
Source
: After NASA (n.d.). Courtesy of NASA.
