Geoscience Reference
In-Depth Information
a temperature inversion level (i.e. a layer of relatively
warm air above a colder layer) and in others by a zone
that is isothermal with height. The troposphere thus
remains to a large extent self-contained, because the
inversion acts as a 'lid' that effectively limits convection
(see Chapter 4E). This inversion level or weather ceiling
is called the
tropopause
(see Note 5 and Box 2.2). Its
height is not constant in either space or time. It seems
that the height of the tropopause at any point is
correlated with sea-level temperature and pressure,
which are in turn related to the factors of latitude, season
and daily changes in surface pressure. There are marked
variations in the altitude of the tropopause with latitude
(Figure 2.16), from about 16 km at the equator, where
there is strong heating and vertical convective turbu-
lence, to only 8 km at the poles.
The equator-pole (meridional) temperature gradi-
ents in the troposphere in summer and winter are
roughly parallel, as are the tropopauses (see Figure
2.16), and the strong lower mid-latitude temperature
gradient in the troposphere is reflected in the tropopause
breaks (see also Figure 7.8). In these zones, important
interchange can occur between the troposphere and
stratosphere, and vice versa. Traces of water vapour
can penetrate into the stratosphere by this means, while
dry, ozone-rich stratospheric air may be brought down
into the mid-latitude troposphere. Thus above-average
concentrations of ozone are observed in the rear of mid-
latitude low-pressure systems where the tropopause
elevation tends to be low. Both facts are probably
the result of stratospheric subsidence, which warms
the lower stratosphere and causes downward transfer
of the ozone.
Figure 2.16
Mean zonal (westerly)
winds (solid isolines, in knots; negative
values from the east) and temperatures
(in °C, dashed isolines), showing the
broken tropopause near the mean
Ferrel jet stream.
Source
: After Boville (from Hare 1962).
Notes
: The term 'Ferrel Westerlies' was
proposed by F. K. Hare in honour of
W. Ferrel (see p. 139). The heavy black
lines denote reversals of the vertical
temperature gradient of the tropopause
and stratopause. Summer and winter
refer to the northern hemisphere.
