Geoscience Reference
In-Depth Information
DISCOVERY OF THE TROPOPAUSE AND
STRATOSPHERE
box 2.2
significant
20th-c. advance
Early scientific exploration of the upper atmosphere began with manned balloon flights in the mid-nineteenth
century. Notable among these was the ascent by J. Glaisher and H. T. Coxwell in 1862. Glaisher last consciousness
due to lack of oxygen at about 8800-m altitude and they barely survived the hypoxia. In 1902 L. Teisserenc de Bort in
France reported a totally unexpected finding: that temperatures ceased decreasing at altitudes of around 12 km. Indeed,
at higher elevations temperatures were commonly observed to begin increasing with altitude. This mean structure is
shown in Figure 2.15.
The terms
troposphere
(turbulent sphere) and
stratosphere
(stratified sphere) were proposed by Teisserenc de Bort
in 1908; the use of
tropopause
to denote the inversion or isothermal layer separating them was introduced in Great Britain
during the First World War. The distinctive features of the stratosphere are its stability compared with the troposphere,
its dryness, and its high concentration of ozone.
2 Stratosphere
atures at about 25 km may jump from -80 to -40°C
over a two-day period. The autumn cooling is a more
gradual process. In the tropical stratosphere, there is
a quasi-biennial (twenty-six-month) wind regime, with
easterlies in the layer 18 to 30 km for twelve to thirteen
months, followed by westerlies for a similar period. The
reversal begins first at high levels and takes approxi-
mately twelve months to descend from 30 to 18 km
(10 to 60 mb).
How far events in the stratosphere are linked with
temperature and circulation changes in the troposphere
remains a topic of meteorological research. Any such
interactions are undoubtedly complex.
The stratosphere extends upward from the tropopause to
about 50 km and accounts for about 10 per cent of the
atmospheric mass. Although the stratosphere contains
much of the total atmospheric ozone (it reaches a peak
density at approximately 22 km), maximum temper-
atures associated with the absorption of the sun's
ultraviolet radiation by ozone occur at the
stratopause
,
where they may exceed 0°C (see Figure 2.15). The air
density is much lower here, so even limited absorption
produces a high temperature rise. Temperatures increase
fairly generally with height in summer, with the coldest
air at the equatorial tropopause. In winter, the structure
is more complex with very low temperatures, averaging
-80°C, at the equatorial tropopause, which is highest
at this season. Similar low temperatures are found
in the middle stratosphere at high latitudes, whereas
over 50-60°N there is a marked warm region with
nearly isothermal conditions at about -45 to -50°C.
In the circumpolar low-pressure vortex over both
polar regions, polar stratospheric clouds (PSCs) are
sometimes present at 20 to 30 km altitude. These
have a nacreous ('mother-of-pearl') appearance. They
can absorb odd nitrogen and thereby cause catalytic
destruction of ozone.
Marked seasonal changes of temperature affect
the stratosphere. The cold 'polar night' winter stratos-
phere in the Arctic often undergoes dramatic
sudden
warmings
associated with subsidence due to circulation
changes in late winter or early spring, when temper-
3 Mesosphere
Above the stratopause, average temperatures decrease
to a minimum of about -133°C (140 K) or around
90 km (Figure 2.15). This layer is commonly termed
the
mesosphere
, although as yet there is no universal
terminology for the upper atmospheric layers. Pressure
is very low in the mesosphere, decreasing from about
1 mb at 50 km to 0.01 mb at 90 km. Above 80 km,
temperatures again begin rising with height and this
inversion is referred to as the
mesopause
. Molecular
oxygen and ozone absorption bands contribute to heating
around 85 km altitude. It is in this region that
noctilucent
clouds are observed on summer 'nights' over high lati-
tudes. Their presence appears to be due to meteoric dust
particles, which act as ice crystal nuclei when traces
