Geology Reference
In-Depth Information
Antarctic ozone minimum (60
- 90
S)
°
°
209
205
205
9/17
10/1510/10
200
189
9/25
1979 - 1992 Nimbus 7 TOMS
1993 - 1994 Meteor 3 TOMS
1995 (no TOMS in orbit)
1996 - 2003 Earth Probe TOMS
180
173
169
9/19
10/18
159
160
154
10/7
146
10/3
10/24
140
140
9/9
128
124
124
120
10/4
117
120
10/7
10/10
10/5
111
10/5
104
10/5
99
100
97
9/24
94
94
92
90
9/26
9/28
88
10/6
9/29
9/5
9/30
9/28
80
1980
1985
1990
1995
2000
2005
Year
Figure 30 Lowest values of ozone measured from satellite each year in the ozone hole.
Global average ozone is about 300 Dobson Units. Before 1980 ozone less than
200 Dobson Units was rarely seen. In recent years ozone near 100 Dobson
Units has become normal in the ozone hole. Ozone in the year 2002 ozone hole
was higher than we have come to expect because of unusually high tempera-
tures in the Antarctic stratosphere (from NASA data)
What happens in Antarctic stratosphere to make it different from
many other places? It is colder than the rest of the earth, ozone depletion
occurs in the Austral spring when it is particularly cold (
801C). Polar
meteorology leads to very strong westerly winds (up to 100 ms
1
)
forming a stable vortex, which prevents mixing of air from lower
latitudes (see Figure 32). The vortex develops a core of very cold air
and it is these temperatures that allow polar stratospheric clouds (PSCs)
to form. When the sun returns in September, the temperatures begin to
rise, the winds weaken and typically this drives the vortex to break down
by November. In a sense the air over the pole is more or less confined
into what can be thought of as a giant chemical reactor. There is a slow
downward circulation (see Figure 32), which draws air down through
the vortex modifying this to a notional flow reactor.
In 1986 and 1987, a huge international expedition was mounted to
examine the chemistry and meteorology that leads to the polar ozone
loss. Many different measurements were made using satellites, aero-
planes and ground-based instruments.
57
The results showed some stark
