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The dust‐free, almost cloudless stratosphere lies above the troposphere. It is characterized by an increase in temperature with altitude, due to the presence of ozone gas which absorbs solar radiation (and protects life on the planet's surface from harmful solar ultraviolet light). This temperature pattern means there is little convection and mixing in the stratosphere, so the layers of air there are quite stable.

The ozone layer is particularly marked above the poles, where the stratosphere becomes much warmer in summer. However, the lack of insolation in winter means that the upper air then becomes very cold, and this frigid air is trapped by polar‐circling winds in the stratosphere.

In the late 1970s, British scientists discovered that the ozone layer over Antarctica was getting thinner every spring. By 1985, it was clear that an “ozone hole” was opening up above the continent. A similar, but smaller, feature was later discovered over the Arctic (Figure 3.8).

These holes last for several months before disappearing. The ozone hole that appears over Antarctica fluctuates in size, normally reaching its widest in the polar spring (September–October). The largest ozone hole, detected over Antarctica in 2006, covered 29.6 million sq. km, an area approximately three times the size of the United States. The deepest ozone hole (i.e. the hole containing least ozone) occurred in September 1994.

One major factor in the creation of these ozone holes was the injection of human‐made pollutants, such as chlorofluorocarbons (CFCs), into the atmosphere. Due to the lack of vertical convection in the stratosphere, these chemicals can stay there for a long time.

When the Antarctic spring arrives, the combination of extremely cold temperatures in the stratosphere and the return of sunlight causes a complex series of chemical reactions involving ice clouds and the atmospheric pollutants. The resultant cocktail of compounds releases chlorine into the air, and this chlorine destroys the ozone gas.

After the introduction of the 1987 Montreal Protocol, which aims to phase out production of ozone‐destroying chemicals, levels of CFCs have leveled off. However, the chemicals can stay in the atmosphere for decades, so it will be a long time before the stratospheric ozone recovers.

Figure 3.8 A series of images showing the changing size and location of the Antarctic ozone hole on October 1 for selected years between 1979 (top left) and 2014 (bottom right). Data from satellites and stratospheric balloons show that the ozone hole (dark blue and purple) grew rapidly from the mid‐1980s. It is expected to take many years for the hole to disappear.

(NASA‐GSFC)