III.

Seasonal Changes in the Ozone Layer

The ozone layer is thinnest near the equator and thickest at the poles. Since ozone formation depends on ultraviolet radiation from the Sun, the amount of ozone present in the atmosphere at any given time and place varies. Also, the lifetime of an ozone molecule in the stratosphere is between several months and several years, so the distribution of ozone is affected by the motion of the atmosphere; ozone molecules can be transported long distances before being destroyed.

There are long-term trends in ozone. Between 1979 and 1991, the ozone in the mid-latitudes (roughly between 25° and 60°) decreased by an average of around 4 per cent per decade. In the northern hemisphere mid-latitudes, the decrease is greater in the winter and spring, and less in the summer and autumn, while the southern hemisphere mid-latitude decrease shows less seasonal variation.