Variability of the ozone layer
Ozone variations with time
The thickness of the ozone layer varies with time. The figure below shows measurements of the ozone layer performed by RMI at Uccle. The red and blue lines are measured daily values for this year and last year. The black line represents the mean values derived from all the measurements performed since 1971. 95% of all the observations are in the shaded area. The day-to-day variability can be as large as 20%. It means that the thickness of the ozone layer can change by more than 20% in one day. This variability is also larger in winter and spring.
Another important fact is the annual cycle. On average, the thickness of the ozone layer is greater in March and April and lowest in October and November. This cycle is related to the general air circulation in the stratosphere at different seasons.
The "thickness" of the ozone layer or total ozone content is measured with specially designed instruments called spectrophotometers and is expressed in Dobson Units (DU). A layer 0.01 mm thick of ozone at 0°C and a pressure of 1 atmosphere (equal to 1013.25 hPa) is defined as one DU. If the Ozone layer over our region were compressed to 0°C and one atmosphere pressure, it would be about 3 mm thick.
Variation of the ozone layer observed at Uccle. The red and the green line represent the daily values of this and last year, respectively. The black line is the mean annual cycle and 95% of the observations are within the grey band.
Ozone variations in space
The thickness of the ozone layer varies with season and location on Earth. Ozone is created naturally in the middle and upper stratosphere at the tropics, due to the high availability of UV radiation there. However, atmospheric circulation (the Brewer-Dobson circulation) transports ozone from its tropical source region into the extratropics, such that, under normal conditions the thickness of the ozone layer is larger at high latitudes than at low latitudes. For example, larger ozone concentrations are usually found above Northern Europe, Canada and Siberia than above the equator. A global view of the ozone layer is given below: the colors represent the thickness of the ozone layer. Regions where the ozone layer is thinner are in blue and regions where it is thicker are in red/purple. One can clearly see that the ozone layer is thinner along the equator and much thicker above the North pole. This can be explained by the fact that the transport of ozone is strongest during late fall and winter (the figure shows the distribution in the middle of January) and in the northern hemisphere (because of the prominent topography and more pronounced land-ocean contrasts in the northern hemisphere).
Total ozone obtained by assimilation of ozone observations from space.