Figure 2.3. Trends in mean annual rainfall in the Sahel (1897-1990).

UNEP (1992).
The number of stations from which data are taken and averaged varies for each year up to about 100. The annual rainfall series for each station is normalized by taking away the long-term (1951-80) mean from each value and dividing the difference by the long-term (1951-80) standard deviation. The smooth curve is a 10-year Gaussian filter that suppresses variations of less than 10 years, giving an indication of trends in the rainfall anomalies.