Exploration & description/Numbers of rainy days

If there is a change in annual seasonal rainfall it is important to know whether there has been any change in the time when the rainy season starts. Thus, we use trellis plots to look at the patterns in average daily rainfall totals and numbers of rainy days for each month. Any signs of curvature are investigated by regression analysis. (GenStat was used as trellis plots and spline smoothing were not yet available in INSTAT).

Only the curve in December (the 3rd graph in the 2nd row) achieved statistical significance (P<0.05) by explaining just 8% of the year-to-year variability in total rainfall. The fitted curve rises from 180 mm in 1922 to a peak of 224mm in 1959, and then falls to 152mm in 2003. Curves in January and February (first two graphs in 3rd row) have similar shapes to December but were not significant.