Third article in a series that discusses the technical aspects of rainfall
World-wide, the climatological approach to the rainfall factor of the weather world is to collect the monthly detail, total it, and compare it to the long-term detail: the mean, write an interpretation of this figure and then publish it. This methodology works satisfactorily where rainfall occurs in a very reliable fashion, on an almost daily basis.

Departures from this narrow range are scarce, but quickly noticeable.
In our world, we have a differing set of factors affecting our rainfall scene.
Within the range of the sub-Tropical High Pressure Belt, the moisture-bearing air has to be imported, by wind flow, from within the surface air layers, the maritime sources: usually brought in by an easterly wind, and within the alto levels, carried south from that very valuable Congo air mass. Either singly or together, these sources will create the right type of weather to bring about rainfall. But this rainfall occurs when the surface heat, insolation, creates ascending air currents, convection at work,   which become visible as cumulus cloud forms, grows and builds into cumulonimbus cloud. Cumulonimbus, in brief, means thunderstorm rains.
As we regularly see, thunderstorm rains are much localised. At the end of a thunderstorm day, there should have been quite a reasonable spread of rainfalls. Very nice, but we are still at a distance from what we need to know. The next morning, as the rainfall reports reach the weather/forecast office, so some sort of picture appears. With a more persistent weather pattern, these same events will recur across the next few days. A better picture of the rainfall spread should evolve. Depending on the weather pattern for the season, the outlines of  a wetter or a dry year appear. The statisticians are now busy comparing totals and means and drawing their conclusions. The editor of the Climate of South Africa series of volumes, B.R. Schulze notes in the General Survey No. 28:  “Although in meteorological literature much emphasis is placed upon the means, it should be stressed that in no one month is this figure matched” .as an epitaph to the application of means and so on to our southern African rainfall story. However, but for the farmer in particular, and many others too, the uncertainties persist.