In wrapping up this series on rainfall, there is only one aspect that still requires more description and that is to provide a formal definition of aridity. Of all the arid areas in the world, we, because of our geography, find ourselves in an arid zone with a considerable range of vegetation types. This obviously determines the type of agriculture that can be practised in a specific region.

Arid definitions are at best vague so various parameters are suggested by various experts.
Evaporation is usually a prime consideration; but real evaporation is very difficult to define. One can only lose the amount of water available. Beyond that amount, evaporation becomes hypothetical.
The evaporation factor is the overall presence of very dry air (relative humidities well below 25%) and it has considerable relevance where open stretches of water, storage dams for example, are concerned.
Humidity is another factor, but one which I see as being very much linked with rainfall. Even in the most arid areas, low percentages of humidity are to be found.
I prefer to use rain (presence or absence) to define aridity, particularly in our context.
Hyper arid
Our closest match to the international definition is the Hyper-Arid Namib, but even then, as environmentalists note, by Hyper-Arid standards, the Namib thrives. Namib rainfall frequently defies international recognition because of the numerous instances where only drops fall, i.e. not measurable according to conventional rainfall data methodologies. Yet the environment has learnt to use these immeasurable values. Hence a Namib ecology has developed within the Hyper-Arid setting. The desert also receives productive falls but only sporadically and it can be years between such occurrences.
Arid
In a typical Arid climate zone, rainfall is erratic. Wet Spells may or may not occur. Occurrences may be limited to single events across the station record. Even 10-day rainy spells may be found in unique circumstances, but the more frequent range lies from a single 6-day duration to 3 or 4-day spells. Wet Days are found occasionally. Productive and Substantial falls do occur, but their regular absence makes drought the norm. Absolute Drought is certainly scarce but should not be discounted. Threshold Drought occurs annually in the dry months and 40-day Droughts often occur during the rain season.
Arid climate defines the area from the Kaokoveld, inland from the escarpment to the western Khomas Hochland and thence further inland, including the interior from around Mariental southward.