Average annual rainfall is calculated for the rainfall season, from the beginning of July of one year to the end of June in the following year. Along the coast and for some way inland rainfall is extremely low and highly variable (figure 3.10). Average rainfall within 40 kilometres of the coast is less than 20 millimetres per year, and most of this typically falls for short periods on only a few rainy days. By contrast, the northeastern areas of Namibia receive on average falls of more than 650 millimetres a year.

The great majority of rain in Namibia results from convective processes whereby heat causes moist air to rise, cool, condense and form dense cumulus clouds. Some inland areas, such as those around Otavi, Tsumeb and Grootfontein, have slightly higher rainfall than their surrounding areas due to orographic rainfall, which results from moist air being forced up over hills or mountains and cooled. Slightly higher patches of rainfall in the far southwest are the result of frontal rain received during the winter months when substantial cold fronts farther south reach into Namibia (figure 3.01).

With the exception of the southwest, rainfall in Namibia is strongly seasonal. As the maps of average monthly rainfall show, from October to February rainfall gradually increases across the country from the east and north, retreating again during March and April.

But what makes Namibia so dry in-between times? As the Intertropical Convergence Zone shifts south in summer, the South Atlantic Anticyclone and Botswana Anticyclone push south, where their impact on Namibia's weather is limited (figure 3.01). However, they dominate weather in winter when they resume their northerly positions west and east of the country. Air descends in the anticyclones, drying as it subsides and spreads across Namibia and the rest of southern Africa. With the warm tropical air effectively blocked, the prevailing anticyclonic conditions produce clear, cloudless skies day after day between May and September.

How does the limited rain in the western parts of Namibia fall – in light showers over many days or in heavier falls on just a few days? Taking a cut-off of 1 millimetre to exclude minor rainfall events, this map shows that the average frequency of rainy days rapidly increases eastwards of the coast. The coastal towns have significant rain on fewer than seven days per season on average while this increases to about 64 rain days at Katima Mulilo. Opuwo, on the escarpment, receives an average rainfall of 285 millimetres which falls over 33 rain days, on average.

Interestingly, Ondangwa in northern Oshana Region receives 175 millimetres less rain on average than Divundu in Kavango East, but its number of significant rain days is similar (50 and 55, respectively). This suggests that the intensity and/or duration of rainfall events increases eastwards in northern Namibia. In general, the pattern of this map mirrors that of annual rainfall (figure 3.07). The higher frequency of rain days in some areas reflects the increase in rainfall caused by local relief, such as around Otavi– Tsumeb–Grootfontein (Otavi Mountains), Windhoek (Auas Mountains) and west of Otjinene (Waterberg area).

With the decline in rainfall from east to west, the frequency of rainfall events becomes more sporadic. This is reflected in the increasing degree of variability in annual rainfall compared to the longterm average, and is expressed as the coefficient of variation. As variability increases, rain-fed cultivation and other land-use practices become more marginal risk increases.

In Namibia's far western areas where variability is greatest, rainfalls may be brief but intense and cause disruption to coastal towns. For example, the longterm average annual rainfall at Lüderitz is just 16 millimetres but, in April 2006, 102 millimetres fell on the town over a three-day period. Such events are rare, irregular and are not expected to occur more than once in a century.20

Rainfall from one season to the next is erratic. This holds true for most places in Namibia and is illustrated in these graphs of monthly rainfall over five consecutive seasons from 2015/16–2019/20 at Khorixas, Tsumkwe and Katima Mulilo. They show a high degree of variation in the amounts of rainfall received each month, in the spread of rainfall over the seasons and in the peak rain months each season. Successful crop farming requires some stability in the timing and consistency of rainfall events, and the inherent variability of rainfall in Namibia often leads to crop failures, or makes dryland cropping impractical.