Namibia extends from 17°S to 25°S: climatically the range of the sub-Tropical High Pressure Belt, arid is the overall climate description descending from the Sub-Humid (mean rain above 500 mm) through Semi-Arid between 300 and 500 mm (embracing most of the waterless Kalahari) and Arid from 150 to 300 mm (these three regions are inland from the western escarpment) to the Hyper-Arid coastal plain with less than a 100 mm mean. What is the climate description for the northern half of Namibia