Here’s a chart showing Windhoek’s average rainfall and temperature for the year. Thanks to BBC and Wikipedia for this info. You can expect temperatures in the north of Namibia to be between 5 and 10 degrees (F) warmer.
As you can see, we expect no rain to fall in August. The temperature, however, is notoriously difficult to pin down. August is known to be windy. I’d recommend dressing in layers. It’ll be chilly/cold at night, and pleasant in the daytime.