NamibWeb.com - The online guide to Namibia:

Temperature in Namibia

Temperature in some major towns and resorts in Namibia. Updated every three days.

Town	Temperature (min/max, C)
Windhoek	10/27
Swakopmund	13/23
Tsumeb	12/29
Grootfontein	10/28
Katima Mulilo	12/30
Gobabis	10/24
Karasburg	12/25
Keetmanshoop	10/26
Mariental	11/27
Luderitz	12/20

These figures also give you an indication of temperatures in the resorts and parks near towns mentioned
(Tsumeb - Etosha Park, Keetmanshoop - Fish River Canyon, etc.)