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	06/25
Swakopmund	13/22
Tsumeb	12/29
Grootfontein	10/28
Katima Mulilo	12/30
Gobabis	03/24
Karasburg	05/25
Keetmanshoop	03/26
Mariental	04/27
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.)