The Klamath Basin with its upper and lower Klamath Lakes and Agency Lake presents a very interesting phenomenon concerning fog conditions. It is most prevalent during the winter months but also occurs in the late evening and early morning hours for the warmer periods of the year. The fog is due to radiation of heat with the attendant ascending water vapor from the extensive water surfaces and adjacent tule swamp lands.

The depth of this so-called Tule Fog mantle is a variable but usually it is quite shallow. As a rule the fog disappears by midday -- the daily period of maximum sun insolation. On the other hand there are days and in some cases longer periods of continuous fog with the resulting poor visibility.