One of the oddities of the nearby river system* is that it can produce a heavy fog bank: warm water mixing with cold air, but limited to a single valley because it is also mixing with a another river branch which will generally have colder water.*

Consequently it isn’t uncommon to start out at a 1000 feet with clear blue sky, then see this ahead and below the next hill:

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Which will become this:

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And end up as this at 400 feet:

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Before clearing out completely in about three miles.

*It isn’t a valley: two main rivers, several minor rivers, four (technically five) reservoirs, and two distinct river types squeezed into a few miles.

*It really depends on how much water is needed and where.