Yes, I guess it depends on the species, but we went to southern California in February 2005 hoping there'd be some snakes around, it being as warm as an average British summer day a lot of the time, but they weren't - British snakes just have to be tougher I guess! Adders come out when it's still really quite cool, less than 60 degrees. Natrix emerge a little later. Of course it depends if they can bask as well, but we saw adders this spring on pretty cool overcast, even rainy days.

I'll have a look for some of your photos - I haven't looked at the field notes section yet.