water temperature formula?

[crankybait]

hey guys, i was wondering if there was a basic formula to guess the approximate temperature of a body of water without going to it... like, just knowing the size and air temperature, sticking it into a forumla and getting within 5 degrees. i know itll vary with depth, sky condition, wind and obstructions but im sure there is a way to get omething near to the actual water temperature without getting up. if there isnt then dangit ill just have to make one playing around with forumlas shouldn't be so hard, ive got all of summer. but i jsut want to know if one already exists... you know. i figure i could fit air temperature, average depth, wind speed and size to get a decent way to figure it out.