I have a fairly accurate (if approximate) sense of the outdoor temperature when I'm at work (making espresso drinks), despite not being able to see outside. The more iced drinks, the warmer it is, and an increase blended drinks (like those rather questionable smoothies-from-a-box) only really begin to sell when it gets into the 70s (Fahrenheit).
So far, I haven't been able to get hard data on the correlation between the proportion of cold drink sales and the outdoor temperature, so this formula is entirely hypothetical, based on my general impression rather than any comprehensive study of sales. Oddly enough, I actually have other things to do when at work... I suspect that k might vary by location - the Seattle standard for a hot day, I've found, isn't quite the same as that of Los Angeles.
s=smoothies and blended drinks
k=the drink-climate constant
And now I know the whole internet is just frantic to go get me a bulk of cafe sales data. Right? Right?