Like KNOWSALOT said you should be making more power, running faster, when it is colder. Could it possibly be someting else, maybe the track isn't hooking up as well. Check your slips and see if your times fall consistantly. You probably get three times see if they fall at the same rate. Check your trap speed against the day time speed, you might be making more power thus loosing some grip on the start. If this is the case you might be able to compinsate by an air pressure change. You may have already done this but I am just trying to cover anything you might of missed.

 

It seems that if I don't run 5 psi in my Extreme haulers they don't bite as hard.
I should know better but I didn't check my air pressure after it got cooler & that causes a pressure loss. I did reduce pressure after it had set in the sun between rounds & it had gained 3/4 of a pound. I'll keep a closer eye on tire press next time!

 

Ward 660R seems to have hit it on the head. I have made close to 1000 passes in the last couple of years in my Mustang. The colder air definitely yields more horsepower with everything else equal. However E.T.s don't always drop accordingly. The MPH will tell whether you are making more horsepower, but e.t.s win the race. In hotter weather the track usually hooks better and produces better e.t.'s than in colder weather. If you can get equal traction in both conditions then the colder air should produce better e.t.'s. Ideal conditions are cool air, low humidity, high pressure, and a tail wind.