Ah, but there's more to it than the outside temperature. The rifle's chamber gets pretty hot after a few shots have been fired. When you shoot a cold round in a cold barrel, the velocity will be low. As the chamber heats up, the heat will transfer through the brass into the powder (only takes a few seconds). Now you have an initial cold temperature followed up by warm, then hot temperature. This extreme will make powder burn at a different rate. The hotter it is, the faster it burns. A 200 degree swing would not be unusual. As the powder changes in burn rate, velocity and accuracy can change radically.
Extreme rated powders like Varget and H4350 are designed to produce a more uniform burn rate across a wide temperature range. I've found Varget to be the best powder available for a 223 Rem when hunting prairie dogs. It's not unusual to pop off a dozen rounds in a fairly short time span. With other powders that produce very accurate groups when shots are spaced every 5 minutes, such as IMR 4895, the chronograph would measure very consistent velocities. With the same powder and the shots are spaced every few seconds, the velocity continues to increase. The chamber pressure also increases as is evidenced by primer flow and difficult extraction. Using Varget under the same conditions produces very consistent velocities and no sign of over pressure.
So, yes josebd, temperature can make a big difference. There is a reference section in Speer Manual #13 (page 719) that has a chart for non-extreme rated powder temperature vs velocity. The typical increase is 200 fps for a 100 degree temperature change. Considering the temperature of the chamber, this could easily double to 400 fps.
P.S. Don't use your "winter" loads in the summer. The increased powder charge coupled with higher temperatures could drive your loads beyond max limits.
