This is a very old issue, in fact to combat the situation, many years ago Hodgdon's came up with "extreme rated" powders that ignite and burn very constant from zero deg to 125 deg F. At temperatures above or below the rated range, they still burn much more consistent than non-extreme rated powders.
Initially, it's the ambient air temperature that determines powder temperature but after a gun has been fired a few times, the chamber will heat up and transfer heat to the powder via the brass case. The same can happen if your ammo or gun is exposed to direct sunlight. Non-extreme rated powder will heat up and increase velocity, which in turn changes the bullet's point of impact. Changing velocities cause vertical stringing. Changes in velocities also affect bullet torque, which in turn affects horizontal stringing.
The solution ….. if you reload, use one of Hodgdon's extreme rated powders. If you shoot factory ammo, you are at the mercy of the ammo manufacturer. Here's a link to extreme rated powders …. click on "take the extreme tour" in any of the listed powders then click on "Continue", then finally click on "Foreward" for actual velocity variations due to temperature changes. Extreme Rifle Powders