The adiabatic lapse rate is the reason. Its 2°C per 1,000ft or 3.5°F. If the plane is cruising at 30,000ft that’s a temperature difference of 105°F from the plane to the ground.

Did some flying for awhile, gets cold up there.

Edit: Actually yeah, if the comment above that the plane was 37k-41k is correct. Let’s says 40k for easy math, thats a 140°F temp difference. If the temp at altitude is -52°F then it would be 88°F on the ground. Math checks out!

You tilt the mirror to align the top of what you’re shooting to the bottom of the compass. When you look at the compass in the mirror, that arrow will be pointing up, towards the object you’re shooting. Watch this video and imagine the arrow on your compass.

https://www.youtube.com/watch?v=99Ezr-8NTuU