Besides mirage between you and the target, you have heat coming off the barrel.
Mirage is your friend, as it gives you an indication of wind direction and velocity.
With the lighter powered scopes, 8 - 12 X, you can offset the mirage caused by heat off the barrel with a barrel band (sometimes called a heat band, heat deflector, or other names).
With extremely high power scopes, even with a barrel band, you are still going to see heat from the barrel. This gives you a false reading.
Another problem with the higher power scopes is, as mentioned, is you wash out the target. Its gonna be "fuzzy" any way, you don't want it so you cant see it.
Why is the target fuzzy, even with a 8 - 10 power scope? Simple, you don't focus on the target, you focus the scope between 1/2 to 2/3s from you and the target. Doing that , you can use the mirage for what its for, that being determining wind direction and velocity.
When focused on the target, you loose the ability to read mirage. PLUS, if you screw up a bit, and get the scope focused beyond the target, you reverse the mirage. Meaning if you are reading a left to right mirage, but your scope is focused beyond the target, you end up with the mirage appearing to be running from right to left. That's gonna cost you big time.
I have no ideal why focusing beyond the target reverses mirage, but it does. I found that out the hard way when I attended the NGB All Guard Coaching Clinic. Couldn't figure out why everyone else was reading X MPH wind one direction and I was getting the same X MPH only the the other direction.
It's embarrassing when the stop shooting and gather all the would be coaches behind your target and use you as an example of what not to do.
I know we are discussing rifle scopes but I'll add a bit about spotting scopes.
When I was coaching the AK NG Rifle Team, I used a 100 MM team scope. It came with several eye pieces, from 16 - 32. 20 or 24 is the best for reading mirage in a spotting scope (remember this isn't mounted on the rifle). My idiot shooters, like some here figured bigger is better, and would stick in the 32X eye piece. I finely stuck in the 20X eye piece and trashed the others.
You can't see bullet holes at 1000 yards anyway, and it don't take much of a scope to see the 3 or 6 inch spotting disk used to mark the location of your bullet holes.
Others have different ideals. I'm not saying they are right or wrong, The way I do it, in NRA 1000 yard shooting is, I still use my spotting scope for reading conditions and use the 10X (mentioned above) for shooting. Even if you don't use the rifle scope for reading mirage, you still need a barrel band.
Last weekend I worked as an RO for another rifle forum's Sniper Challenge.
I use a cheap pair of Bushnell 10X50 field glasses for spotting hits or misses. Even though they are cheap, the are extremely clear. I could read mirage better then most of the shooters and their high dollar scopes. I could see a boil and know if the shooter was going to miss by shooting in a boil.
I also didn't have any problem seeing the 6 to 8 inch diamond targets we were using.
Never shoot in a boil, a boil doesn't mean "no wind", a boil means its shifting and shifting right now. There is no such thing as NO WIND.
If you're using a extremely high powered scope (like 40x someone mentioned) and its focused on the target, you're going to miss the boil. If its focused to read mirage, then you wont be able to see a 6 or 8 inch target at 1000 + yards.
This is easy to check. Take your spotting scope and observe a small target at 1000 or so yards. Lets say you have a 16-60 variable scope. Put it one 60X and observe the target. If its not washed out by mirage, you wont be able to read mirage. If its focused to read mirage at mid to two thirds range, you're target will be too washed out to see.