On a warm, sunny day, you'll see heat waves, or mirage, shimmying off the ground. Mirage is especially pronounced on flat, hard surfaces that collect heat, such as pavement and steel rails. Fire your rifle repeatedly, and mirage from the heated barrel will distort your sight picture.
Watch distant mirage through your spotting scope or high-powered rifle-scope, and you'll see it "run" or "boil." The flatter the mirage, the stronger the wind and the greater your bullet deflection. When the waves get tall and slow, deflection will be less. Boiling mirage appears to climb straight up. A boil may indicate a let-up in the wind, or a reversal. In either case, if you shoot during a boil when zeroed for a breeze, you'll miss to the leeward side. If a reversal occurs while you're squeezing the trigger, you'll miss badly.
Mirage is most useful as a wind indicator, but it can also visually displace distant targets. In heavy mirage you'll be aiming at a bullseye that's not where you see it. A zero correction may be in order, if such a condition is expected to remain during a match.
Savvy shooters focus their spotting scopes at mid-range, so they can read mirage not only at the target where the rifle-scope is focused, but halfway along the bullet's path, where conditions can be quite different. A bullet's course "bent" by wind near the muzzle does not re-align with the bore unless an opposite wind downrange nudges it back.