When ammunition sits in direct sunlight, the burn rate of powder is increased, resulting in greater muzzle velocity and a higher point of impact. A major key to consistency in your shooting is keeping aware of your environment and knowing how it's changes effect your shots. Humidity varies along with the altitude and temperature . The Sniper can encounter problems if drastic humidity changes occur in his area of operation or from the day or time he zeroed his weapon. If humidity goes up any time after the rifle is zeroed, then the impact of the bullet goes down.
Humid air is les dense which in turn gives the bullet less lift.