Temperature affects the firer, ammunition, and density of the air. When ammunition sits in direct sunlight, the bum rate of powder is increased, resulting in greater muzzle velocity and higher impact. The greatest effect is on the density of the air. As the temperature rises, the air density is lowered. Since there is leas resistance, velocity increases and once again the point of impact rises. This is in relation to the temperature at which the rifle was zeroed, If you zero your rifle at 50 degrees and you're now firing at 90 degrees, the point of impact rises considerably. How high it rises is best determined by recording past shots fired. The general role, however, is that when the rifle is zeroed, a 20-degree increase in temperature will raise the point of impact by one minute; conversely, a 20-degree decrease will drop the point of impact by one minute.
Humidity varies along with the altitude and temperature. You can always encounter problems if drastic humidity changes occur in the area of the shot. Remember, if humidity goes up, impact goes down; if humidity goes down, impact goes up. As a rule of thumb, a 20-percent change will equal about one minute, affecting the point of impact.
At least thats what my uncle always told me. He was a USMC Scout Sniper during Vietnam.