For the long-range shooter, though, barometric pressure is a significant factor in making solid hits. Barometric Pressure, also called Atmospheric Pressure, is the amount of pressure present in the air at any given place at any given time. Think of it this way: the Earth is surrounded by a layer of gasses, called the "atmosphere." At sea level, this layer is at its thickest (from the ground to atmosphere's end), and this equates to lower barometric pressure, compared to the top of a mountain where the pressure is less (less air mass above the top of the mountain) and therefore creates higher barometric pressure.
Weather fronts moving in or out can also greatly affect barometric pressure.
Barometric pressure is often measured in inches. The higher the number, the higher the barometric pressure, and vice-versa.
What does all this mean for the long-distance shooter? Well, at a higher barometric pressure, a bullet experiences less resistance. Therefore, it will drop less than the same bullet moving at the same speed through air with a lower barometric pressure.
For example, consider a rifle chambered in 6.5 Creedmoor, the ammunition loaded with a 140-grain Hornady ELD Match bullet. Muzzle velocity is 2,710 feet per second, and the rifle is set with a 100-yard zero.