Weather

For the highly trained Sniper, the effects of weather are the main causes of error in the strike of the bullet. Wind, mirage, light, temperature, and humidity affect the bullet, the Sniper, or both. Some effects are minor; however, Sniping is often done in extremes of weather and all effects must be considered.

Wind classification

Wind poses the biggest problem for the sniper. The effect that wind has on the bullet increases with range. This is due mainly to the slowing of the bullet's velocity combined with a longer flight time. This allows the wind to have a greater effect on the round as distance increases. The result is a loss of stability.

Since the Sniper must know how much effect the wind will have on the bullet, he must be able to classify the wind. The best method is to use the clock system. With the Sniper at the center of the clock and the target at 12 o'clock, the wind is assigned the values: full, half, and no value. Full value means that the force of the wind will have a full effect on the flight of the bullet. These winds come from 3 and 9 o'clock. Half value means that a wind at the same speed, but from 1,2,4,5,7,8,10, and 11 o'clock, will move the bullet only half as much as a full-value wind. No value means that a wind from 6 or 12 o'clock will have little or no effect on the flight of the bullet.
 

Chart: Wind

Before adjusting the sight to compensate for wind, the Sniper must determine wind direction and velocity. He may use certain indicators to accomplish this. These are range flags, smoke, trees, grass, rain and the sense of feel. However, the preferred method of determining wind direction and velocity is reading mirage. In most cases, wind direction can be determined simply by observing the indicators.

Example: Reading wind with a range flag - A common method of estimating the velocity of the wind during training is to watch the range flag. The Sniper determines the angle between the flag and the pole, in degrees, then divides the angle by the constant number 4. The result gives the approximate wind velocity in miles per hour. If no flag is visible, the Sniper can use paper, grass, or some other light material held at shoulder level  The sniper then drops it and points directly at the spot where it lands.  He divides the angle between his body and arm by the constant number 4.

The most desired method of determining wind is by reading mirage. A mirage is a reflection of the heat through several layers of air at different temperatures and density. A Sniper can see the mirage as long as there is a difference in ground and air temperature. In order to see this mirage the Sniper must focus his spotting scope on an object at midrange, then place the scope back onto the target without readjusting the focus. As observed through the telescope, the mirage appears to move with the same velocity and direction as the wind, except when blowing straight into or away from the scope. Then the mirage gives the appearance of moving straight upward with no lateral movement. This is called a " boiling mirage." In general, changes in wind, up to 12 mph can be readily and accurately determined by observing the mirage.

Conversion of wind velocity to minutes of angle

All telescopic sights have windage adjustments that are graduated in minutes of angle or fractions thereof. A minute of angle (MOA) is 1/60th of a degree. This equals 1 inch at 100 meters (1.145 inches). Snipers use min. of angle (MOA) to determine and adjust the elevation and windage needed on the weapon's scope. After finding wind direction and velocity in MPH, the Sniper must then convert it into MOA using the following formula. This formula is a rule of thumb only and is used as a starting point. Practice and keeping a good data book are the ultimate goals and tools.

RANGE (1st digit) x VELOCITY (MPH) = MOA (for a full value wind)

CONSTANT ( for a half value wind divide this answer by 2)

The constant depends on the targets range:

100-500 meters "c"=15

600 meters "c" =14

700-800 meters "c"= 13

900 meters "c"= 12

1000 meters "c"=11

Chart: Wind

Effects of light

Light does not affect the trajectory of the bullet; however, it does affect the way the Sniper sees the target through the scope. This can be helped by practice and a good data book.
 

Effects of temperature

Temperature affects the shooter, ammunition, and air density. 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 general rule; note the air temperature when you zero your rifle, a 20 degree increase in temperature at any time after your zero will raise the point of impact by 1 MOA. A 20 degree decrease at any time after your zero will drop the bullet 1 MOA. A major key to consistency in your shooting is keeping aware of your environment and knowing how it's changes effect your shots.
 

Effects of humidity

Humidity varies along with the altitude and temperature. The military and most schools teach Humidity as a factor. In further testing and information gathering, it has been determined that for most sniper operations, humidity is a NON-factor. At 1000 yards, a 100% humidity change will make less difference then most rifles and shooters can correct for.

Barometric pressure IS important and often changes with the rise and fall of humidity, which may help explain why it was previously thought that humidity was a factor.