5.3  Wind Effects

5.3  Wind Effects

 A bullet’s reaction to a wind depends very much on both the speed of the wind and the direction in which it blows. In the first place, a headwind or a tailwind causes a vertical deflection (a change in drop) of the bullet, while a crosswind causes a horizontal deflection. It also happens that a crosswind causes a much larger deflection than does a headwind or tailwind of the same speed. As an example, take the .308 Winchester cartridge loaded with the Sierra 190 grain HPBT MatchKing bullet at a muzzle velocity of 2500 fps. At a range of 600 yards, a 5 mph headwind would cause the bullet to strike the target 0.30 inch lower than it would with no wind. A 5 mph tailwind would cause the bullet to shoot high by the same amount. In either case, a well-aimed shot certainly would stay within the X-ring on the 600 yard target. But a 5 mph crosswind would deflect the bullet 14.35 inches to one side, with a devastating effect on the shooter’s score if he hadn’t made a windage correction.

 This example is very typical of the relative importance of a crosswind as opposed to a headwind or tailwind. As we will show a little later, the effects of a headwind or tailwind can be neglected for almost all shooting situations, except perhaps for extra long range (1000 yard) target shooting in strong winds. Crosswinds, though, are important for hunting as well as target shooting. The crosswind deflections in the Ballistics Tables show how much crosswinds affect each Sierra bullet, and it is clear that both hunters and target shooters need to be aware of the importance of crosswinds.

 Although a crosswind has a strong effect on bullet flight, it turns out to be much less than what might be expected from rough reasoning. Actually, a flying bullet tends to “resist” the wind to a large degree. It does not simply follow the sideward motion of the air mass through which it flies. This is an important point to understand, and we will develop it further a little later.

 Of course, Mother Nature seldom obliges a shooter with a wind that blows only along or only across the line of sight to his target, so we are really concerned with winds from any direction. It turns out, though, that a wind from any direction can always be regarded as made up of two component winds, one parallel to the shooter’s line of sight and one perpendicular to it. Figure 5.3-1is an example of how this happens. The vector representing the true wind points in the direction in which the wind is blowing, and the length of the vector is scaled to the true wind speed. The crosswind component always has a direction perpendicular to the shooter’s line of sight, and the other component (headwind or tailwind component) is always parallel to the shooter’s line of sight. The two component vectors always are made just long enough so that, when they add together in the head-to-tail fashion shown in Figure 5.3-1 , they start at the tail of the true wind vector and end at its head. Then the lengths of these component vectors represent their speeds.

 The number table in Figure 5.3-1 can help you calculate the two component winds if you know the speed and direction of the true wind. Some well-equipped target ranges, like Camp Perry, Ohio, have wind meters which display this information. As an example of how to use the number table, suppose that the true wind speed is 10 mph and that it makes an angle of 35 degrees to the line of sight. Looking down the number table to 35 degrees, we see that the headwind component has a speed of .819 x 10 = 8.19 mph, and the crosswind component has a speed of .574 x 10 = 5.74 mph.

 This may all seem very complicated, but it is important to understand how a true wind is made up of the two physical components for the following reasons:
1. A components acting simultaneously.
2. Thnent causes a horizontal deflection.
3. Fod v ts!

 The following three subsections will describe these wind effects in a little more detail, and examples will be given to verify the key observations made in this introduction to wind effects. Before proceeding, we should note that it is possible to have vertical wind components, as well as headwinds, tailwinds, and crosswinds. Vertical winds are mainly encountered in hunting mountainous or hill terrain; they are very seldom important in target shooting. The effect of a vertical wind component is just like the effect of a crosswind component, except that it causes a vertical, rather than a horizontal, deflection. Hunters should be aware that vertical wind components happen in canyons and close to steep hillsides and that they can cause a bullet to shoot high or low, just as a crosswind will cause it to shoot left or right.