5.3.3  Winds from Any Direction

5.3.3  Winds from Any Direction

 As explained earlier, a wind blowing from any general direction can always be regarded as made up of a crosswind and a headwind or tailwind. These components then cause both a horizontal and a vertical bullet deflection simultaneously. We already know what deflection to expect from each component wind acting alone, and we now want to see if any changes occur when both act together.

 We can clearly expect that when two wind components exist simultaneously, there will be some interaction between them to produce deflections that are at least a little different from those produced by each component acting alone. For example, we know from the previous subsection that the horizontal deflection caused by a crosswind depends on the time of flight. We also know from Section 5.3.1 that a headwind or tailwind causes a change in the time of flight. Therefore, we must expect that the horizontal deflection due to a crosswind acting together with a headwind or tailwind will be different from the deflection caused by the same crosswind acting alone.

 It turns out that this interaction is really quite small. Table 5.3-3 has been prepared to show this. It contains five of the bullets listed in Tables 5.3-1 and 5.3-2 . For each bullet it shows the vertical and horizontal wind deflections (at maximum range) for three wind conditions, (1) a 30 mph headwind only, (2) a 30 mph crosswind only, and (3) both acting together (a true wind of 42.4 mph blowing at 45 degrees to the line of sight).

 Consider the 150 grain FN bullet listed first in Table 5.3-3 . When this bullet is fired at 2200 fps muzzle velocity in a 30 mph crosswind alone, it has a horizontal deflection of 101.84 inches at 300 yards. If it were fired in a 30 mph headwind instead, it would shoot 2.03 inches lower at 300 yards. When both wind components act together, the bullet shoots 2.04 inches lower (instead of 2.03) and 106.43 inches to the side (instead of 101.84). The interaction of the two components therefore causes the bullet to shoot 0.01 inch lower and 4.59 inches farther to the side, but these are relatively small additional effects.

  Table 5.3-3 only shows the effects of the interaction at maximum range for each bullet entry. At shorter ranges these effects are much smaller. 

 Also, if a tailwind occurs instead of a headwind, the horizontal deflection will be decreased by about the same small amount when it interacts with the crosswind.

 Two conclusions may be drawn from Table 5.3-3 . First, when a crosswind interacts with a headwind (or tailwind), there is almost no effect on the vertical deflection caused by the headwind (or tailwind) acting alone. Second, the interaction does cause a change in the horizontal deflection, but it is small enough to be neglected for almost all shooting situations, unless the range is very long and the wind is very strong.

 Although the data in Table 5.3-3 are for a wind blowing at 45 degrees to the line of sight, the conclusions apply to winds blowing in any direction. In general, we can always separate a wind into its to components. Then we can calculate the vertical and horizontal deflections caused by each component independently, forgetting their interaction. Neglecting the interaction will cause an error in the vertical deflection which is totally negligible, and it will cause an error in the horizontal deflection small enough to be neglected in almost all shooting situations.