Topic of the Month

A headwind or a tailwind causes a vertical deflection in the bullet or change in drop, while a crosswind causes a horizontal displacement of the bullet on it’s way to the target. The vertical deflection caused by wind is usually small, except at long ranges. For this topic we will be discussing primarily the effect of crosswinds on the bullet.

Wind blowing at right angles to the bullet have the most effect while wind blowing at an angle to the bullet will have less effect, depending on the angle it approaches the bullet. We often give the wind direction as related to shooting across a clock face from 6 o’clock to 12 o’clock. The wind at 3 o’clock is blowing from the right at 90 degrees to the bullet while a wind at 4 o’clock approaches the bullet at an angle of 60 degrees from the right. A wind at 5 o’clock approaches the bullet at an angle of 30 degrees from the right. The same logic can be applied to the wind blowing from the shooters left. The wind at 9 o’clock is blowing from the left at 90 degrees to the bullet while a wind at 8 o’clock approaches the bullet at an angle of 60 degrees from the left. A wind at 7 o’clock approaches the bullet at an angle of 30 degrees from the left. The same methodology can be applied to headwinds at 10 o’clock and 11 o’clock or 1 o’clock and 2 o’clock.

Estimating wind deflection is nearly always a problem because it depends on wind velocity and wind direction. To make matters worse the wind velocity is rarely constant and often comes in gusts. It can also change direction from place to place. Target shooters can get a feel of the wind from flags set along the range and the hunter can judge the wind from the movement of tall grass or small branches on bushes. Experience and intuition can be a big help in doping and compensating for the wind. Wind speeds of 1 to 10 mph are usually not a big problem, but as the wind approaches 20 mph or more an accurate shot at the longer ranges become extremely difficult and require a high level of skill for success.

The wind displacement can be calculated mathematically if the information is available. The most common formula is represented by:

The factor (F) is the sine of the angle at which the wind approaches the bullet. For example the factor for a 90 degree crosswind is 1.0 since the sine of 90 degrees is 1.0. See Table I below for other factors.

The time of flight in a vacuum is the range distance (ft.) divided by the muzzle velocity (f.p.s.). The bullet velocity remains constant in a vacuum since there is no air to slow it down. The wind velocity is usually measured in m.p.h. To convert to f.p.s. multiply by 1.467.

The time of bullet flight in air is a little more difficult to determine unless you have a ballistic computer program to do this for you or you have access to Ingall’s ballistics tables or similar tables. Robert Rinker in his book "Understanding Firearm Ballistics" gives the following formula to estimate time of flight:

This formula requires only the basic information that can be obtained from either a chronograph or bullet manufacturers ballistic tables.

ANGLE OF APPROACH	FACTOR (F)
0 deg.	0.000
5	0.087
10	0.174
15	0.259
20	0.342
25	0.423
30	0.500
35	0.574
40	0.643
45	0.707
50	0.766
55	0.819
60	0.866
65	0.906
70	0.940
75	0.966
80	0.985
85	0.995
90	1.000

For an example, assume that a 150 gr. bullet is fired from a 30-30 Win. at 2330 f.ps. The bullet has a BC of 0.409 and a 30 mph tailwind is blowing at 30 degrees to the shooters left. What is the bullet displacement at 100 yards caused by the wind? Time of flight is 0.1344 sec.

Watch our web site for the next topic of interest "Terminal Ballistics." Until then, shoot safely and know where your bullets are going.

Topic of the Month: November 2001