A Scientific Basis For Evaluating Variable Crosswinds
A Scientific Basis for Evaluating Variable Crosswinds
By Paul Carter
Although we frequently discuss wind as if it is constant in both magnitude and direction, thatís seldom true in the real shooting world, where features of terrain make accurate wind doping much more challenging. For instance, itís possible that a significant wind experienced at the shooterís location may be completely blocked somewhere downrange by a hill or other physical entity. Of course, the converse can be true, also. In that event, the bulletís flight could be influenced by a wind which is present closer to the target but absent at the shooting location.
An interesting and practical wind-doping problem arises regarding the previously mentioned wind-blocked-by-hill scenarios. Specifically, when considering the amount of wind drift that must be corrected for, does it make any difference when the bullet experiences wind? In other words, is the bullet exposed to wind early in flight deflected more than, less than or the same amount as a bullet which starts its journey under calm conditions but is exposed to wind near the end of its flight?
In order to investigate this matter, letís establish some conditions. The range will be 1,000 yards and weíll be dealing with a 10-mph crosswind from 9 oíclock. In the first instance (Wind Early), weíll assume this wind is present for the first half of the bulletís journey (0-500 yards). Thereafter, the wind will be completely blocked. In the second scenario (Wind Late), the wind will be blocked until the bullet reaches the 500-yard mark. For the second half of its flight (500-1,000 yards) the bullet will be exposed to the crosswind.
At first glance, it might seem reasonable to assume that wind drift under both conditions would be equal. After all, the same bullet is exposed to a wind of the same strength and direction over the exact same distance. Unfortunately, itís not that simple! There are additional factors at play. Chief among them is the scientific concept of momentum. Once an object (bullet) is acted upon by a force (in this case the wind) and put into motion, it tends to stay in motion unless acted upon by another force. Sir Isaac Newton said it best: ďA body at rest tends to stay at rest; a body in motion tends to stay in motion.Ē When a bullet encounters a crosswind upon exiting the muzzle, that wind will cause the bullet to drift as long as the wind is acting upon the projectile. However, even if the windís influence is subsequently removed, the bullet will continue to move laterally, resulting in additional drift by the time it reaches the target.
To better illustrate this phenomenon consider a boat with a motor at a lake. The boat starts from a stationary position. Once the engine is running the boat begins to move in a particular direction. If the motor is turned off the boat does not come to an immediate stop. Instead, it continues to drift in the direction of travel. How far the boat travels is largely a function of the boatís speed and the frictional force exerted on the boat by the water which surrounds it. For a bullet in flight, the wind fulfills the same role as the boat motor, while the atmosphere replaces water as the medium of travel.
Returning to our 1,000-yard shooting experiment, letís do some calculations and see what conclusions can be drawn. For a baseline, letís start by calculating the expected bullet drift for a thousand-yard shot, where a 10-mph crosswind is acting upon the bullet over the entire range. Weíll be shooting a 185 grain Berger VLD bullet (B.C. = .569) out of a .300 Winchester magnum at a muzzle velocity of 2,865 feet per second. In order to calculate the drift, weíll need the time it takes for the bullet to travel 1,000 yards and an appropriate formula. From the Sierra Infinity ballistic software (Version 6), we find that the time of flight to travel 1,000 yards is 1.4376 seconds. A formula which describes the deflection of a bullet in a crosswind has been developed. In the absence of a headwind for a level-fire scenario, this formula is:
Where Z is the cross-range deflection in inches,
Vew is the cross-range wind velocity in inches/ second (1 mph = 17.60 inches/ second),
t is the bulletís true time of flight in seconds,
X in the range to the target in feet, and
Vo is the muzzle velocity of the bullet in feet/ second.
Plugging the relevant values into the equation 1, we get: