ATMOSPHERIC RIVERS

THEY’RE INVISIBLE, INEXORABLE. THEY’RE THE ONLY THING BETWEEN YOU AND A KILLING HIT. WELL, ALMOST.

BY WAYNE VAN ZWOLL

The canyon squeezed air like the roiling Snake funneled water through its chutes. Turbulence was here on the mountain too, in the bobs and flares of chokecherry leaves across the gulf. The buck read the currents. In short steps he descended the opposite slope: winks of eye, leg and antler in shadowed brush. He slipped into aspens on a bench. I was 350 yards away, prone, sling taut.

The river of wind between us was visibly stronger in the chokecherries than on my side. But there was nothing to signal air flow in the great middle, above canyon’s floor.

Leaf-shudder in the ‘cherries pulled my eye to the scope. The shoulder crease came clear. Sliding the reticle at back level, I paused it over the brisket, six inches to windward. At the Weatherby’s snap, the deer fell from sight. A stiff breeze barreling from the peaks hit me on the crosscanyon hike, so it was no shock to find the bullet had drifted twice as far as expected. Sent at 3,300 fps, the .257 Barnes had barely caught both lungs. A less capable cartridge, or another 100 steps for wind to work its mischief, and my hit would have been too far back.

Mountain landscapes suggest long shots. Looking across a drainage, you’ll spot game that would be hidden by brush or the earth’s curves, were it on your side. The most open shot, and the least urgent, is also commonly from one rise to the next. The stone in this pudding is distance. It magnifies error in range estimation and shot execution. And wind drift.

The effect of gravity on a bullet can be predicted. But wind is neither constant nor knowable. The influence of a given windor let-off, or reversal— is distance specific, because the bullet is decelerating. It spends more time aloft for any span the farther it gets from the muzzle.

The argument over where wind affects bullets the most has two correct answers. Drift, like drop, begins at exit, and its deviation from bore-line is irreversible. Distance increases the deviation. A bullet that meets a gust at 250 yards after cleaving still air has lost some speed to drag, so any wind has greater effect between, say, 250 and 300 yards than it would over the first 50 yards of flight. But a gust near the muzzle alters the flight path over its entirety. That bullet won’t “straighten up” or correct to fly parallel with bore-line, any more than a bullet deflected by a twig will then resume its original course.

The effect of wind on a bullet’s path is like that of gravity, applied laterally. A constant wind, like the tug of gravity, puts the bullet on a parabolic track. Unlike gravity, of course, wind is not constant, so that arc is unknowable. The best we can do is try to read the angle and intensity of wind at the time of the shot across the bullet’s path. Clouds, dust, smoke, rain, snow, grass, leaves, limbs—all that’s unanchored or supple—obeys wind. The most ballistically capable bullets are slave to it. Updrafts and downdrafts can add a vertical component, as does the bullet’s spin. My match rifle with right-twist rifling gives bullets a lift to 10-o’clock in a 3-o’clock breeze. Wind from the left sends those bullets to 4 o’clock.

Full-value wind, perpendicular to a bullet’s path, has greater effect than does oblique wind. Wind angling toward you from 10 or 2 o’clock, or flanking you from 4 or 8, must be reckoned with if the target is far away —say, beyond 250 yards—and wind speed over 15 mph. Unless the animal is very far, you can ignore all but gales from 11 to 1 o’clock, and 5 to 7. The cross-flight-path component of steeply oblique wind amounts to only about a third of full-value force.

Wind from behind you (6 o’clock) or in your face (12 o’clock) has negligible effect on the bullet over ordinary shot distances. It adds or subtracts insignificant pressure. A bullet exiting at Mach 3 meets terrific wind resistance regardless of wind conditions. It generates its own 2,000-mph head-wind, so even a 25-mph blow along the same path hardly registers. Only at extreme range will a stiff head-wind or tailwind demonstrably affect bullet flight. Unlike head-weighted arrows, bullets oriented to hit distant targets travel slightly nose-up until they lose stability. A descending bullet is like a football thrown long—though because more of the bullet’s weight lies in its tail than in its nose, it is less likely to “porpoise.” A head-wind gives slightly more push to the bullet’s belly than to its top; a tail-wind favors the top.

Once, having hit a mountain goat hard with an ‘06 bullet at about 270 yards, I watched in dismay as he put his big billy pants on and scrambled away and up dark rock toward glowering clouds. Second and third shots seemed on course, but didn’t faze him. Wind and the reports bouncing off the mountain nixed any sound of impact; the rock showed none. Prone, but now struggling with uphill aim, I began to doubt my range and wind estimates. The temptation to shift my hold was strong. Instead, I fed two more cartridges to the follower, shading high but hewing to center, laterally, into a facing wind. The billy fell.

Correcting for errant shots makes sense when you can see strikes. Not so much when you can’t. Time for new wind reads expires quickly after a hit!

Afield, this claptrap on head- and tail-winds is useless. But it confirms what wind is: a transient difference in air pressure. Fore and aft, you can ignore it. From the side, it causes drift.

I first met drift in smallbore rifle matches. Moving from indoor to outdoor ranges, I felt as if I’d been plucked from a hot-tub and dropped into the ocean. Flags and “windicators” (light weathercocks on ball bearings, with nose fans to show speed) pivoted to air currents at the firing line. But often the 50and 100-yard flags told another tale. The fan might hum lazily, tail to 2 o’clock, while flags hung limp at 50 and lifted to 9 o’clock at 100! That invisible gauntlet of air took bullets on a ziz-zag course. Firing during mixed signals was perilous. The patter of shots would die as savvy marksmen awaited decipherable signs. The clock, however, didn’t pause. Tricky conditions would bring a desperate fusilade as time ran out.

Light, capricious breeze was devilish. Barely detectable as a wiggle of mirage in a 20x scope, it could shove .22 bullets from the X-ring. A let-off was as deadly as a shift in direction or a gust. On “still” mornings, thermal currents would appear as mirage as air near the ground warmed. Shooters watched for a dominant condition, perhaps adjusting sights for center hits on “sighter” targets in a modest 3 o’clock flow. Woe to the poor sod who missed or ignored a left-running mirage that braked to a boil. Absent the wind’s nudge from 3 o’clock, his next bullet would hop right…. Land form directs air. It’s a lot like liquid. Canyons typically accelerate it—the “venturi effect” tapped in carburetor design. Air can be twisted in direction by hills and blocks of timber. Tall trees rake strong wind from the sky, so it splashes to the forest floor. One range on which I often competed is on a river-bank. Wind typically angles across the line from 7 or 8 o’clock, bounces off the bank, then hits the 100-yard targets from 4 o’clock. Shooters who minded only wind at the line would be ready to sell their rifles by the end of the match. Hunting, you won’t have a chance to learn the effect of local features on wind. But before a hunt, you can fire under different wind conditions in places where wind from various directions must negotiate various obstacles. Use paper targets, to show exact points of impact.

Zeroing in still conditions, holding the rifle as you expect to on a hunt, is a pious idea. That way, you have a point of reference from which to peg any shading or “hold-off” to windward. I prefer not to use the windage dial afield. I can shade 6 inches as accurately as I can estimate 6 inches of drift. Because wind is so changeable, running a dial to chase it can easily get you lost.

Hunting bullets drive through most wind without significant deflection at normal hunting ranges. Even a flatnose 170-grain .30-30 bullet drifts less than 2 inches in a 10-mph full-value wind at 100 yards. A 25-mph wind, strong enough to set the tree-tops wagging, shoves it only 4 inches. Pointed bullets from the likes of the

.270 and .30-06 fare better. Even gusts that lift small raccoons and trash can lids are best ignored for shots inside 150 yards or when the angle is acute. As with compensating for bullet drop, you can over-think the effect of wind. But as its arc becomes steeper the farther a bullet travels, so deflection increases at long range.

A 130-grain .270 bullet at 3,000 fps drifts only about 3/4 inch at 100 yards in 10-mph wind. But at 200 yards, it yields 3 inches to that wind—four times as much! At 300 yards it’s out 7 inches. At 400 it drifts 12 ½. Why? This bullet reaches 100 yards very fast, in about 1/10 second. (It doesn’t drop much at 100 either!) Between 100 and 200 yards it slows considerably, multiplying drift. Deceleration rate falls as range increases. The relationship of drift to distance is remarkably constant to 400 yards for loads popular with sheep hunters. This rule of thumb is easy to remember:

Given a 10-mph crosswind, allow for an inch of drift at 100 yards and thrice that at 200. Assume double the 200 drift at 300, double the 300 drift at 400.

Here’s how that rule works for a sleek 180-grain bullet sent at 2,960 fps from any of the popular .300 magnums:

The rule is darned near spot on! But won’t drift be greater if you’re using, say, a .30-06? Here’s how that same pointed 180-grain bullet measures up when sent at 2,700 fps from an ’06:

The rule still brings you within an inch of actual drift. It works for most pointed mid-diameter big game bullets hurled at 2,600 to 3,100 fps. For traditional lever-action loads for “deer cartridges” like the .30-30, allow 2 inches at 100; triple that at 200. Drift at 300 is about 2 ½ times the 200-yard drift.

Drift much beyond 400 yards is hard to pattern and more specific to the load. Drift for our .270 bullet (BC .460) at 500 yards is about 21 inches, or 65 percent greater than at 400. It drifts 31 inches at 600, about 33 percent more than at 500.

Vulnerability to wind depends on bullet velocity and ballistic coefficient. Bullets of similar BC show about the same drift. Nosler’s 130-grain .270 Partition has roughly the same profile as its 140-grain 7mm, 165-grain .308 and 210-grain .338. BCs range from .390 to .440. Launched at 3,000 fps, all drift about 6 inches at 200 yards in a 20-mph crosswind. Drop BC to .300, and you get half again as much drift with the same starting speed. Increase BC to .600 with a 140-grain Berger Hybrid OTM in a 6.5 PRC, and 200yard drift is only about 4 inches.

While a sharp nose contributes to a high BC and helps a bullet battle wind, it’s only one element of shape. The bullet’s shank and ogive—the section between tip and shank—matter a great deal. A short, lightweight bullet sent fast decelerates at a high rate despite a pointed nose because its sectional density (SD, the ratio of its weight in pounds to the square of its diameter in inches) is low. Such a bullet works as hard piercing air as does a longer (heavier) blunt bullet. A bullet that effectively resists drag also resists drift. That is, a bullet that best retains speed keeps the truest course through wind. In this example, speed quickly loses its 700-fps velocity edge, and its fight with drag. The slower 30-caliber bullet never drifts more, and after 300 yards, drifts less. Its greater weight and SD offset its relatively slow start.

30-mph blow that shoves the flat-base 7mm bullet 17 inches at 350 moves the boat-tail 15 ½ inches. A halfminute difference in 30-mph wind is hard to hold!

Truisms to keep in mind: Wind speed and drift are directly and forever linked. Double the wind speed and you double the drift. Halve wind speed, and you halve the drift. Changing wind angle changes drift proportionately. Change shot distance, and the math gets more complicated