How to read the wind

SOME EL­E­MENTS OF RI­FLE SHOOT­ING ARE SCIENCE, SOME ARE ART. COR­RECTLY JUDG­ING WIND IN­VOLVES BOTH

Outdoor Life - - CONTENTS - BY BRAD FITZ­PATRICK

Are­cent uptick in the pub­lic’s fas­ci­na­tion with long-range shoot­ing has rev­o­lu­tion­ized hunt­ing. Not so long ago, el­e­va­tion ad­just­ments were holdover es­ti­mates based on bul­let drop as it re­lated to the an­i­mal’s body size; hold dead-on a white­tail buck at 200 yards, just un­der his spine at 300, and just over his back at 400, and you’re in busi­ness. Thank­fully, modern op­tics that in­cor­po­rate bal­lis­tic tur­rets, cus­tom bal­lis­tic apps, and rangefind­ers make el­e­va­tion ad­just­ments much sim­pler and far more ex­act, but es­ti­mat­ing windage is still a chal­lenge. Your rangefinde­r and tur­ret may give you the dope for el­e­va­tion, but you’ll still have to fig­ure out how to beat the breeze. When you’re shoot­ing an elk or deer across a windy canyon, you must deal with a mo­saic of ther­mals, cross cur­rents, and topo­graph­i­cal fea­tures that make plac­ing your bul­let ex­actly where you want it to strike a blend of science and art.

THE SCIENCE OF WIND DOP­ING

Un­der most field con­di­tions, you will dial for range and hold for wind. But how can you pos­si­bly know ex­actly how much windage is re­quired? The first tool at your dis­posal should be a wind dope chart that is spe­cific to your load. Based upon your bul­let’s ve­loc­ity and bal­lis­tic co­ef­fi­cient, and external fac­tors like el­e­va­tion, you should be able to de­ter­mine wind drift.

For ex­am­ple, us­ing an online bal­lis­tic cal­cu­la­tor, I in­put data for a stan­dard .308 Win. load with a 165-grain bul­let with a G1 bal­lis­tic co­ef­fi­cient of .435 and a muz­zle ve­loc­ity of 2,700 fps. The re­sults tell me that this par­tic­u­lar load will drift 2.9 inches at 200 yards in a 10 mph cross­wind. At 400 yards un­der the same con­di­tions, that bul­let drifts just over a foot. Windage val­ues are also given in MILS and MOA, so if you have sta­dia lines in your scope’s ret­i­cle, you can ad­just ac­cord­ingly. An anemome­ter will help you as­sess wind speed, but only at your lo­ca­tion. Cal­iber and bul­let se­lec­tion can help min­i­mize wind drift: Long, heavy-for­cal­iber bul­lets with high BCS will be less af­fected by wind.

THE ART OF READ­ING WIND

Learn­ing to read wind speed by ob­serv­ing en­vi­ron­men­tal fac­tors is a critical skill for shoot­ers. For starters, as­sess the wind’s ef­fect on grass and fo­liage at the tar­get. This will pro­vide not only an in­di­ca­tion of the wind’s speed but also its di­rec­tion. A 5 mph wind makes long grass sway and a 10 mph breeze will move more dense veg­e­ta­tion lightly.

Mi­rage is an­other vi­tal wind-es­ti­ma­tion tool. As heat waves rise above the ground, they drift with wind cur­rents, and the an­gle of mi­rage travel is in­dica­tive of wind speed and di­rec­tion. In light winds— less than 3 or 4 miles an hour—mi­rage drift will be min­i­mal. Be­tween 5 and 10 miles per hour, ex­pect to see the mi­rage an­gle at about 45 de­grees. And in winds above 12 miles an hour, the mi­rage will drift par­al­lel to the tar­get or dis­ap­pear al­to­gether. Don’t be fooled into read­ing wind speed just above the ground, though; eval­u­ate the wind’s ef­fect on mi­rage along the path of the bul­let’s flight. In­con­sis­tent mi­rage—which appears in your scope as a pat­tern of lines mov­ing at dif­fer­ing an­gles—is in­dica­tive of winds of vary­ing speeds be­tween your lo­ca­tion and the tar­get.

The key is not to iden­tify ex­act wind speeds but rather to learn to bracket winds—to rec­og­nize winds in the 0 to 5 mph range, 5 to 10, 10 to 15, and calculate your holds ac­cord­ingly.

Wind drift is most pro­nounced when cur­rents are mov­ing at a 90-de­gree an­gle to the bul­let’s path. A per­pen­dic­u­lar wind re­sults in max­i­mum drift; the im­pact of wind on bul­let place­ment lessens as the an­gle de­creases. In a zero-fac­tor wind (blow­ing di­rectly from the shooter to the tar­get, or vice versa), wind drift will be min­i­mal.

To be­come truly skilled in these wind-es­ti­mat­ing tech­niques, dou­ble check your guesses with a hand­held wind me­ter and by ob­serv­ing your ac­tual bul­let drift on tar­gets.

Tak­ing a read­ing in high winds dur­ing an an­te­lope hunt in South Dakota.

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