It’s already well known that Trump turning his head to his right at the last second likely prevented serious injury. But there’s another factor that needs to be taken into account: wind.

 At the time of the shooting (6:10PM), the wind in the area was blowing from the W at ~8 mph (image from met. Jim Eberwine):

 That W wind (light blue arrow in image below) was perpendicular to the trajectory of the bullet, which was N to S (purple arrow)(image from met. Jim Eberwine):

 As hunters know, a perpendicular wind affects the trajectory more than any other. In this case, the wind blew the bullet to Trump’s right. For a perpendicular wind check out the following wind drift chart for a 7 mm bullet: 

  This image is table 1 from here: https://www.backcountrychronicles.com/wind-drift-missed-shot/

 The gunman was ~135 yards from Trump. The wind drift was a little <1” per this table. This will vary for different bullets and other things. However, other tables still suggested ~1”.

 So, assuming the shooter didn’t adjust for wind drift, the wind likely blew the bullet ~1” to the left of where he was aiming or ~1” to Trump’s right. Thus, with the bullet just grazing his ear, the 8 mph W wind very likely altered the course of the bullet just enough to prevent a potentially much more serious injury! Had he not turned his head and had the wind been lighter or from a different direction, Trump very likely would have sustained a much more serious injury. So, a seemingly measly 8 mph W wind likely drastically changed history!