Reading wind direction and speed is something that is learned by experience and trial and error. On extended shots over open terrain the wind speed can and often does change in direction and speed over the distance the bullet must travel to impact the target. It's not at all uncommon to have a 3 mph. from the west at your position and a 6 mph. wind from the east at the target. Heat rising from the ground is also an important factor at extended range similar to shooting fish in the water with an arrow. Reflection and refraction must be weighed and considered before you drop the hammer. At 3 to 5 mph. wind will cause leaves to rustle slightly, at 5 to 7 there will be constant movement, at 8 to 10 the limbs will sway and anything above 10 mph. you better craw a bit closer unless you are experienced and very familiar with your rifle and bullet selection.