Fishtail Wind
A wind that switches back and forth near the six or twelve o'clock line, producing low deflection but proving very hard to time correctly.
A fishtail wind is one that swings back and forth across the line of fire near the six or twelve o’clock position, named for the way a flag or the grass appears to wag like a fish tail. Because the wind is blowing roughly head-on or tail-on, its wind value is low and the actual horizontal deflection on the bullet stays small. The challenge is not the size of the correction but the constant reversal of which way the small push is coming from.
These conditions frustrate shooters during wind reading because the direction can flip from a slight left bias to a slight right bias between shots, turning a tiny windage error into a visible spread on paper. Many precision shooters wait for the fishtail to pause at a repeatable angle, or watch the downrange mirage for a steady drift, before breaking the shot. Patience usually beats trying to chase every twitch of the flag.