Answer
If the wind at high altitudes is blowing faster toward the listener (compared to how it is blowing near the ground), this effectively speeds up sound at high altitudes. This will cause the waves to refract (bend) downward as shown in Figure 20.9b, making the sound more easily heard at long distances.
Wind conditions opposite to the ones listed (i.e., wind at high altitudes blowing slower toward the listener) would refract sound waves upward, away from the listener. This would make the sound less easily heard at long distances.
