The above is the most correct answer but an additional factor is that if you are flying into a reasonably brisk wind and then rapidly turn downwind you will tend to temporarily lose airspeed. This is due to the momentum of the aircraft being related to groundspeed whilst the windspeed goes from a positive to a negative value. A large groundspeed acceleration is required to recover the 'equilibrium' airspeed.
Some people will argue that this effect does not exist, saying that the airspeed will not vary (only true in light winds with gentle turns). If this effect did not exist, neither would windshear.
Doing tight low level field circuits in a heavy helo with a strong wind, the effect is very noticeable, requiring significant nose down and power to recover airspeed on turning downwind.
Point me at one authorititive text which confirms the "Down Wind Turn".
An aircraft flys with reference to the air around it. The fact that that air may itself be moving is completeley irrelevent.
You make the mistake because you have an erroneous idea as to what momentum is. Monentum is kinetic energy which is mass x velocity.
Ah! you will say, but velocity is relative to the earths surface.
No it isn't.
It is relative to whatever you want it to be relative to. For an aircraft in flight, the only momentum that matters is that relative to the air around it.Does a space ship in inter-steller space have to look back at the earths surface to find out it's velocity?
Look at it another way- the earth is turning at arond 600mph at the equator. Does this mean if an east-bound aircraft turn it starts flying backwards bacause of all the momentum it has due to the earths rotation?
Go flying some time on a windy day. Get some height, and by reference to the ASI alone, try and determine the wind dirsction- I assure you it won't be possible
Wind shear is a different thing. It occurs when the aircraft encounters a change in wind speed.