Why angle of attack is always shown against the relative wind parallel to horizon? I have a question about Stall, which I have difficulty to understand.
According to the theory, stall happens when :
1-  Speed is slow then a certain limit.
2-  Angle of attack is greater than a certain limit.
In the case of angle of attack, there is being spoken about relative wind along the wings.
But here is my confusion.
Relative wind is always shown parallel to the horizon, which strikes the wings.
While in my understanding, in the sky there is equal air and wind everywhere. We create our own hard wind for our wings, by moving fast through the air. Right?
So it should not make any difference in which direction we move, along the horizon, or at a steep angle upwards or downwards, relative to the horizon. The relative hard wind will be created straight in opposite direction where we move fast. Because we are also moving in the same direction as our wings angle of attack.
As I have shown with blue line in my illustration above.
If so, then there is no question about angle of attack at all. Because we always create our own wind by moving fast in any direction in the sky.
It would be different if we are moving parallel to the horizon, but only our wings have a greater angle of attack relative to the fuselage and the horizon. (As it is often explained in the example of, hand out of a car. While car is moving horizontally, but only the angle of our hand changes relative to the car). Then this stall theory is understandable.
If we look at the fighter jets and acrobatic airplanes, then we see that they can climb vertically up against the horizon, at 90 degree bank angle and fly upside down too. And they do no stall, because they create their own relative straight opposite wind by moving fast in any direction in the sky.
So why in normal planes we do have to think about straight only horizontally coming relative wind and accordingly the angle of attack against it?
I think this has been at least hinted at in other answers, but, just to put it succinctly, the diagrams aren't actually intended to show the airflow as always being parallel to the horizon. The airstream is just shown flowing along the horizontal axis of the graph because it's convenient to illustrate it that way. There is no implication intended in the diagram that the horizontal axis of the diagram is actually parallel to the Earth's surface.
The behavior of the airfoil is exactly the same regardless of the actual orientation of the wing and airflow relative to the horizon. All that matters is how the airfoil is oriented relative to the airflow. You can rotate those diagrams any way you want relative to the horizon and what they depict will remain true.
The blue airflow lines you've drawn on the diagrams seem to be assuming that the airplane is always climbing at an angle roughly equal to the angle between the horizon and the wing's chord line, but this is not true. You can fly with a high pitch-up angle while maintaining level flight if you fly slowly enough, for example. You will practice this (called &quot;slow flight&quot;) when training for a private pilot certificate. You can (and often do) even descend with a nose-up attitude. And, if you put the nose up too far, you can descend very rapidly with a nose-up attitude after your wings stall. For example, Air France 447 was falling 10,000 feet per minute with a nose-up attitude as one of the pilots held the stick back.