ORIGINAL: Shoe
OK, I get it, you can choose any reference frame and the kinetic energy will be different. If your position is that kinetic energy can take on any value you choose, then it's not a very useful quantity. The concept of kinetic energy is only useful if you stick to one reference frame for your analysis. In the reference frame of the earth, the kinetic energy of the "windy glider" went up. I think you agree with that point. I think we also both agree that the extra kinetic energy had to come from somewhere. You are trying to dismiss the question without answering where that energy came from.
All correct, except the concept that darock cleared up about energy taking on any value I choose. It's of course dependent on the frame in question.
I meant no disrespect by sidestepping your question about where the energy comes from; I did that because I assumed you were trying to use that question to suggest that kinetic energy was "absolute". That was an assumption, and if incorrect, I apologize. In my defense, I did ask directly whether that was what you meant, but I wasn't sure I had received a direct answer. In any case, It sounds as if we all understand each other. I have enjoyed this thread.
The energy comes from the forces that cause the air to move over the surface of the earth (or the earth under the air
): coriolis, local heating (diurnal, thermal, from the sun, and the earth), pressure gradients, etc. You already know all that stuff.