WHY CANT WE SEE WIND

Air is defined as a mixture of nitrogen,oxygen,and minute amounts of other gases that surrounds the earth and forms it atmosphere.

Air is transparent to our eyes because we have evovled retinas that are sensitive to the wavelengths of light that pass through it unobstructed-otherwise we wouldn't be able to see anything at all.

Wind is just moving air, caused by pressure differences in the atmosphere. 

Low-pressure air is slightly less dense than high-pressure air and so has a lower refractive index, which affects the path of light rays slightly : stars "twinkle" in the sky because of tiny distortions in the atmosphere, but you can't observe the effect with ground level objects, because they are much nearer & your brain processes signals from your eyes to present as stable an image as possible. 

But you can see wind in indirect ways, from the movement of hair, trees & clouds. The funnel of a tornado is visible because the pressure drop in the center causes water vapour to precipitate.