Asked by: Brian Atkinson, Bromley
Air is transparent to our eyes because we have evolved retinas that are sensitive to the very 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 the tiny distortions in the atmosphere, but you can’t observe the effect with ground level objects, because they are much nearer and 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 and clouds. The funnel of a tornado is visible because the pressure drop in the centre causes water vapour to precipitate.