The movement of water is obvious and easy to observe, but this is not the case with air. We can feel air but cannot see it.

A flag moves in reaction to the movements of air, and its motion gives us an visual interpretation of the motion of air that we are otherwise unable to see.

This is part of the fascination of flags.

See Ipernity Group FLAG:

Vlaggen, kentekens, emblemen***Markierungsfahnen, Abzeichen, Embleme***σημαίες, διακριτικά, εμβλήματα***깃발, 기장, 상징***флаги, эмблемы***bandierine, distintivi, emblemi***drapeaux, insignes, emblèmes***旗、バッジ、紋章***旗子, 徽章, 象徵***banderas, divisas, emblemas