(a) Wind is moving–––––––– air. If there is any motion in the air, it can be called as wind.
(b) Near the earth's surface warm––––––– air rises up whereas cold––––– air comes down.
The reason for this is that the warm air is lighter than cold air.
(c) Air moves from a region of high––––– pressure to a region of low–––– pressure. Winds flow due to the pressure difference in the direction from higher pressure regions to lower pressure regions.