What causes wind?

Wind is simply air in motion, and it is driven by differences in air pressure, which in turn come from the sun heating the Earth unevenly. When the sun warms a patch of ground or ocean, the air above it heats up, expands, and becomes lighter, so it rises, leaving an area of lower air pressure at the surface. Meanwhile, cooler, denser air over adjacent areas has higher pressure. Air naturally flows from areas of higher pressure to areas of lower pressure, trying to equalize the difference, and that horizontal movement of air rushing from high pressure toward low pressure is what we feel as wind. The bigger the pressure difference over a given distance (what meteorologists call the pressure gradient), the faster the air moves and the stronger the wind, which is why steep pressure contrasts produce gales and gentle ones produce breezes.

The uneven heating that starts it all happens at every scale, which is why there are winds of every size. On a small local scale, a sea breeze forms on a sunny day because land heats up faster than water, so warm air rises over the land, and cooler air flows in off the sea to replace it. On a global scale, the equator receives far more direct sunlight than the poles, setting up vast pressure differences that drive planet-wide wind systems, like the trade winds and jet streams. The Earth's rotation adds a twist, literally: the Coriolis effect deflects moving air (to the right in the Northern Hemisphere, left in the Southern), which is why large weather systems and winds curve and spiral rather than flowing in straight lines, and why hurricanes and pressure systems rotate. Local geography matters too, mountains funnel and block wind, and temperature contrasts between surfaces channel it. So at its heart, wind is the atmosphere balancing itself: the sun heats the Earth unevenly, that creates areas of higher and lower air pressure, and air flowing from high to low pressure, bent by the planet's spin, is the wind, from a light breeze to a howling storm.