Fan yourself with your hand. Feel that? Something is striking your face, but you can’t see what it is. That invisible substance is air. But what exactly is air, and what is its structure?

Air, like all matter, consists of atoms and molecules.  Because air is a gas, these atoms and molecules are not packed as tightly together as they would be in a solid or liquid. In fact, air is about 800 to 900 times less dense on average than solids or liquids. The average density of seawater, for example, is 1027 kg/m3, whereas the average density of air at sea level is 1.2 kg/m3. This difference means that there is 855 times more matter in one cubic meter of seawater than in one cubic meter of air.

The atoms and molecules in air are constantly moving. As they move, they spread out and mix evenly, ensuring that the atoms and molecules form a single, well-mixed substance.

All gases and gas mixtures, like air, lack a definite shape. If you’ve ever played with a balloon, blowing it up, twisting it or squeezing it, you have observed this important property of air. Like a liquid, a gas will always take on the shape of its container. This property allows air to move around Earth wherever the wind pushes it. This also makes it possible for convections to develop within weather systems.

 

The other important characteristic of air is that it lacks a definite volume. By reducing the volume of a parcel of air, you increase its pressure, and by increasing the volume of a parcel of air, you reduce its pressure. Atoms and molecules of air will fill the volume they are given.

 

Air’s lack of shape and indefinite volume, combined with the uneven heating of Earth, produce high- and low-density parcels of air. These parcels are known as high and low pressure areas in the atmosphere. Air moves from areas of higher density (higher pressure) to areas of lower density (lower pressure). It is the expansion, contraction, and movement of air from areas of high density to areas of low density that produces wind.