In order to understand how air particles move, it is first necessary to understand what air is made of and how it acts. While the exact composition of air differs at any given place and time, the components generally fall under two different categories: what we think of as the air itself and particles floating through the air. Air itself is composed of a mixture of atoms, mostly nitrogen and oxygen, with many other elements also present. These atoms compose most of the atmosphere and are always present, literally the fluid through which all other particles travel. The other category includes the larger particles that float through the air, heavier than the atoms but still very small, including water vapor and dust.
These other particles can again be divided into two categories: coarse and fine. Coarse particles are the largest that can stay afloat and the most visible to humans, such as smoke, pollen, and dust. Fine particles are much smaller, often only a molecule in size, and include many organic compounds and other chemical particles. The earth's gravity is trying to pull both fine and course particles down to the surface, just like it pulls everything with the gravitational produced by its mass, but air particles can still stay afloat because of their microscopic size. Even the largest particles like dust can stay up for days before finally getting pulled down, and finer particles may circulate in the atmosphere for years until they are chemically changed into another substance.
The reason these particles can float and move about is thermal motion, the process that controls most activity in the atmosphere. Essentially, the air is a gas and as a gas there is a lot of space between its atoms, which is why it is such a light fluid. This space gives the atoms and molecules plenty of room to gain energy and bounce back and forth off of its other. The warmer the air is, the more energy it has and the faster its atoms are moving, while cooler air has slower-moving atoms, but they are always constantly ricocheting against each other in far-flung vibrations.
Small particles are greatly effected by this constantly motion. As they float through the air they are being struck constantly by atoms which move, in an average building, at a speed of 500 meters per second. The force the vibrating gas elements exerts on the particles propel through the air, keeping them from falling to the ground in a manner similar to how the wind keeps a feather floating in the air. The random movement of air particles caused by the excited gas particles is known as Brownian motion, an entire branch of physics.
Because of thermal motion, certain areas of the atmosphere are high pressure and certain areas are low pressure, caused by differences in temperature and heat transfer. Low pressure areas tend to move particles upward more easily, while high pressure areas can have the opposite effect.
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