Density of Air Vs. Temperature
- All matter, including the substances that make up the air people breathe, exists in three different phases: solids, liquids and gases. Solids are hard and dense, and have a rigid physical form. Liquids are less dense, and have no form, though their molecules still touch each other. Gases (such as air) are even less dense than liquids, and the molecules of gases are not bound to each other at all. The differences among solids, liquids and gases are due to temperature.
- Air is heated differently in different areas. Wherever the sun is shining, the ground is warming up. The warming ground in turn warms the air above it. Dark soil or rock tends to warm the air above it much better than ground covered by bushes. Open fields fall in between those extremes. Ground under shadow or at night cools down, cooling the air above it.
- Energy is always conserved. When photons of heat moving through the atmosphere are absorbed by air molecules, those air molecules add the photons' motion to their own. This means that when a gas like air absorbs heat, its molecules start moving faster. The hotter the air gets, the faster the air molecules move around.
- Because they are moving faster, they travel a greater distance in the same amount of time. This makes them spread out more. Because the same number of molecules fill a greater volume, the density of the hotter air has decreased. Density is, after all, just the ratio of the amount of matter to the amount of space it fills.
- This tendency of air to get less dense as it gets hotter and get more dense as it gets colder is what causes winds to form. As warm air rises, it pulls air from across the surface of the Earth to fill the space it leaves behind. As cold air sinks, it pushes the excess air out across the surface of the Earth. This constant motion of air as it warms and cools gives the Earth's atmosphere a complex, always-moving nature.