| A | B |
|---|
| mixture | Two or more substances that are mixed together but not chemically combined. |
| element | A substance composed of a single kind of atom that cannot be broken down into other substances by chemical and physical means. |
| compound | A substance in which two or more elements are chemically joined in a set ratio. |
| air mass | A huge body of air that has similar temperature, pressure, and humidity throughout. |
| pressure | The amount of force pushing on a surface or area. |
| altitude | Elevation above sea level. |
| pressure | The amount of force pushing on a surface or area. |
| moisture | The amount of water vapor in the air. |
| humidity | A measure of the amount of water vapor in the air. |
| atmosphere | The mixture of gases that surrounds Earth. The outermost of the four spheres into which scientists divide Earth. |
| troposphere | The lowest layer of Earth’s atmosphere where weather occurs. |
| stratosphere | The second-lowest layer of Earth’s atmosphere; the ozone layer is located in the upper stratosphere. |
| mesosphere | The middle layer of Earth’s atmosphere; the layer in which most meteoroids burn up. |
| thermosphere | The outermost layer of Earth’s atmosphere. |
| temperature | The average amount of energy of motion in the molecules of a substance. |
