| A | B |
|---|
| atmosphere | a mixture of gases that surrounds the Earth |
| nitrogen | the most abundant gas (78%) in Earth’s atmosphere |
| oxygen | the second most abundant gas (21%) in Earth’s atmosphere, produced mainly by plants |
| air pressure | the measure of force with which air molecules push on a surface |
| gravity | Earth’s atmosphere is held around the Earth by this. |
| altitude | the height of an object above the Earth’s surface |
| decreases | As altitude increases, air pressure does this. |
| increases | As altitude decreases, air pressure does this. |
| troposphere | lowest and densest layer of the atmosphere |
| stratosphere | the atmospheric layer above the troposphere; air is thin and contains little moisture |
| ozone | a gas molecule made of 3 oxygen atoms; absorbs ultraviolet radiation from the sun that can cause skin cancer |
| mesosphere | the coldest layer of the atmosphere |
| thermosphere | the uppermost layer of the atmosphere |
| exosphere | the atmospheric layer that is an extension of the thermosphere |
