Page 22 - Curriculum Visions Dynamic Book

The atmosphere
The Earth’s atmosphere is a mixture of nitrogen (78%) and oxygen (21%), with argon, water vapor, and carbon dioxide making up most of the rest. Notice that hydrogen and helium are both almost absent.
The shells of gases that envelop the Earth are more
dense than on some planets (for example, Mars) but far less dense than on others (for example, Jupiter). But the most extraordinary thing about them is that they contain water
in all of its states: solid, liquid, and gas. Furthermore, the gases are largely transparent to sunlight (although an ozone layer blocks harmful Ultraviolet radiation). So the heat energy in sunlight is not absorbed by the air but instead goes directly through it to heat the surface, both land and water.
Exosphere (above 600 km from the Earth’s surface). Air molecules are very rare at these levels, and helium is the most common gas.
Thermosphere (about 500 km thick). Extremely thin air. Readily absorbs ultraviolet radiation. Within this layer is the ionosphere, the place that bounces back medium (MW) and short (SW) rAdio wAves, allowing them to travel large distances around the world.
 The atmosphere is the envelope around the Earth that still contains a significant amount of gas. Only the lowest layer, the troposphere, has enough oxygen to support life.
Mesosphere (about 50 km thick). Transparent to the Sun’s rays. Temperature decreases with height.
Stratosphere (about 30 km thick). The air is very “thin” but contains important ozone gas. Temperature increases with height.
Troposphere (10–12 km thick—thickest over the equator, thinnest at the poles). The layer that contains the clouds. It
is mainly transparent to the Sun’s rays. The temperature decreases with height.
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