The atmosphere, or “skin” of gas that surrounds the Earth, consists of six layers that are differentiated by temperature:

1. The troposphere is the lowest level. It averages about 7 miles (11 kilometers) in thickness, varying from 5 miles (8 kilometers) at the poles to 10 miles (16 kilometers) at the equator. Most clouds and weather form in this layer. Temperature decreases with altitude in the troposphere.

2. The stratosphere ranges between 7 and 30 miles (11 and 48 kilometers) above Earth’s surface. The ozone layer, which is important because it absorbs most of the Sun’s harmful ultraviolet radiation, is located in this band. Temperatures rise slightly with altitude to a maximum of about 32°F (0°C).

3. The mesosphere (above the stratosphere) extends from 30 to 55 miles (48 to 85 kilometers) above the Earth. Temperatures here decrease with altitude to –130°F (-90°C).

4. The thermosphere (also known as the hetereosphere) is between 55 to 435 miles (85 to 700 kilometers) above Earth’s surface. Temperatures in this layer range to 2,696°F (1,475°C).

5. The ionosphere is a region of the atmosphere that overlaps the others, reaching from 40 to 250 miles (65 to 400 kilometers). In this region, the air becomes ionized (electrified) from the Sun’s ultraviolet rays. It is divided into three subregions: 1) the D Region (40 to 55 miles [65 to 90 kilometers]); 2) the E Region (also called the Kennelly-Heaviside layer) at 56 to 93 miles (90 to 150 kilometers); and 3) the F Region (93 to 248 miles [150 to 400 kilometers]), which is further separated into the F1 layer and the F2 layer (also called the Appleton layer), with the dividing line being at about 150 miles (240 kilometers) above sea level.

6. The exosphere lies above the thermosphere and includes everything above 435 miles (700 kilometers) high. In this layer, temperature no longer has any meaning.