The Atmosphere and the Weather

The Layered Atmosphere

Earth's atmosphere contains several different layers that can be defined according to air temperature or chemical composition.

A brief look at air pressure and air density

Air molecules are held near Earth by the force of gravity.

This strong, invisible force pulling down on the air above squeezes air molecules close together, which causes their numbers on a given volume to increase.

The more air above a level, the greater the squeezing effect (or compression).

Since air density is the number of air molecules in a given space (volume), air density is greatest at the surface and decreases as we move up in the atmosphere.

Air molecules have mass and therefore weight. The weight of the air molecules acts as a force upon Earth. The amount of force excerted over an area of surface is called atmospheric pressure or air pressure.

As we climb in elevation, fewer air molecules are above us; hence, atmospheric pressure always decreases with increasing height.

Layers of the atmosphere

Troposphere:

Located from the surface of Earth to approximately 11 kilometers in altitude.

Contains about 75 % of the total mass of the atmosphere.

Maximum air temperature occurs near Earth's surface and drops with increasing height at a rate of approximately 6.5 degrees Celsius per 1000 meters.

Tropopause:

Isothermal layer that exists somewhere from 11 to 17 kilometers depending on season and location.

Layer in the atmosphere where the jet stream exists.

Stratosphere:

Extends from 17 to 50 kilometers above the surface.

Temperature generally increases with altitude because ozone gas molecules, present in this layer, absorb ultraviolet sunlight creating heat energy.

This layer of ozone is also called the ozone layer (depicted in yellow in the Figure on the left) which protects life from the harmful effects of the sun's ultraviolet radiation.

Stratopause:

Isothermal layer separating the mesosphere from the stratosphere.

Mesosphere:

The atmosphere reaches its coldest temperatures in this layer (about -90 degrees Celsius) at a height of approximately 80 kilometers.

Mesopause:

Isothermal layer above the mesosphere.

Thermosphere:

Final or topmost atmospheric layer.

Hottest layer in the atmosphere (can reach 1300-1800 degrees Celsius).

Heat is generated from the absorption of solar radiation by oxygen molecules.