Atmosphere -

The gaseous outer envelope of the earth. - click here for a picture  with explanations

It is divided vertically into

• the troposphere
• the stratosphere
• the mesosphere
• the thermosphere

The boundaries between these segments are places where the temperature changes

• In the troposphere the temperature decreases from Earth’s surface to a height of about 15-18 km.
• Temperature then starts to increase. The location of this change (the tropopause) separates the troposphere from the stratosphere. The stratosphere is from approximately 15 km to approximately 50 km.
• Between the stratosphere and the mesosphere is the stratopause. Above the stratopause temperature starts to decrease again.

The atmosphere is warmed by

1. radiation from the sun
2. radiation from the earth warmed by the sun
3. energy trapped from incoming solar energy by atmospheric gas molecules subsequently released as heat (the "greenhouse effect").

 Temperature on Earth’s surface is not uniform. It is hotter near the equator and colder at the poles.

Hot air expands and rises, cold air contracts and sinks. Overall hot air rises at the equator and moves poleward to colder regions and cold air sinks at the pole and moves toward hot regions. On its way toward the poles, hot air cools and sinks. On its way toward the equator the cold polar air warms and rises. This convection circulation is modified by the Coriolis force, which results in the path of any object moving above the surface of the solid earth being deflected to the right when viewed in the direction of its motion in the northern hemisphere (to the left in the southern hemisphere).

 The net result on global atmospheric circulation is six circulation cells - three in the Northern Hemisphere and three in the south. These are located approximately 0^o-30^o N and S, 30^o-60^o N and S and 60^o-90^o N and S.

Water vapor is transported upward with rising warm air. As the air cools the water vapor condenses and forms clouds and rain. As it condenses it also returns to the atmosphere as heat the energy it took to evaporate the water. Thus evaporation and condensation of water moves thermal energy around on earth.

 wind

red = warm, blue = cold

At the ocean surface ocean currents are driven by the winds and so have the same pattern as the winds, modified by interference by the continents.

Also in the ocean are currents driven by the temperature gradient in the oceans, cold at the poles and hot at the equator, and by density differences caused in part by varying salinity. The more saline and cold the water is, the denser. Dense water sinks, lighter water floats. This thermohaline (temperature and salt) circulation is considered by some to be a main control on climate.

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