The atmosphere is a blanket of air and moisture that surrounds Earth. Mostdense at sea level, where the molecules are pressed together by weight ofthe air above, the atmosphere becomes less dense as the height above sealevel increases. About 500 km up, there are hardly any molecules, and thevacuum of space begins. The atmosphere consists of several layers, eachwith distinct properties. All the layers are thicker above the equator thanabove the poles. The air at the equator is warmer, so it expands and takesup more space than the cold air at the poles.
The troposphere is theatmospheric level closest to the Earth’s surface. It contains most of theatmospheres moisture and is responsible for most of our weather systems. Where the troposphere ends there is a thin boundary called tropopause. Above the tropopause there is the stratosphere, a dry atmospheric layerthat contains higher concentrations of ozone than any other layer. Themiddle layer in the atmosphere is called the mesosphere. Above themesosphere is the thermosphere, where the density remains low but moleculeshave higher energy, producing higher temperatures than in the mesosphere.
The thermosphere is also called the “ionosphere” because in this layer,high-energy radiation from the sun causes particles to become electricallycharged ions. The ions produce the beautiful light displays called auroras,the Northern Lights and the Southern Lights. Above the thermosphere is theexosphere, the thin outermost layer of the atmosphere. Atmospheric pressureis the pressure the air exerts as gravity pulls it toward the center ofEarth.
It is the greatest at sea level, where the molecules are closesttogether. At higher altitudes, atmospheric pressure decreases. Pressuregradient is the measure of the amount the atmospheric pressure changesacross a set distance. Pressure gradients can be vertical or horizontal. By Christopher Morris
