The atmosphere is a blanket of air and moisture that surrounds Earth. Most
dense at sea level, where the molecules are pressed together by weight of
the air above, the atmosphere becomes less dense as the height above sea
level increases. About 500 km up, there are hardly any molecules, and the
vacuum of space begins. The atmosphere consists of several layers, each
with distinct properties. All the layers are thicker above the equator than
above the poles. The air at the equator is warmer, so it expands and takes
up more space than the cold air at the poles. The troposphere is the
atmospheric level closest to the Earth's surface. It contains most of the
atmospheres 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 layer
that contains higher concentrations of ozone than any other layer. The
middle layer in the atmosphere is called the mesosphere. Above the
mesosphere is the thermosphere, where the density remains low but molecules
have 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 electrically
charged ions. The ions produce the beautiful light displays called auroras,
the Northern Lights and the Southern Lights. Above the thermosphere is the
exosphere, the thin outermost layer of the atmosphere. Atmospheric pressure
is the pressure the air exerts as gravity pulls it toward the center of
Earth. It is the greatest at sea level, where the molecules are closest
together. At higher altitudes, atmospheric pressure decreases. Pressure
gradient is the measure of the amount the atmospheric pressure changes
across a set distance. Pressure gradients can be vertical or horizontal.