As we said earlier, each layer of the atmosphere has distinct
characteristics. There are 5 main layers within the atmosphere, which we will
discuss in turn. They are the troposphere, the stratosphere, the mesosphere, the
thermosphere, and the exosphere.
The troposphere is the lowest layer of the atmosphere. This is the layer where
most weather takes place. Most thunderstorms don't go much above the top of the
troposphere (about 10 km) . In this layer, pressure and density rapidly decrease
with height, and temperature generally decreases with height at a constant rate.
The change of temperature with height is known as the lapse rate. The
standard lapse rate for the troposphere is a decrease of about 6.5 degrees
Celsius (C) per kilometer (km) (or about 12 degrees F). Near the surface, the
lapse rate changes dramatically from hour to hour on clear days and nights.
Sometimes the temperature does not decrease with height, but increases. Such a
situation is known as a temperature inversion. Persistent temperature
inversion conditions, which represent a stable layer, can lead to air pollution
The other main characteristic of the troposphere is that it is
well-mixed. The name troposphere is derived from the Greek tropein, which
means to turn or change. Air molecules can travel to the top of the troposphere
(about 10 km up) and back down again in a just a few days. This mixing
encourages changing weather.
The troposphere is bounded above by the tropopause, a boundary marked as the
point where the temperature stops decreasing with height and becomes constant
with height. Any layer where temperature is constant with height is called
isothermal. The tropopause has an average height of about 10 km (it is
higher in equatorial regions and lower in polar regions). This height
corresponds to about 7 miles, or at approximately the 200 mb (20.0 kPa) pressure
level. Above the troposphere is the stratosphere.
The stratosphere is the layer above the troposphere, characterized primarily as
a stable, stratified layer (hence, stratosphere) with a large temperature
inversion throughout (see chart above). The main impact the stratosphere has on
weather is that its stable air prevents large storms from extending much beyond
The other main impact important to life deals with ozone. Ozone is the
triatomic form of oxygen that absorbs ultraviolet(UV) light and prevents it from
reaching the earth's surface at dangerous levels. The stratosphere contains the
ozone layer that has been such a hot topic as of late. The maximum
concentrations of ozone are at about 25 km (15 miles) above the surface, or near
the middle of the stratosphere. The interaction between UV light, ozone, and the
atmosphere at that level releases heat, warming the atmosphere and helping to
create the temperature inversion in this layer.
The stratosphere is bounded above by the stratopause, where the atmosphere
again becomes isothermal. The average height of the stratopause is about 50 km,
or 31 miles. This is about the 1 mb (0.1 kPa) pressure level. The layer above
the stratosphere is the mesosphere.
The mesosphere is the middle layer in the atmosphere (hence, mesosphere). There
are two key points about the mesosphere. First, temperature in the mesosphere
decreases with height. At the top of the mesosphere, air temperature reaches its
coldest value, around -90 degrees Celsius (or -130 degrees Fahrenheit). The
second point is that the air is extremely thin at this level. Over 99.9 percent
of the atmosphere's mass lies below the mesosphere. However, the proportion of
nitrogen and oxygen at these levels is about the same as at sea level.
The mesosphere is bounded above by the mesopause. The average height of the
mesopause is about 85 km (53 miles), where the atmosphere again becomes
isothermal. This is around the 0.005 mb (0.0005 kPa) pressure level. Above the
mesosphere is the thermosphere.
The thermosphere is a warm layer above the mesosphere. In this layer, there is a
significant temperature inversion. The few molecules that are present in the
thermosphere receive extraordinary amounts of energy from the sun, causing the
layer to warm. Though the measured temperature is very hot, if you exposed your
skin to the thermosphere, the perceived temperature would be very cold. Because
there are so few molecules present, there would not be enough molecules
bombarding your body to transfer heat to your skin. Temperature is a measurement
of the mean kinetic energy , or average speed of motion, of a molecule. So
although there are only a few molecules, each has a huge amount of kinetic
Above the thermosphere is the exosphere. Unlike the layers discussed
previously, there is no well defined boundary between the thermosphere and the
exosphere (i.e., there is no boundary layer called the thermopause).
The exosphere is the region where molecules from the atmosphere can overcome the
pull of gravity and escape into outer space. The atmosphere slowly diffuses into
the viod of space. The exosphere usually begins about 500 km up (notice, this is
well off the chart above), but there is no definable boundary to mark as the end
of the thermosphere and the beginning of the exosphere. Even at heights of 800
km, the atmosphere is still measurable. However, molecule concentrations are
very small and considered negligible.