Layers in the Atmosphere
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.
(Click on image to view in separate window)
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
episodes as we will discuss in Session
6's Focus on Air Quaility.
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 tropopause.
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 energy.
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.