structure of the atmosphere
envelope of gases that surround our Earth and allow life, as we know
it, to exist is called "the atmosphere". The current atmosphere
is the product of 4.6 billion years of changes in the geology of the
Earth coupled with the evolution of life in its many forms. This topic
investigates the physical structure and properties of the atmosphere,
while the topic The
composition of the Earth's atmosphere, investigates the origins
and gases of our atmosphere.
The atmosphere extends some 1000 km into space from
the Earth's surface. It is made up of five physical layers - the troposphere,
stratosphere, mesosphere, thermosphere and exosphere, each with distinctly
individual properties. Two other regions - the ionosphere and magnetosphere
- exist within these layers. Both are the product of interaction between
the Sun and Earth's atmosphere.
factor best distinguishes one layer from another - temperature. Each
layer behaves differently from its neighbours with respect to the temperature
and how temperature varies within the layer.
These layers and regions will now be investigated starting with the
layer closest to the Earth's surface, the one we are most familiar with
because we live in it - the troposphere.
The troposphere contains over half of the air in our atmosphere and
extends up to 20 km above
the Earth's surface at the equator. It is not the same depth in all
places, being only about 8 km thick at the poles.
All life and most of the weather, clouds and pollution exist in this
layer. The air temperature drops from an average of 20°C at the
surface to a low of -55°C as you rise through the troposphere.
the top of the troposphere to about 50 km, this layer contains the very
important "ozone layer". The tops of large thunderstorms and
dust from volcanoes can penetrate the lower part of the stratosphere,
but generally there is little interaction with the lower troposphere.
Highflying jets also travel in its lower reaches.
Temperature rises in this layer from -55°C at the interface with
the troposphere to 0°C near the top of the stratosphere.
From 50 km to 90 km, this 40 km thick layer again decreases in temperature
from 0°C to -90°C with altitude. Shooting stars or meteors,
burn up in this layer.
90 km to about 300 km, the thermosphere dramatically increases in temperature
with altitude, reaching 1200°C. The thermosphere is greatly affected
by interactions with the solar wind from the Sun and is where the spectacular
"Southern and Northern Lights" or auroras occur.
Reaching from 300 km to a 1000km or so, the exosphere is the less dense
part of the Earth's atmosphere. In this layer, the air is so thin that
satellites travel around the Earth, within its boundaries, without much
effect on their motion.
The temperature in the exosphere drops off dramatically to that of
near absolute zero in outer space.
The ionosphere is a region within the upper mesosphere and thermosphere
where solar radiation and particles in the "solar wind" strip
electrons from atoms in the atmosphere and create an electrically charged
zone of ions.
use the ionosphere to communicate long distances by radio. Radio waves
bounce off the
ionosphere in much the same way that light is reflected by a mirror.
The ionosphere changes its position from day to night, rising in the
night and lowering in the day. This is why radio signals from far off
places can be detected at night, while only close by transmissions are
detectable during the day.
Contained within the Earth's thermosphere, the magnetosphere is the
region where the Earth's magnetic field interacts with the charged particles
coming from the Sun in the solar wind. These particles become trapped
in the magnetic field of the Earth and circulate around the Earth following
the Earth's magnetic field. Large bombardments of these charged particles
cause the auroras in the ionosphere as they follow the Earth's magnetic
field through the upper atmosphere towards the magnetic poles.