Notes of Atmosphere
Composition of the Atmosphere Nearly 99% of the volume of atmosphere is composed of various gases.
Nitrogen and oxygen are the most predominant gases. The major gases that form
the bulk of the volume of dry air are as follows: Nitrogen 78.088%, Oxygen 20.949%, Argon 0.930% and Carbon
dioxide 0.030%. The remaining gases are neon, helium, krypton, xenon, ozone,
hydrogen, methane and nitrous oxide. Together they form the remaining 0.003% of
the volume of air.
The troposphere is the lowest layer of
Earth's atmosphere. It starts at Earth's surface and goes up to a height of 7
to 20 km (23,000 to 65,000 feet) above sea level. Most of the mass (about
75-80%) of the atmosphere is in the troposphere. Nearly all of the water vapor
and dust particles in the atmosphere are in the troposphere. That is why most
clouds are found in this lowest layer, too. In fact most weather phenomenon
such as thunder, lightening, precipitation etc are observed in
troposphere. The bottom of the
troposphere, right next to the surface of Earth, is called the "boundary
layer". In places where Earth's surface is "bumpy" (mountains,
forests) winds in the boundary layer are all jumbled up. In smooth places (over
water or ice) the winds are smoother. The winds above the boundary layer aren't
affected by the surface much.
between the top of the troposphere and the stratosphere (the layer above it) is
called the tropopause. The height of the tropopause depends on latitude,
season, and whether it is day or night. Near the equator, the tropopause is about
20 km (65,000 feet) above sea level. In winter near the poles the tropopause is
much lower. It is about 7 km (23,000 feet) high. The jet stream is just below
The top of the stratosphere occurs at 50 km altitude.
Ozone is relatively abundant in the
stratosphere, at about 25 to 30 km from the surface of the earth. This portion
of troposphere is also called Ozonosphere.
It absorbs energy from incoming ultraviolet radiation from the Sun. In the lower stratosphere the
temperature is practically constant or increases slightly with altitude. Within
the ozone layer the temperature rises more rapidly, and the temperature at the
upper boundary of the stratosphere, almost 50 km above sea level, is about the
same as the temperature at the surface of the earth. Commercial jet
aircrafts fly in the lower stratosphere to avoid the turbulence which is common
in the troposphere below.
The stratosphere is very dry; air there
contains little water vapor. Because of this, few clouds are found in this
layer; almost all clouds occur in the lower, more humid troposphere. Polar
stratospheric clouds (PSCs) are the exception. PSCs appear in the lower
stratosphere near the poles in winter. They are found at altitudes of 15 to 25
km and form only when temperatures at those heights dip below -78° C.
starts at 50 km above Earth's surface and goes up to 85 km high. As you get
higher up in the mesosphere, the temperature gets colder. The top of the
mesosphere is the coldest part of Earth's atmosphere. The temperature there is
around -90° C (-130° F)!
Scientists know less about the
mesosphere than about other layers of the atmosphere. The mesosphere is hard to
study. Weather balloons and jet planes cannot fly high enough to reach the
mesosphere. The orbits of satellites are above the mesosphere. We don't have
many ways to get scientific instruments to the mesosphere to take measurements