The Structure of the Atmosphere

The atmosphere is composed of nitrogen, oxygen, argon, water vapor, and a number of trace gases
(Table 1). This composition has remained relatively constant throughout much of Earth's history.
Chemical reactions maintain the ratios of major constituents of the atmosphere to each other. For
example, oxygen is released into the atmosphere by photosynthesis and consumed by respiration.
The concentration of oxygen in the atmosphere is maintained by a balance between these two
processes:
Photosynthesis: CO2 + H2O + light ® CH2O" + O2
Respiration: CH2O + O2 ® CO2 + H2O + energy

"CH2O" denotes the average composition of organic matter.
Almost all weather occurs in the troposphere, the lowest layer of the atmosphere, which extends
from the surface up to 8 to 16 kilometers above Earth's surface (lowest toward the poles, highest
in the tropics). Earth's surface captures solar radiation and warms the troposphere from below,
creating rising air currents that generate vertical mixing patterns and weather systems, as detailed
further below. Temperatures decrease by about 6.5°C with each kilometer of altitude. At the topof the troposphere is the tropopause, a layer of cold air (about -60°C), which forms the top of the
troposphere and creates a "cold trap" that causes atmospheric water vapor to condense.
The next atmospheric layer, the stratosphere, extends upward from the tropopause to 50 kilometers.
In the stratosphere temperatures increase with altitude because of absorption of sunlight by
stratospheric ozone. (About 90 percent of the ozone in the atmosphere is found in the stratosphere.)
The stratosphere contains only a small amount of water vapor (only about one percent of total
atmospheric water vapor) due to the "cold trap" and the tropopause, and vertical air motion in this
layer is very slow. The stratopause, where temperatures peak at about -3°C, marks the top of the
stratosphere.
In the third atmospheric layer, the mesosphere, temperatures once again fall with increasing altitude,
to a low of about -93°C at an altitude of 85 kilometers. Above this level, in the thermosphere,
temperatures again warm with altitude, rising higher than 1700°C.
The atmosphere exerts pressure at the surface equal to the weight of the overlying air. Figure 1 also
shows that atmospheric pressure declines exponentially with altitude—a fact familiar to everyone who
has felt pressure changes in their ears while flying in an airplane or climbed a mountain and struggled
to breathe at high levels. At sea level, average atmospheric pressure is 1013 millibars, corresponding
to a mass of 10,000 kg (10 tons) per square meter or a weight of 100,000 Newtons per square meter
(14.7 pounds per square inch) for a column of air from the surface to the top of the atmosphere.
Pressure falls with increasing altitude because the weight of the overlying air decreases. It falls
exponentially because air is compressible, so most of the mass of the atmosphere is compressed
into its lowest layers. About half of the mass of the atmosphere lies in the lowest 5.5 kilometers (the
summit of Mt. Everest at 8850 m extends above about roughly two-thirds of the atmosphere), and 99
percent is within the lowest 30 kilometers.

Atmosphere (Introduction)

Earth's atmosphere is a critical system for life on our planet. Together with the oceans, the
atmosphere shapes Earth's climate and weather patterns and makes some regions more habitable
than others. But Earth's climate is not static. How variable is it, and how quickly does it change? What
physical factors control climate, and how do they interact with one another?
To see how and why climate fluctuates, we need to learn about the basic characteristics of the
atmosphere and some physical concepts that help us understand weather and climate. This unit
describes the structure of the atmosphere and examines some of its key functions, including
screening out harmful solar radiation, warming Earth through the natural greenhouse effect, and
cycling carbon. It then summarizes how physical processes shape the distributions of pressures and
temperatures on Earth to create climate zones, weather patterns, and storms, creating conditions
suitable for life around the planet.
The atmosphere is a complex system in which physical and chemical reactions are constantly taking
place. Many atmospheric processes take place in a state of dynamic balance—for example, there
is an average balance between the heat input to, and output from, the atmosphere. This condition
is akin to a leaky bucket sitting under a faucet: when the tap is turned on and water flows into the
bucket, the water level will rise toward a steady state where inflow from the tap equals outflow
through the leaks. Once this condition is attained, the water level will remain steady even though
water is constantly flowing in and out of the bucket.
Similarly, Earth's climate system maintains a dynamic balance between solar energy entering
and radiant energy leaving the atmosphere. Levels of oxygen in the atmosphere are regulated
by a dynamic balance in the natural carbon cycle between processes that emit oxygen through
photosynthesis and others that consume oxygen, such as respiration. The strength of atmospheric
circulation is also controlled by a dynamic balance. Some parts of the planet receive more energy
from the sun than others, and this uneven heating creates wind motions that act to move heat from
warm to cold regions. (The process by which differential heating triggers atmospheric motion is
discussed below in Section 5, "Vertical Motion in the Atmosphere.")
Today human actions are altering key dynamic balances in the atmosphere. Most importantly,
humans are increasing greenhouse gas levels in the troposphere, which raises Earth's surface
temperature by increasing the amount of heat radiated from the atmosphere back to the ground.
The broad impacts of global warming are discussed in Unit 12, "Earth's Changing Climate," but it
should be noted here that climate change will alter factors that are key determinants of environmental
conditions upon which ecosystems depend. As the following sections will show, changing global
surface temperatures and precipitation patterns will have major impacts on Earth's climate and
weather.

Thermosphere

—  The thermosphere goes from 90km to 300km.

—  It’s the hottest layer in the atmosphere.

—  The Ionosphere is found in the thermosphere.

—  Auroras(natural light display

            in the sky ) occur in this layer.

n  Curtains of light called auroras occur in this layer.

The Ionosphere is found in the thermosphere(The ionosphere is defined as the layer of the Earth's atmosphere that is ionized by solar and cosmic radiation)

n  Auroras occur in this layer.

Charged particles from space collide with atoms and molecules.

Those atoms and molecules shed this excess energy by emitting photons of light, which we see as colorful auroral displays.

 

Mesosphere

—  The Mesosphere goes from 46km to 90km.

—  This layer is known as the

             Coldest layer of the atmosphere.

—  Radio waves are reflected

             back to earth in the mesosphere.

—  Mesopause is seen at the top of this

             layer.

  At mesopause temperature is about -90°C.

Thermosphere.

 

Stratosphere

—  The stratopause, where temperatures peak at about -3°C, marks the top of the

—  The stratosphere goes from 16km to 46 km.

—  The temperature increases

            with altitude.

—  Aeroplanes fly in this layer.

—  Protective Ozone is presented

             in this layer.

—  Stratopause is seen at the top of this layer.

What is ozone?

It’s a layer that protects the earth  from the ultraviolet radiation of the sun. At lower levels, ozone becomes a major pollutant.

From the fig we can observe that the harmful radiations coming to earth is absorbed by the ozone layer & due to the CFC ‘s coming from the earth surface will cause the destruction of that layer. Thus the U.V rays enter to the earth.