Composition and Structure of the Atmosphere

Atmosphere is a mixture of gases that surrounds the earth and is held in place by Earth’s gravitational force. It is this atmosphere that helps make life possible on earth. It reaches over 560 kilometers from the surface of the Earth. It is the atmosphere that provides us the air to breathe, shields us from most of the harmful ultraviolet (UV) radiation coming from the Sun, warms the surface of our planet by about 33° C (59° F) via the greenhouse effect, and prevents extreme differences between daytime and nighttime temperatures. The other planets in our solar system also have an atmosphere, but none of them have the same ratio of gases and layered structure as Earth’s atmosphere.

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Each of the planets in the solar system has an uniquely structured atmosphere. The atmosphere of Mercury is extremely thin and is somewhat similar to the vacuum of space. Jupiter, Saturn, Uranus and Neptune have very thick, deep atmospheres. Earth, Venus and Mars have much thinner atmospheres hovering above their solid surfaces.

Earth’s Atmosphere and it’s Composition

Earth’s atmosphere is composed of about 78% nitrogen, 21% oxygen, and 0.93% argon. The remainder, less than 0.1%, contains trace gases such as water vapor, carbon dioxide, neon, helium, hydrogen, etc. and ozone. All of these trace gases have important effects on Earth’s climate. There are also many small particles – solids and liquids – “floating” in the atmosphere. These particles, which scientists call “aerosols”, include dust, spores and pollen, salt from sea spray, volcanic ash, smoke, and more. The composition of the atmosphere is not static and it changes according to the time and place.

Even though nitrogen gas, N2, it is unavailable to most living things, it is needed and used by living things to make proteins. Nitrogen dilutes oxygen and prevents rapid or instantaneous burning at the Earth’s surface, as oxygen gas is a necessary reactant of the combustion process. Oxygen is used by all living things to make molecules that are essential for life. It is also essential for aerobic respiration as well as combustion. Argon is a non-reactive gas and is used in light bulbs, double-pane windows etc. Carbon dioxide is used by plants for photosynthesis and also acts as a blanket that prevents the escape of heat into outer space. Ozone acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth. Dust particles are found in the form of sand, smoke-soot, oceanic salt, ash, pollen, etc. generally in the lower layers of the atmosphere. These dust particles help in the condensation of water vapor.

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Layers of Atmosphere

The atmosphere grows thinner as one moves upward from Earth’s surface and gradually gives way to the vacuum of outer space. The atmosphere can be divided into five vertical layers based on thermal characteristics (temperature changes), chemical composition, movement, and density. Each layer has characteristic temperatures, pressure, and phenomena. The five layers are:

1. Troposphere
2. Stratosphere
3. Mesosphere
4. Thermosphere
5. Exosphere

The layer closest to the surface is the troposphere where life exists. It extends from the surface of the earth up to roughly 18 km above the surface. The thickness of the troposphere is greatest at the equator because heat us transported to great heights by strong convectional currents. It contains over 80% of the atmospheric mass and nearly all the water vapor. It is the layer where clouds are found and almost all weather occurs. The troposphere is the wettest layer of the atmosphere. The boundary between the troposphere and the stratosphere, the next layer, is called the tropopause.

The next layer, the stratosphere, contains most of the atmosphere’s ozone, which absorbs high-energy radiation from the sun and makes life on the surface possible. It extends from the tropopause up to about 50 km to 53 km above the Earth’s surface depending on location. The stratosphere is very dry air and contains little water vapor. Some jet aircraft fly in this layer which contains the jet streams and the ozone layer. The ozone, which is relatively abundant in the stratosphere, heats this layer as it absorbs energy from incoming ultraviolet radiation from the Sun. The top of the stratosphere is bound by a boundary known as the stratopause.

Above the stratosphere is the mesosphere. It extends to about 85 km above the Earth’s surface. Temperatures reach the lowest in the mesosphere, because there are almost no air molecules there to absorb heat energy. Temperature decreases with height throughout the mesosphere. The sky also changes from blue to black in the mesosphere, because there are so few molecules for light to refract off of there. Meteors or falling stars occur in this layer. Most meteors vaporize in the mesosphere. Some material from meteors lingers in the mesosphere, causing this layer to have a relatively high concentration of iron and other metal atoms. The upper limit of the mesosphere is known as mesopause.

The thermosphere is the farthermost layer. It is the widest layer of the atmosphere lying between about 85 km and 600 km. It absorbs much of the harmful radiation like X – ray, ultraviolet radiation etc. that reaches Earth from the Sun. Because of this absorption, the temperature in the thermosphere increases with height and can reach as high as 2,000°C (3,600°F) near the top depending on solar activity. This layer contains electrically charged particles known as ions, and hence, it is known as the ionosphere.

The exosphere is the uppermost region of Earth’s atmosphere. This layer represents the transition from Earth’s atmosphere to space. Some scientists think that the exosphere is just part of space while some other scientists consider the exosphere part of our planet’s atmosphere. Gases are very sparse in this sphere due to the lack of gravitational force. Therefore, the density of air is very less here.

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The last two layers, mesosphere and thermosphere, include regions of charged atoms and molecules, or ions. The upper mesosphere and lower thermosphere are called the ionosphere. This region is important to radio communications, because radio waves can bounce off this layer and travel great distances.

Quiz on Atmosphere

This gas makes up 78% of the atmosphere.

a) Nitrogen

b) Oxygen

c) Carbon dioxide

d) Water vapor

Ans: a) Nitrogen

The layer of the atmosphere that contains the ozone is ____________.

a) Troposphere

b) Stratosphere

c) Mesosphere

d) Thermosphere

Ans: b) Stratosphere

The layer of the atmosphere where the weather occurs is _____________.

a) Troposphere

b) Stratosphere

c) Mesosphere

d) Thermosphere

Ans: a) Troposphere

In which layer do auroras (e.g. northern lights) occur?

a) Troposphere

b) Stratosphere

c) Mesosphere

d) Thermosphere

Ans: d) Thermosphere

What frequencies of electromagnetic radiation are absorbed by the earth’s ozone layer?

a) Ultra violet rays

b) Infra red rays

c) Radio waves

d) Visible lights

Ans: a) Ultra violet rays

What instrument is used to measure air pressure?

a) Thermometer

b) Voltmeter

c) Barometer

d) Seismograph

Ans: c) Barometer

Which of these is not part of the atmosphere?

a) Troposphere

b) Stratosphere

c) Mesosphere

d) Ionosphere

Ans: d) Ionosphere

In the troposphere, temperature _______________ with altitude.

a) increases

b) decreases

c) remains constant

d) changes randomly

Ans: b) decreases

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