PLANET EARTH AND ITS ATMOSPHERE

Earth is the only planet in the solar system with an atmosphere that can sustain life. The blanket of gases not only contains the air that we breathe but also protects us from the blasts of heat and radiation emanating from the sun. It warms the planet by day and cools it at night.

Earth's atmosphere is about 300 miles (480 kilometers) thick, but most of it is within 10 miles (16 km) the surface. Air pressure decreases with altitude. At sea level, air pressure is about 14.7 pounds per square inch (1 kilogram per square centimeter). At 10,000 feet (3 km), the air pressure is 10 pounds per square inch (0.7 kg per square cm). There is also less oxygen to breathe.

COMPOSITION OF THE ATMOSPHERE

Compound	Distribution
Nitrogen	78%
Oxygen	21%
Argon	0.9%
Water vapour	0.4% (around 1% at the surface)
Carbon dioxide	0.03%

Atmosphere layers

Earth's atmosphere is divided into five main layers, the exosphere, the thermosphere, the mesosphere, the stratosphere and the troposphere. The atmosphere thins out in each higher layer until the gases dissipate in space. There is no distinct boundary between the atmosphere and space, but an imaginary line about 68 miles (110 kilometers) from the surface, called the Karman line, is usually where scientists say atmosphere meets outer space.

The troposphere is the layer closest to Earth's surface. It is 4 to 12 miles (7 to 20 km) thick and contains half of Earth's atmosphere. Air is warmer near the ground and gets colder higher up. Nearly all of the water vapor and dust in the atmosphere are in this layer and that is why clouds are found here.

The stratosphere is the second layer. It starts above the troposphere and ends about 31 miles (50 km) above ground. Ozone is abundant here and it heats the atmosphere while also absorbing harmful radiation from the sun. The air here is very dry, and it is about a thousand times thinner here than it is at sea level. Because of that, this is where jet aircraft and weather balloons fly.

The mesosphere starts at 31 miles (50 km) and extends to 53 miles (85 km) high. The top of the mesosphere, called the mesopause, is the coldest part of Earth's atmosphere with temperatures averaging about minus 130 degrees F (minus 90 C). This layer is hard to study. Jets and balloons don't go high enough, and satellites and space shuttles orbit too high. Scientists do know that meteors burn up in this layer.

The thermosphere extends from about 56 miles (90 km) to between 310 and 620 miles (500 and 1,000 km). Temperatures can get up to 2,700 degrees F (1,500 C) at this altitude. The thermosphere is considered part of Earth's atmosphere, but air density is so low that most of this layer is what is normally thought of as outer space. In fact, this is where the space shuttlesflew and where the International Space Station orbits Earth. This is also the layer where the auroras occur. Charged particles from space collide with atoms and molecules in the thermosphere, exciting them into higher states of energy. The atoms shed this excess energy by emitting photons of light, which we see as the colorful Aurora Borealis and Aurora Australis.

The exosphere, the highest layer, is extremely thin and is where the atmosphere merges into outer space. It is composed of very widely dispersed particles of hydrogen and helium.

Climate and weather

Earth is able to support a wide variety of living beings because of its diverse regional climates, which range from extreme cold at the poles to tropical heat at the Equator. Regional climate is often described as the average weather in a place over more than 30 years. A region's climate is often described, for example, as sunny, windy, dry, or humid. These can also describe the weather in a certain place, but while the weather can change in just a few hours, climate changes over a longer span of time.

Earth's global climate is an average of regional climates. The global climate has cooled and warmed throughout history. Today, we are seeing unusually rapid warming. The scientific consensus is that greenhouse gases, which are increasing because of human activities, are trapping heat in the atmosphere.

OTHER LAYERS AND BOUNDARIES OF THE ATMOSPHERE

Ozone layer

It is contained within the stratosphere at about 10 – 50 km above the Earth’s surface

About 90% of the ozone layer is present in the stratosphere

The ozone layer absorbs 93-99% of harmful ultraviolet light

Ozone is formed when UV light strikes oxygen in the stratosphere to split the oxygen atoms, which then reform as ozone

The ozone layer was discovered by the French physicists Charles Fabry and Henri Buisson in 1913

British meteorologist GMB Dobson established a worldwide network of ozone monitoring stations between 1928 and 1958 that continues to operate today. He also developed a spectrophotometer (called the Dobsonmeter) to measure stratospheric oxygen from the ground. The Dobson unit, a measure of ozone density is named in his honour

Ionosphere

Stretches from the thermosphere to the exosphere (100 km – 700 km)

This is caused due to ionization by solar UV radiation

Responsible for radio propagation by reflecting radio waves back to the Earth’s surface thereby enabling long-distance communication

Plays an important part in atmospheric electricity (like lightning)

Responsible for auroras

Homosphere and Heterosphere

Homosphere is the part of the atmosphere where gases are well mixed due to turbulence

This includes the troposphere, stratosphere and mesosphere

Heterosphere is the part of the atmosphere where gases are not well mixed

This usually happens above the turbopause (100 km) where distance between particles is large due to low density

This causes the atmosphere to stratify with heavier gases like oxygen and nitrogen present in the lower layers and lighter gases like hydrogen and helium in the upper layers

Planetary boundary layer

Part of the troposphere closest to the Earth’s surface and most influenced by it

Friction with the earth’s surface causes turbulent diffusion

Ranges from 100 m to about 2 km

Magnetosphere

A mix of free ions and electrons from solar wind and the Earth’s atmosphere

It is non-spherical and extends to more than 70,000 km

It protects the Earth from harmful solar winds

Mars is thought to have lost most of its former oceans and atmosphere to space due to the direct impact of solar winds. Similarly Venus is thought to have lost its water due to solar winds as well

Karman line

Defines the boundary between the Earth’s atmosphere and outer space

Lies at an altitude of 100 km above mean sea level

At this altitude the atmosphere becomes too thin for aeronautical purposes

However, there is no legal demarcation between a country’s air space and outer space

Van Allen Belt

It is a region of energetic charged particles (plasma) around the Earth held in place by the Earth’s magnetic field

Extends from about 200 km to 1000 km

Has important implications for space travel because it causes radiation damage to solar cells, integrated circuits, sensors and other electronics

PHYSICAL PROPERTIES OF THE ATMOSPHERE

Pressure and thickness

Atmospheric pressure at sea level is 1 atmosphere (around 14.7 psi)

50% of atmospheric mass is below an altitude of 5.6 km

90% of atmospheric mass is below 16 km

99.99% of atmospheric mass is below 100 km

Density and mass

Atmospheric density decreases with height

Density at sea level is about 1.2 kg/cu.m

OPTICAL PROPERTIES OF THE ATMOSPHERE

Scattering

When sun’s rays pass through the atmosphere, photons in light interact with the atmosphere to produce scattering

Eg: on overcast days there are no shadows because light reaching the surface is only scattered, indirect radiation, with no direct radiation reaching the earth

Scattering is responsible for blue appearance of the sky, and for red appearance of sunset

Absorption

The atmosphere absorbs radiation of different wavelengths, allowing only certain ranges (UV to IR) to pass on to the earth’s surface

Emission

The atmosphere absorbs and emits IR radiation

Earth cools down faster on clear nights than on cloudy nights because clouds absorb IR radiation from the Sun during the day and emit IR radiation towards the Earth at night

Greenhouse effect is directly related to emission, where certain greenhouse gases (carbon dioxide) prevent IR radiation from the earth’s surface to exit back to space

WATER VAPOUR IN THE ATMOSPHERE

99.9% of water vapour is contained in the troposphere

Condensation of water vapour into liquid or ice is responsible for rain, snow etc

The latent heat released during condensation is responsible for cyclones and thunderstorms

Water vapour is also a potent greenhouse gas

Water vapour is most common gas in volcanic emissions (around 60%)

CARBON DIOXDE IN THE ATMOSPHERE

It is an important greenhouse gas

Natural sources of carbon dioxide in the atmosphere include volcanic activity, combustion of organic matter, respiration, decay of forests etc

Current carbon dioxide levels (0.0384%) are around 35% higher than the levels in 1832

The concentration of carbon dioxide is higher in the northern hemisphere because it has greater land mass and plant mass than the southern hemisphere

Carbon dioxide concentrations peak in May (just after the end of winter in the Northern Hemisphere) and reach a minimum in October (at the end of summer in Northern Hemisphere, when the quantity of plants undergoing photosynthesis is greatest)