Atmosphere- Everything you need to know!

What is atmosphere?

 The word atmosphere comes from the Ancient Greek language, where ‘atmós‘ means ‘vapour’ or ‘steam’, and ‘sphaîra‘ means ‘sphere’.

Today when we use this word we mean, a layer of gas or layers of gases that envelope a planet and is held in place by the gravity of the planetary body.

Some important characteristics of atmosphere are:

Thin line of the earth’s atmosphere and the setting sun
  • It is a mixture of different gases
  • It regulates Insolation
  • 99% of the atmospheric mass is within 32 km
  • Earth when scaled to an apple, its atmosphere is no thicker than the peal
  • Atmosphere is fluid
  • Has a base (surface) but no top
  • High oxygen content and low carbon dioxide content
  • The right amount of Greenhouse gases makes the surface temperature livable

Why is the atmosphere blue?

One word answer is ‘Rayleigh Scattering’

Sunlight reaches Earth’s atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered more than the other colors because it travels in shorter, smaller waves. This is why we see a blue sky most of the time.

spaceplace.nasa.gov

Read more about ‘Rayleigh Scattering’

Role of Earth’s Atmosphere

Atmosphere is essential for all the life existing on earth. It acts as a protective shield, giving all the life forms the prerequisites to sustain and flourish.

By blocking the harmful radiations of the sun in the day and by not letting the warmth of the earth escape  in the night with the help of Greenhouse Gasses the atmosphere regulates the required temperature for all the biophysical processes. Various gasses like oxygen, carbon-dioxide, nitrogen etc. are required by plants animals and other organisms.

Ozone (O3) composed of three oxygen atoms which is present in the upper atmosphere absorbs the harmful ultra-violet (UV) radiations .

The hydrological cycle, which operates in the lower limits of the atmosphere is responsible for all the fresh water movement.

The atmosphere exists because…

Earth is in the ‘Goldielocks Zone’ and can support liquid water.

van Ellen, Layla. (2018). Building on Mars.

Goldielocks zone is the habitable zone around a star, where it is not too hot and not too cold for liquid water to exist on the surface of surrounding planets.

Earth’s average distance from the Sun is 149,600,000 kilometers (92,900,000 miles), which is also called an astronomical unit (or 1 AU).


Composition of atmosphere

Figure shows the concentration of gases by molecular count.

When we casually use the word air we actually mean a mixture of gases. So our atmosphere which consists of air is actually a mixture of many gases. Air consists of gases like nitrogen, oxygen, carbon dioxide, etc. Table 1 shows the composition of dry air.

Gas NameChemical FormulaPercent Volume
NitrogenN278.08%
OxygenO220.95%
*WaterH2O0 to 4%
ArgonAr0.93%
*Carbon DioxideCO20.0360%
NeonNe0.0018%
HeliumHe0.0005%
*MethaneCH40.00017%
HydrogenH20.00005%
*Nitrous OxideN2O0.00003%
*OzoneO30.000004%
Table 1: Average composition of the atmosphere up to an altitude of 25 km.

When we say “atmosphere” we refer to the envelope of air that surrounds the earth and other than these gases two more important constituents of the atmosphere are water vapour and aerosols.

>Water vapor: It varies with time and space and is an important atmospheric constituent.

>Aerosols: These are solid particles of sand, sea salt, dust, pollen grains, soot.etc.,  suspended in the atmosphere.

If the water vapour and aerosols are not considered then observations show that the proportion of gases is constant and thus dry air will be stable up to the height of 80 km. The atmosphere is held close to earth due to its gravitational pull, this is the reason it is densest near the surface and rarefies as we rise up.

In the higher layers of the atmosphere, not only density but the composition of gases also vary, so much so that at the height of 120 km oxygen is almost negligible whereas CO2 and water vapour are found only up to 90 km above sea level.

Gasses of our atmosphere:

Oxygen: It is the most important and second most abundant ( 21% by volume ) gas of the atmosphere. All organisms need oxygen for metabolic functions of the body to produce energy from food and the release of energy, and the release of carbon dioxide as a by-product. Oxygen is also required for the process of combustion. Besides oxygen in combination with other elements forms oxides.

Nitrogen: 78% by volume of our total atmosphere is nitrogen. A comparatively inert gas that forms the basis of the ‘Nitrogen Cycle’. Nitrogen is made available to living organisms through fixation and is required by the most important processes of our body as it is the major part of amino acids which forms protein, DNA…etc. Plants get nitrogen through the soil. Nitrogen dilutes oxygen which helps in controlling combustion.

Carbon Dioxide: 0.03% by volume this is another important gas in our atmosphere. Carbon dioxide ( CO2) is a ‘Greenhouse Gas’ and is a product of the combustion of carbon-based material, photosynthesis, and respiration. Due to its heat absorbing capability, it constitutes significantly to the heat budget, having a major effect on the world climate and global climate change.

Formation of Ozone (O3)

Ozone:  Ozone or O3 is concentrated mainly between 15 to 35 km. In the upper layers of the atmosphere, the Oxygen i.e. O2 molecule is broken by the ultraviolet radiation of the sun.

  O2    =  +  O

These free oxygen atoms combine with the O2 molecule to form ozone (O3)

This O3 molecule itself is unstable and is affected by ozone-depleting substances (ODS) like chlorofluorocarbons (CFCs) entering the upper atmosphere due to human activities and is the main cause of ozone depletion.

The ozone layer forms less than 0.00005% of the atmosphere but is crucial in blocking the deadly ultraviolet radiations from the sun.

Ozone Depletion


The figure on the left shows an Ozone hole over the Antarctic. It is a zone of extremely depleted ozone gas.


Water Vapour: The content of water vapor in the atmosphere is closely related to the temperature of air and therefore is highly variable with latitude and altitude, but there are exceptions to this generalisation. Usually, in low latitudes, the humidity will be highest in summers but the same may not hold true if humidity is observed at a deserted place. Water vapour is variable from 0.02% in cold dry air to 4% in hot and humid air by volume. Water vapor is important for precipitation and cloud formation.

Aerosols:  Minute particles suspended in the atmosphere are known as aerosols. If these particles are sufficiently large they scatter and absorb the sunlight, resulting in reduced visibility.

Aerosols enter the atmosphere through natural as well as man-made activities. Volcanic activity, wind-raised dust, sea sprays, forest fires, pollution, agricultural practices, etc are some of its sources.

Aerosols affect the atmosphere by reflecting back sun’s radiations back into space. They also form the cloud condensation nuclei around which the cloud formation occurs.

When man-made chemical aerosols (CFCs found in spray cans and refrigerants) reach the stratosphere during winter region in the polar regions due to a phenomenon known as the ‘Polar Vortex’ they can react with stratospheric ozone causing ‘Ozone depletion’.


Permanent and Variable Gases

Permanent GasesVariable gases
NitrogenWater Vapour
OxygenCarbon Dioxide
ArgonMethane
NeonNitrous Oxide
HeliumOzone
HydrogenParticles
XenonChlorofluorocarbons

Permanent or fixed gases are those gases that do not vary with time in our lifetime. However they may vary in geological time or in billions of years.

Variable gases on the other hand change constantly even with the season. Such gases in fact may have an impact on the climate and weather of local areas.

The atmospheric concentration of COhas increased from 280 ppm in 1750 to 413 ppm in 2020.



Atmospheric Cycles & Residence Time of Gases 

Image Source

The gases in our atmosphere keep changing. As we read in the above section, not all gases are constant, throughout the geologic history of our planet, the composition of these gases has changed. But in our lifetime some gases fluctuate at a faster pace, while others may take billions of years. Such fluctuations affect the ecosystems as every gas is not only a product but also a contributor to the biotic and abiotic cycle.

Image Source

Since every gas goes through a certain process, each gas takes time to get absorbed either by an organism or by a geologic process. This is known as the residence time of gas. Due to anthropogenic causes, the carbon cycle is disturbed leading to unnatural concentrations of in the atmosphere. Due to human interference caron dioxide is now categorised as a variable gas.


Greenhouse gas, 386 ppm, Residence time is 300-1000 Years


What are Atmospheric State Variables?

Answer: Pressure, Temperature & Density

Atmospheric state variables are related to one another by Ideal Gas Law(IDL).

Ideal Gas Law states that the pressure, temperature & volume of a gas are related to each other.

IDL is also referred to as the “Equation of State”

Read more about ‘Atmospheric State Variables’

Vertical Structure of Atmosphere

Since we move on the surface of the earth horizontally almost all the time, we practically miss the vertical variation of our atmosphere. But when we study this vertical structure we understand the uniqueness of our atmosphere. Due to the layered vertical structure of the atmosphere, Sun’s harmful radiations are blocked and the right temperature is maintained. Our atmosphere mainly consists of 5 layers. Each layer has different properties and is divided by a “pause”. The word pause literally means a stagnancy in temperature, beyond which the characteristics change again.

Layers of Atmosphere

Troposphere

  • It is the atmospheric layer between the earth’s surface and an altitude of 8 km at the poles and 18 km at the equator.
  • The thickness is greater at the equator because the heated air rises to greater heights.
  • The troposphere ends with the Tropopause.
  • The temperature in this layer, as one goes upwards, falls at the rate of 6.5°C per kilometer, and reaches -45°C at the poles and -80°C over the equator at Tropopause (greater fall in temperature above the equator because of the greater thickness of troposphere – 18 km).
  • The fall in temperature is called the ‘lapse rate’.
  • The troposphere is marked by temperature inversion, turbulence and eddies.
  • It is also meteorologically the most significant zone in the entire atmosphere (Almost all the weather phenomena like rainfall, fog, and hailstorm, etc. are confined to this layer).
  • It is also called the convective region since all convection stops at Tropopause.
  • The troposphere is the theatre for the weather because all cyclones, anticyclones, storms, and precipitation occur here, as all water vapours and solid particles lie within this.
  • The troposphere is influenced by seasons and jet streams.

Stratosphere

  •  It lies beyond the troposphere, up to an altitude of 50 km from the earth’s surface.
  • The temperature in this layer remains constant for some distance but then rises to reach a level of 0°C at 50 km altitude.
  • Very dry contains less water vapor
  • This rise is due to the presence of ozone (harmful ultraviolet radiation is absorbed by ozone).
  • This layer is almost free from clouds and associated weather phenomena, making conditions ideal for flying airplanes. So airplanes fly in the lower stratosphere, sometimes in the upper troposphere where the weather is calm.
  • Sometimes, cirrus clouds are present at lower levels in this layer.

Mesosphere

  • This is an intermediate layer beyond the ozone layer and continues up to an altitude of 80 km from the earth’s surface.
  • The temperature gradually falls to -100°C at 80 km altitude.
  • Meteorites burn up in this layer on entering from space.

Thermosphere

  • In the thermosphere, temperature rises very rapidly with increasing height.
  • The ionosphere is a part of this layer. It extends between 80-400 km.
  • This layer helps in radio transmission. In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
  • A person would not feel warm because of the thermosphere’s extremely low pressure.
  • The International Space Station and satellites orbit in this layer. (Though the temperature is high, the atmosphere is extremely rarified – gas molecules are spaced hundreds of kilometers apart. Hence a person or an object in this layer doesn’t feel the heat)
  • Auroras are observed in the lower parts of this layer.

What are auroras?

Latitude and Longitude

Insolation and Heat Budget



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