Atmosphere- Everything you need to know!

What is atmosphere?

 The term “atmosphere” originates from Ancient Greek, where “atmós” signifies “vapour” or “steam,” and “sphaîra” refers to “sphere.” Today when we use this word we actually mean: a layer/layers of gas/gasses, enveloping a planet, held in place by the gravity of that planet. The atmosphere is a crucial aspect of our planet, playing a vital role in regulating temperature, weather patterns, and air quality. Atmosphere of Earth is a delicate balance of gases, composed mainly of nitrogen (78%) and oxygen (21%), as well as trace amounts of other gases such as carbon dioxide, argon, and neon. In this article, we will explore our atmosphere in detail. We will discuss its composition, structure, and functioning.

Some important characteristics of the atmosphere are:

atmosphere and the setting sun
A 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
  • When the Earth is scaled down to the size of an apple, surprisingly its atmosphere is as thin as the apple’s peel.
  • 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 colours 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

What is atmospheric science?

Atmospheric science is the study of the Earth’s atmosphere and the processes that occur within it. This field encompasses a wide range of topics, including meteorology (the study of weather and climate), atmospheric chemistry, atmospheric physics, and atmospheric dynamics. Researchers in atmospheric science use a variety of tools and techniques. Chiefly, computer models, satellites, and ground-based observations, in order to study the atmosphere and predict weather patterns and climate change.

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 during the day and by not letting the warmth of the earth escape at night with the help of Greenhouse gases, the atmosphere regulates the required temperature for all the biophysical processes. Organisms such as plants, animals, and others require various gases, including oxygen, carbon dioxide, and nitrogen.

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

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

Why does our atmosphere exist?

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

Located within the “Goldilocks Zone,” Earth resides in an environment that allows for liquid water to exist on its surface. The Goldilocks Zone refers to the habitable region around a star, that is where temperatures are neither too hot nor too cold for liquid water. Earth’s average distance from the Sun, which is 149,600,000 kilometres (92,900,000 miles) and referred to as 1 Astronomical Unit (AU), places it in the ideal location within this zone.

Functions of atmosphere

The atmosphere plays a crucial role in protecting and sustaining life on earth. Some of the key functions of the atmosphere include:

  • Regulating the temperature: The atmosphere acts as a thermal blanket, trapping heat from the sun and keeping the earth warm enough for life.
  • Protecting from harmful radiation: The ozone layer in the stratosphere helps protect the earth from harmful ultraviolet radiation from the sun.
  • Providing air to breathe: The atmosphere contains oxygen, which is necessary for respiration and combustion.
  • Cycling water and nutrients: The atmosphere plays a key role in the water cycle, helping to distribute water and nutrients around the earth.
  • Moderating weather patterns: The atmosphere helps regulate weather patterns by transporting heat and moisture around the earth.

Composition of atmosphere

concentration of gases of atmosphere by molecular count
Figure shows the concentration of gases by molecular count.

When we casually use the word air, we essentially mean a mixture of gases. Therefore our atmosphere which consists of air is in fact 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
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)

ozone depletion
Ozone Depletion

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 hole

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 vapour 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 vapour 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 human 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 artificial 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’.


What are Permanent and Variable Gases

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

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.

rise in co2 concentration

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



Atmospheric Cycles & Residence Time of Gases 

the nitrogen cycle
Nitrogen in the atmosphere 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.

co2 cycle (carbon cycle)
Carbon dioxide in the atmosphere 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. Human activities have disrupted the natural carbon cycle, resulting in unusual levels of carbon dioxide in the atmosphere. This anthropogenic influence has led to carbon dioxide being classified 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

layers of atmosphere

The troposphere is the lowermost layer of the Earth’s atmosphere and extends from the surface up to an altitude of 8 km at the poles and 18 km at the equator. It is thicker at the equator due to the rising of heated air to greater heights. The troposphere is bounded by the Tropopause and its temperature decreases with increasing altitude at a rate of 6.5°C per kilometre, reaching -45°C at the poles and -80°C over the equator at the Tropopause (with a greater fall in temperature above the equator due to its greater thickness). This decrease in temperature is known as the ‘lapse rate’.

The troposphere is characterized by a temperature inversion, turbulence, and eddies, and is meteorologically the most significant zone in the atmosphere, as almost all weather phenomena like rainfall, fog, hailstorms, etc. are confined to this layer. It is also known as the convective region as all convection stops at the Tropopause. The troposphere is the region where weather occurs, with all cyclones, anticyclones, storms, and precipitation taking place here because all water vapour and solid particles are present in this layer. It is influenced by seasons and jet streams.

Stratosphere

The stratosphere is a layer of the Earth’s atmosphere that lies beyond the troposphere and extends up to an altitude of 50 km from the surface. It is characterized by a constant temperature for some distance, followed by a rise to 0°C at 50 km altitude, due to the presence of ozone which absorbs harmful ultraviolet radiation. The stratosphere is very dry and contains less water vapour compared to other layers of the atmosphere. This layer is almost cloud-free, making it ideal for flying aeroplanes. Occasionally, cirrus clouds can be found at lower levels in the stratosphere. Aeroplanes often fly in the lower stratosphere or in the upper troposphere where weather conditions are calm.

Mesosphere

The mesosphere is a layer of the Earth’s atmosphere that lies beyond the ozone layer and extends up to an altitude of 80 km from the surface. The temperature in the mesosphere gradually decreases to -100°C at an altitude of 80 km. This layer is also where meteorites burn up upon entering the Earth’s atmosphere from space.

Thermosphere

The thermosphere is characterized by a rapid increase in temperature with height. It encompasses the ionosphere, which lies between 80-400 km and aids in radio transmission by reflecting radio waves back to Earth. Despite the high temperature, the extremely low pressure in the thermosphere means that it does not feel warm. The International Space Station and satellites orbit in this layer, where the atmosphere is extremely thin with gas molecules spaced hundreds of kilometres apart, so objects or people do not experience the heat. Auroras can be observed in the lower parts of the thermosphere.


The Role of the Atmosphere in Climate Change

What are auroras?

Latitude and Longitude

Insolation and Heat Budget



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