Earth Atmosphere: Although the atmospheres of other planets and moons in our solar system exist, none of them is capable of supporting life as we know it. None of them have much oxygen, the priceless gas that we Earth animals need every minute, either because they are either too thick (as on Venus) or not dense enough (as on Mars).
Some scientists describe three stages in the evolution of Earth’s atmosphere as it is today.
A thick layer of gases covering our planet extends thousands of kilometers above the surface. The Earth atmosphere is the term for this gaseous layer that covers the earth.
The atmosphere is a fundamental component of the earth, just like the lithosphere and the hydrosphere.
The atmosphere appears to be simply a relatively thin layer of gases when compared to the radius of the world. It is connected to the earth, nevertheless, due to the pull of gravity.
The weight of the air puts pressure on the surface of the earth, creating atmospheric pressure. The Earth Atmospheric pressure is the name given to this force. The primary climatic factor is atmospheric pressure. At sea level, there are 1034 grams of air pressure per square centimeter.
Origin of Earth Atmosphere
- Earth is believed to have formed about 5 billion years ago.
- In the first 500 million years, a dense atmosphere emerged from the vapor and gases that were expelled during degassing of the planet’s interior.
- During the cooling of the earth, gases and water vapor were released from the interior solid earth. This started the evolution of the present atmosphere. This process is called degassing.
- These gases may have consisted of hydrogen (H2), water vapor, methane (CH4), and carbon oxides.
- Prior to 3.5 billion years ago the atmosphere probably consisted of carbon dioxide (CO2), carbon monoxide (CO), water (H2O), nitrogen (N2), and hydrogen.
- The hydrosphere was formed 4 billion years ago from the condensation of water vapor, resulting in oceans of water in which sedimentation occurred.
- The absence of free oxygen was the primary characteristic of the prehistoric environment. Early rock formations that include many elements, such as iron and uranium, in their reduced states, conceal evidence of such an anaerobic reducing atmosphere. The rocks from the mid-Precambrian and younger periods, which are older than 3 billion years, do not contain any elements in this form.
- Early aquatic organisms known as blue-green algae started utilizing solar energy to divide H2O and CO2 molecules and recombine them into chemical compounds and molecular oxygen one billion years ago (O2). Photosynthesis is the process of converting solar energy. Some of the oxygen produced during photosynthesis interacted with organic carbon to produce CO2 molecules. With regard to early anaerobic species that were already there, the residual oxygen collected in the atmosphere and set off a major ecological catastrophe. CO2 decreased as the amount of oxygen in the atmosphere rose.
- High in the atmosphere, some oxygen (O2) molecules absorbed energy from the Sun’s ultraviolet (UV) rays and split to form single oxygen atoms. These atoms combine with remaining oxygen (O2) to form ozone (O3) molecules, which are very effective at absorbing UV rays. The thin layer of ozone that surrounds Earth acts as a shield, protecting the planet from irradiation by UV light.
- The amount of ozone required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence 600 million years ago. At this time, the oxygen level was approximately 10% of its present atmospheric concentration.
- Prior to this period, life was restricted to the ocean. The presence of ozone enabled organisms to develop and live on land. Ozone played a significant role in the evolution of life on Earth and allows life as we presently know it to exist.
Role of Earth Atmosphere
Numerous gases, including oxygen, carbon dioxide, nitrogen, etc., are present in the atmosphere.
Animals and many other species need oxygen to thrive, while plants need carbon dioxide to survive. These gases are provided by the atmosphere.
To carry out their biophysical processes, all life forms require a specified range of temperature and a specific range of solar radiation frequencies. Some solar radiation frequencies are absorbed by the atmosphere, while others are let through. In other words, the atmosphere controls how much solar radiation enters the atmosphere.
Additionally, the atmosphere regulates the temperature over the surface of the world. Temperature extremes would exist between day and night over the surface of the world without the atmosphere. If there was no atmosphere, specifically ozone in the stratosphere, harmful ultraviolet rays would get through.
The atmosphere also protects extraterrestrial objects like meteorites, which are destroyed by friction as they travel through the atmosphere (or mesosphere, to be more specific).
Another significant phenomenon that controls the course of many natural and artificial activities, including plant development, agriculture, soil formation, human settlements, etc. is the weather. Weather is the result of numerous climatic elements working together.
Composition of Earth Atmosphere
- There are numerous gases in the atmosphere. In addition, it comprises a sizable amount of aerosols, which are both solid and liquid particles.
- Some of the gases could be thought of as enduring atmospheric elements that continue to exist in a fixed ratio to the total gas volume.
- The number of other components varies from time to time and from location to place. Dry air is extremely stable throughout the entire planet up to an altitude of roughly 80 kilometers if water vapor, suspended particles, and other variable gases were omitted from the atmosphere.
- At a height of 120 km, oxygen will essentially be nonexistent due to a change in gas proportions in the upper layers of the atmosphere. Similarly, carbon dioxide and water vapor are found only up to 90 km from the surface of the earth.
- Nitrogen and oxygen make up nearly 99% of the clean, dry air. The remaining gases are mostly inert and constitute about 1% of the atmosphere.
- Besides these gases, large quantities of water vapor and dust particles are also present in the atmosphere. These solid and liquid particles are of great climatic significance.
- Different constituents of the atmosphere, with their individual characteristics, are discussed below.
- Oxygen, although constituting only 21% of the total volume of the atmosphere, is the most important component among gases. All living organisms inhale oxygen. Besides, oxygen can combine with other elements to form important compounds, such as oxides. Also, combustion is not possible without oxygen.
- Nitrogen accounts for 78% of total atmospheric volume. It is a relatively inert gas and is an important constituent of all organic compounds. The main function of nitrogen is to control combustion by diluting oxygen. It also indirectly helps in the oxidation of different kinds.
- The third important gas is Carbon Dioxide which constitutes only about 03% of the dry air and is a product of combustion. Green plants, through photosynthesis, absorb carbon dioxide from the atmosphere and use it to manufacture food and keep other bio-physical processes going.
- Being an efficient absorber of heat, carbon dioxide is considered to be of great climatic significance. Carbon dioxide is considered to be a very important factor in the heat energy budget.
- With the increased burning of fossil fuels – oil, coal, and natural gas – the carbon dioxide percentage in the atmosphere has been increasing at an alarming rate.
- More carbon dioxide in the atmosphere means more heat absorption. This could significantly raise the temperature at lower levels of the atmosphere thus inducing drastic climatic changes.
- Ozone (03) is another important gas in the atmosphere, which is actually a type of oxygen molecule consisting of three, instead of two, atoms. It forms less than 0.00005% by volume of the atmosphere and is unevenly distributed. It is between 20 km and 25 km altitude that the greatest concentrations of ozone are found. It is formed at higher altitudes and transported downwards.
- Ozone plays a crucial role in blocking harmful ultraviolet radiation from the sun.
- Other gases found in almost negligible quantities in the atmosphere are neon, helium, hydrogen, xenon, krypton, methane, etc.
- Water Vapour is one of the most variable gaseous substances present in the atmosphere – constituting between 02% and 4% of the total volume (in cold dry and humid tropical climates respectively). 90% of moisture content in the atmosphere exists within 6 km of the surface of the earth. Like carbon dioxide, water vapor plays a significant role in the insulating action, of the atmosphere.
- It absorbs not only the long-wave terrestrial radiation (infrared or heat emitted by the earth during the night) but also a part of the incoming solar radiation.
- Water vapor is the source of precipitation and clouds. On condensation, it releases latent heat of condensation —the ultimate driving force behind all storms.
The moisture–carrying capacity of air is directly proportional to the air temperature.
- The Solid Particles present in the earth atmosphere consist of sand particles (from weathered rocks and also derived from volcanic ash), pollen grains, small organisms, soot, and ocean salts; the upper layers of the atmosphere may even have fragments of meteors which got burnt up in the atmosphere. These solid particles perform the function of absorbing, reflecting, and scattering radiation.
- The solid particles are, consequently, responsible for the orange and red colors at sunset and sunrise and for the length of dawn (the first appearance of light in the sky before sunrise) and twilight (the soft glowing light from the sky when the sun is below the horizon, caused by the reflection of the sun’s rays by the atmosphere. Dusk: the darker stage of twilight). The blue color of the sky is also due to selective scattering by dust particles.
- Some of the dust particles are hygroscopic (i.e. readily absorbing moisture from the air) in character, and as such, act as nuclei of condensation. Thus, dust particles are an important contributory factor in the formation of clouds, fog, and hailstones.
Major Greenhouse Gases in Earth Atmosphere
Carbon dioxide is meteorologically a very important gas as it is transparent to incoming solar radiation but opaque to outgoing terrestrial radiation. It absorbs a part of terrestrial radiation and reflects back some part of it towards the earth’s surface. It is largely responsible for the greenhouse effect.
Ozone is another important greenhouse gas. But it is very small proportions at the surface.
Water vapor is also a variable gas in the atmosphere, which decreases with altitude. Water vapor also decreases from the equator towards the poles. In the warm and wet tropics, it may account for four percent of the air by volume, while in the dry and cold areas of desert and polar regions, it may be less than one percent of the air. It also absorbs parts of the insolation from the sun and preserves the earth’s radiated heat. It thus acts like a blanket allowing the earth neither to become too cold nor too hot. Water vapor also contributes to the stability and instability of the air.
One of the most important greenhouse gases. It is produced from the decomposition of animal wastes and biological matter.
Structure of Earth Atmosphere
- The atmosphere can be studied as a layered entity – each layer having its own peculiar characteristics. These layers are systematically discussed below.
- 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 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 is because of the greater thickness of troposphere – 18 km).
- The fall in temperature is called the ‘lapse rate’. (more about this in future posts)
- 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, 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 vapors and solid particles lie within this.
- The troposphere is influenced by seasons and jet streams.
- The topmost layer of the troposphere.
- It acts as a boundary between the troposphere and the stratosphere.
- This layer is marked by constant temperatures.
- 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.
- 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.
- It lies at an altitude between 30 km and 60 km from the earth’s surface and spans the stratosphere and lower mesosphere.
- Because of the presence of ozone molecules, this layer reflects harmful ultraviolet radiation.
- The ozonosphere is also called the chemosphere because a lot of chemical activity goes on here.
- The temperature rises at a rate of 5°C per kilometer through the ozonosphere.
- 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.
- In the thermosphere, the 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.
- The 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.
- This layer is located between 80 km and 400 km and is an electrically charged layer.
- This layer is characterized by the ionization of atoms.
- Because of the electric charge, radio waves transmitted from the earth are reflected back to the earth by this layer.
- Temperature again starts increasing with height because of radiation from the sun.
- This is the uppermost layer of the atmosphere extending beyond the ionosphere above a height of about 400 km.
- The air is extremely rarefied and the temperature gradually increases through the layer.
- Light gases like helium and hydrogen float into space from here.
- The temperature gradually increases through the layer. (As it is exposed to direct sunlight)
- This layer coincides with space.
The speed of sound follows the temperature profile
This is because the speed of sound is directly proportional to temperature as we move away from earth.