Structure of Earth & Atmosphere

The Earth has 4 spheres:

•         Lithosphere: (land) This includes all the rocks that make up the Earth, from the partially melted rock under the crust to grains of sand on a beach. The geosphere also provides natural resources and a place to grow food.

 •        Biosphere: (living things) This is the narrow zone where land, water, and air come together.

 •        Hydrosphere: (water) This is the area of the Earth’s surface that is covered by water.

 •        Atmosphere: (air) This is the gaseous layers that surround the Earth.

Atmosphere

The envelope of gas and or dust particles that surrounds the various celestial bodies of the universe, due to the force of gravity is called atmosphere.

Earth’s atmosphere is composed of about 78% nitrogen, 21% oxygen, and 1% other gases.

Nitrogen (N2): It is the most plentiful gas in the air. It is one of the primary nutrients critical for the survival of all living organisms.

 (O2): Humans and animals take oxygen from the air as they breathe. Green plants produce oxygen during photosynthesis. In this way oxygen content in the air remains constant.

Carbon dioxide (CO2): It is an important heat-trapping gas, or, that comes from the extraction and burning of fossil fuels.

Layers of atmosphere based on vertical temperature profile

Air temperature throughout atmosphere shows complicated vertical profile and may be divided into five main distinguished layers, namely troposphere, stratosphere, mesosphere, thermosphere and exosphere. They are layered one atop the other and each one is very much needed to support and protect the life on the earth. The atmospheric also contains transition layers between two successive main layers. The nomenclature of these transitional layers zones contains the term – pause at its end. These are tropopause (from the end of troposphere and till the beginning of stratosphere), stratopause (boundary separating stratosphere from mesosphere), mesopause (transition region between mesosphere and thermosphere) and thermopause (between thermosphere and exosphere)

Troposphere

The Earth’s troposphere extends from the surface up to an average height of about 12 kilometers, varying between 18 km at the equator and 8 km at the poles. The depth of the troposphere is influenced by latitude, season, and time of day. The reasons for this non-uniform thickness include:

1. High insolation at the equator.
2. Higher gravitational pull on atmospheric gases at the poles.
3. The highest centrifugal force due to Earth’s rotation at the equator.

The troposphere contains the air necessary for photosynthesis and respiration, as well as about 99% of all water vapor and aerosols (minute solid or liquid particles suspended in the atmosphere). Temperature in the troposphere decreases with height.

Above the troposphere is the tropopause, an isothermal layer where the lapse rate becomes zero and air temperature remains constant (around -60°C). This layer marks the boundary between the troposphere and the stratosphere.

The troposphere is the densest atmospheric layer due to the weight of the atmosphere above it. Most of Earth’s weather occurs here, and almost all weather-related clouds are found within this layer. Most aviation also takes place in the troposphere, including in the transition region between the troposphere and the stratosphere.

The lapse rate, or the rate of temperature decrease with altitude, is approximately -6.5°C per kilometer (1°C per 155 meters).

Stratosphere

The stratosphere is located between approximately 12 and 50 kilometers above Earth’s surface and is best known for containing the ozone layer. In this region, temperature increases with height due to the heat produced during the formation of ozone. The stratosphere is nearly free of clouds and weather, although polar stratospheric clouds can sometimes be found at its lowest, coldest altitudes during winter at high latitudes. It is also the highest part of the atmosphere that jet planes can reach.

Mesosphere

The mesosphere extends from about 50 to 80 kilometers above Earth’s surface and becomes progressively colder with altitude. The top of the mesosphere is the coldest part of the Earth’s atmosphere, with average temperatures around -85°C. The scarce water vapor present at the top of the mesosphere forms noctilucent clouds, the highest clouds in Earth’s atmosphere. Most meteors burn up in this layer. Sounding rockets and rocket-powered aircraft can reach the mesosphere. The boundary separating the mesosphere from the stratosphere is called the stratopause.

Thermosphere

The thermosphere is located above the mesosphere, extending from about 80 to 700 kilometers above Earth’s surface. In this layer, temperature generally increases with altitude, ranging from 600 to 3000°F (600-2000 K) depending on solar activity. This increase is due to the absorption of intense solar radiation by the sparse molecular oxygen present. At these extreme altitudes, gas molecules are widely separated.

Above 100 kilometers from Earth’s surface, the chemical composition of air varies significantly with altitude, with the atmosphere becoming enriched with lighter gases such as atomic oxygen, helium, and hydrogen. At this altitude, Earth’s atmosphere becomes too thin to support aircraft, requiring vehicles to travel at orbital velocities to remain aloft. This boundary between aeronautics and astronautics is known as the Karman Line.

Above about 160 kilometers, atomic oxygen becomes the major atmospheric component. At very high altitudes, residual gases stratify according to molecular mass due to gravitational separation. The lowest part of the thermosphere contains the ionosphere.

This layer is both cloud- and water-vapor-free, with temperatures increasing due to the very low density of molecules. The aurora borealis (Northern Lights) and aurora australis (Southern Lights) can sometimes be seen in the thermosphere. The International Space Station (ISS) orbits within this layer.

Ionosphere

The ionosphere is not a distinct layer like the others mentioned above; instead, it overlaps the mesosphere, thermosphere, and exosphere. This very active part of the atmosphere expands and contracts based on the energy it absorbs from the sun.

The ionosphere is an electrically conducting region capable of reflecting radio signals back to Earth. The electrically charged atoms and molecules formed in this region are called ions, which give the ionosphere its name and special properties. The ionosphere is defined by atmospheric effects on radiowave propagation due to the presence and variation in the concentration of free electrons in the atmosphere.

The ionosphere is divided into several regions based on altitude and electron density:

• D-region: Located at about 60 to 90 km in altitude, this region disappears at night.
• E-region: Located at about 90 to 140 km in altitude, also known as the Heaviside-Kennelly layer.
• F-region: Found above 140 km in altitude. During the day, it splits into two regions:
  • F1-region: Located from about 140 to 180 km in altitude.
  • F2-region: Where the concentration of electrons peaks, ranging from 250 to 500 km in altitude.

The combined F1 and F2 regions are known as the Appleton Layer.