Structure of Atmosphere
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Structure of the Atmosphere
The atmosphere has 4 layers:
- the troposphere that we live in near the surface of the earth;
- the stratosphere that houses the ozone layer;
- the mesosphere, a colder and lower density layer with about 0.1% of the atmosphere;
- The thermosphere, the top layer, where the air is hot but very thin.
Mnemonic to remember this figure: T.S.M.T.I.E – The Sun & Moon Trust In Earth.
Troposphere
- The troposphere is the lowermost layer of the atmosphere. Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator.
- Thickness of the troposphere is greatest at the equator because heat is transported to great heights by strong convectional currents. This layer contains dust particles and water vapour.
- This is where we live and all changes in climate and weather take place in this layer
- The temperature in this layer decreases at the rate of 1°C for every 165m of height. This is the most important layer for all biological activity.
- The zone separating the troposphere from stratosphere is known as the tropopause.The temperature here is nearly constant, and hence, it is called the tropopause. The jet stream sits at this level and it marks the highest point that weather can occur.
- The height of the troposphere varies with location, being higher over warmer areas and lower over colder areas.The air temperature at the tropopause is about -80º Celsius over the equator and about -45º Celsius over the poles.
Stratosphere
- The stratosphere is found above the tropopause and extends up to a height of 50 km.
- In this layer the temperature increases with height. This is because the stratosphere houses the ozone layer. The ozone layer is warm because it absorbs ultraviolet (UV) rays from the sun.
- By absorbing ultra-violet radiation it shields life on the earth from intense, harmful form of energy there by protecting us from diseases like Cancer, skin burn etc.
- Its non-turbulence and stable, non-convection character makes it possible for the jets to cruise easily, hence they are flown here.
- When Volcanic eruptions occur, the ejected material reaches as high as Stratosphere and it stays there for long period, as it doesn’t allow the circulation, there by leading to stratifying the volcanic particles and cooling down of the Earth surface.
- There is also an idea which the scientists are considering that could result in the slowing down of the Earth’s heating, i.e., by adding the man-made materials to stratosphere. Though the feasibility of this idea is yet to be verified.
Mesosphere
- The mesosphere lies above the stratosphere, which extends up to a height of 80 km. In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to-100°C at the height of 80 km.The temperature decreases with height here just like it does in the troposphere.
- This layer also contains ratios of nitrogen and oxygen similar to the troposphere, except the concentrations are 1000 times less and there is little water vapour there, so the air is too thin for weather to occur.
- The upper limit of mesosphere is known as the mesopause
Thermosphere
- The ionosphere is located between 80 and 400 km above the mesopause. It contains electrically charged particles known as ions, and hence, it is known as ionosphere
- Radio waves transmitted from the earth are reflected back to the earth by this layer
- Temperature here starts increasing with heightbecause it is being directly heated by the sun.
Exosphere
- The uppermost layer of the atmosphere above the thermosphere is known as the exosphere
- This is the highest layer but very little is known about it.
Why Do Temperature and Pressure Decrease with Height?
Pressure Decrease
- The molecules that make up the atmosphere are pulled close to the earth’s surface by gravity. This causes the atmosphere to be concentrated at the Earth’s surface and thin rapidly with height.
- Air pressure is a measure of the weight of the molecules above you. As you move up in the atmosphere there are fewer molecules above you, so the air pressure is lower.
- For example, at 15 km up, 90% of the atmosphere is below you. At the peak of Mount Everest, as shown, the air pressure is 70% lower than it is at sea level. This means when mountain climbers breathe air on top of the mountain, they are only inhaling 30% of the oxygen they would get at sea level. It is no surprise that most climbers use oxygen tanks when they climb Mt. Everest.
Temperature Decrease
Temperature decreases with height in the troposphere.
- Even though the sun’s energy comes down from the sky, it is mostly absorbed by the ground. The ground is constantly releasing this energy, as heat in infrared light, so the troposphere is actually heated from the ground up, causing it to be warmer near the surface and cooler higher up.
- Another reason is the decreasing air pressure with height. If the warm air at the surface gets blown upward into the cooler air above it, the surface air will continue to rise. As air rises into areas of lower pressure it expands because there are less molecules around it to compress it. The molecules in the air use some of their energy to move apart from each other, causing the air temperature to decrease. The constantly decreasing air pressure alongwith the ground-up heating keeps the temperature in the troposphere decreasing with height.
COMPOSITION OF THE ATMOSPHERE
- The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth
Gases
- The permanent gases whose percentages do not change from day to day are nitrogen, oxygen and argon. Nitrogen accounts for 78% of the atmosphere, oxygen 21% and argon 0.9%.
- Carbon dioxide is meteorologically a very important gas as it is transparent to the incoming solar radiation but opaque to the outgoing terrestrial radiation therefore called Greenhouse gas.
- Greenhouse gases percentages vary daily, seasonally, and annually.This is why scientists are watching the observed increase in greenhouse gases like carbon dioxide and methane carefully, because even though they are small in amount, they can strongly affect the global energy balance and temperature over time.
- Ozone is another important component of the atmosphere found between 10 and 50 km above the earth’s surface and acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.
Water Vapour
- Water vapour is also a variable gas in the atmosphere, which decreases with altitude.
- In the warm and wet tropics, it may account for four per cent of the air by volume, while in the dry and cold areas of desert and Polar Regions; it may be less than one per cent of the air.
- Water vapour also decreases from the equator towards the poles
- 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 vapour content is very important in predicting weather.
Dust Particles
- Atmosphere has a sufficient capacity to keep small solid particles, which may originate from different sources and include sea salts, fine soil, smoke-soot, ash, pollen, dust and disintegrated particles of meteors
- Dust particles are generally concentrated in the lower layers of the atmosphere; yet, convectional air currents may transport them to great heights
- The higher concentration of dust particles is found in subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions
- Dust and salt particles act as hygroscopic nuclei around which water vapor condenses to produce clouds.
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