■ IONOSPHERE
#Atmosphere
◇ This layer is located between 80 km and 400 km and is an electrically charged layer.
◇ This layer is characterized by 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.
■ STRATOSPHERE
#Atmosphere
◇ The layer just above the troposphere is called stratosphere.
◇ On an average the upper limit of the stratosphere is taken to be 50 km.
◇ The upper limit of the stratosphere which is known as stratopause.
◇ Stratosphere is more or less devoid of major weather phenomena but there is circulation of feeble winds and cirrus cloud in the lower stratosphere.
◇ The lower part of this layer is very important for life-forms in the bio spheric ecosystem because there is concentration of ozone between the height of 15-30 km though ozone has been discovered upto the height of 80 km.
◇ The lower portion of the stratosphere having maximum concentration of ozone is called ozonosphere, which is confined between the height of 15 km to 35 km from sea level though the upper limit has been fixed at 55 km.
◇ Vapour is almost transparent for incoming shortwave Solar radiation so that the electromagnetic radiation waves reach the earth's surface without much obstacles but vapour is less transparent for outgoing shortwave terrestrial radiation and therefore it helps in heating the earth's surface and lower portion of the atmosphere because it absorbs terrestrial radiation.
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■ GASES IN ATMOSPHERE
#Atmosphere
◇ Nitrogen (78%) and oxygen (21%) are major gases which constitute 99% of the total gaseous composition of the atmosphere. The remaining one per cent is represented by argan (0.93%), carbon dioxide (0.03%), neon (0.0018%), helium (0.0005 %), ozone (0.00006%), hydrogen (0.00005%), krypton (trace), xenon (trace), methane (trace) etc.
◇ Oxygen is the most important gas from the stand point of living organisms because they inhale it for their survival. Oxygen is also essential for combustion of burning matter.
◇ Nitrogen acts as dilutant and is generally chemically inactive.
◇ Carbon dioxide is used by green plants for photosynthesis. It absorbs most of radiant energy from the earth and reradiates it back to the earth. Thus, carbon dioxide, a greenhouse gas, increases the temperature of the lower atmosphere and the earth's surface.
◇ Ozone gas absorbs most ultraviolet rays radiated from the sun and thus prevents the earth from becoming too hot.
□ Reasons:-
◇ Global warming as the most significant factor of coral bleaching causing large-scale coral death.
◇ El Nino phenomenon has also been related to coral bleaching.
◇ The outbreaks of coral diseases (black band disease, coral plague, aspergillosis and white band disease) cause coral death.
◇ Local factors like increase in siltation of sea waters due to mass flux of sediments and nutrients brought by the steams from the erosion of high islands consequent upon land use changes; pollution of sea waters caused by industrial effluents, urban sewage and oil slicks; destructive fishing practices, overfishing; clearing of marine forests around coral reefs; filling of wetlands (marine forests and wetlands trap sediments and filter pollutants and thus save corals from degradation); mining of coral rocks for building materials; collection of rare coral species etc. are also responsible for coral degradation at local and regional levels.
■ ATOLL
#coralreefs
◇ A ring of narrow growing corals of horseshoe shape and crowned with palm trees is called atoll. It is generally found around an island or in elliptical form on a submarine platform.
◇ Atolls are divided into 3 types. E.g. (i) true atoll characterized by circular reef enclosing a shallow lagoon but without island, (ii) island atoll having an island in the central part of the lagoon enclosed by circular reef, and (iii) coral island or atoll island does not have island in the beginning but later on island is formed due to erosion and deposition by marine waves.
◇ Shallow lagoon reefs are minor reef features which are annular in shape and are found in epi-continental seas like Indonesian Sea, South China Sea etc. The lagoon is a small pool. Faros are chains of small atolls having shallow small lagoons. Coral banks are isolated shapeless reefs. Coral pinnacles are small ridges which rise within the lagoons.
■ FRINGING REEF
#Coralreefs
◇ Coral reefs developed along the continental margins or along the islands are called fringing reefs.
◇ The seaward slope is steep and vertical while the landward slope is gentle.
◇ The upper surface is uneven and corrugated.
◇ Though fringing reefs are usually attached to the coastal land but sometimes there is gap between them and land and thus lagoon is formed between the fringing reef and the land. Such lagoon is called boat channel.
◇ Coral reefs are generally long but narrow in width. The continuity of coral reefs is broken wherever rivers drain into the seas and oceans.
◇ Coral reefs are basically of two types e.g. (i) coral reefs facing open ocean and (ii) coral reefs protected by a barrier. Such fringing reefs are found along Sakau island, southern Florida, Mehetia island (of Society Group of Islands) etc.
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■ MESOSPHERE
#Atmosphere
◇ Mesosphere extends between 50 km and 80 km. Temperature decreases with increasing height. In fact, the rise of temperature with increasing height in the stratosphere stops at stratopause.
◇ At the uppermost limit of mesosphere (80 km) temperature becomes -80°C. This limit is called mesopause above which temperature increases with increasing height.
■ THERMOSPHERE
◇ The part of the atmosphere beyond mesopause is known as thermosphere wherein temperature increases rapidly with increasing height. It is estimated that the temperature at its upper limit (height undecided) becomes 17°C.
◇ It may be pointed out that this temperature cannot be measured by ordinary thermometer because the gases become very light due to extremely low density. That is why one does not feel warm when one stretches one's arm in the air.
■ TROPOSPHERE
#Atmosphere
◇ The lowermost layer of the atmosphere is known as troposphere and is the most important layer because almost all of the weather phenomena (e.g. fog, cloud, dew, frost, rainfall, hailstorm, storms, cloud-thunder, lightning etc.) occur in this layer.
◇ Temperature decreases with increasing height at the rate of 6.5°C per 1000m. This rate of decrease of temperature is called normal lapse rate. There is seasonal variation in the height of troposphere.
◇ Temperature at the top of tropopause is lowest over the equator (-70°C) and is relatively high over the poles. Since temperature decreases upward at the rate of 6.5°C per 1000m and hence it is natural that temperature at the height of 17 km over the equator becomes much lower than at the height of 9-10 km over the poles.
◇ The word troposphere literally means 'zone or region of mixing' whereas the word tropopause means 'where the mixing stops'.
■ WATER VAPOUR
#Atmosphere
◇ The vapour content in the atmosphere ranges between zero and 5 per cent by volume.
◇ The atmospheric vapour is received through the evaporation of moisture and water from the water bodies (like seas and oceans, lakes, tanks and ponds, rivers etc.), vegetation and soil cover.
◇ Vapour depends on temperature and therefore decreases from the equator poleward.
◇ The content of vapour decreases upward. More than 90 per cent of the total atmospheric vapour is found upto the height of 5 km.
◇ The moisture content in the atmosphere creates several forms of condensation and precipitation e.g. clouds, fogs, dew, rainfall, frost, hailstorm, ice, snowfall etc.
■ ATMOSPHERE
#Atmosphere
◇ The atmosphere is a thick gaseous envelope which surrounds the earth from all sides and is attached to the earth's surface by gravitational force.
◇ The atmosphere is a significant component of the bio spheric ecosystem because the life on the earth's surface is because of this atmosphere otherwise the earth would have become barren like moon.
◇ Besides providing all necessary gases for the sustenance of all life forms in the biosphere, it also filters the incoming solar radiation and thus prevents the ultraviolet solar radiation waves to reach the earth's surface and thus protects the earth from becoming too hot.
◇ The height of the atmosphere is estimated between 16 to 29 thousand kilometres from the sea level. It is estimated that 97 per cent of the effective atmosphere is upto the height of 29 km.
◇ The atmosphere is composed of (i) gases, (ii) vapour and (iii) particulates.
■ CORAL BLEACHING
#coralrelief
◇ Coral bleaching refers to loss of algae from the corals resulting into the white colour which is indicative of death of corals.
◇ Global warming has been reported as the major factor of coral bleaching.
◇ Clive Wilkinson identified four overlapping levels of coral bleaching.
• Catastrophic bleaching adversely affecting 95 per cent of shallow water corals.
• severe bleaching accounting for 50-70 per cent death of corals.
• moderate bleaching resulting into 20-50 per cent coral mortality but with quick recovery.
• insignificant bleaching or no bleaching.
■ BARRIER REEF
#Coralreefs
◇ The largest coral reefs off the coastal platforms but parallel to them are called barrier reefs.
◇ Barrier reefs are the largest, most extensive, highest and widest reefs of all types of coral reefs.
◇ The average slope is about 45° but some barrier reefs are characterized by 15–25-degree slope.
◇ There is extensive but shallow lagoon between the coastal land and barrier reef. Barrier reefs are seldom found as continuous chains rather they are broken at many places and thus the lagoons have contact with the open seas and oceans through tidal inlets.
◇ Sometimes, tidal inlets are so wide that ships enter the lagoons through them.
◇ Sometimes, the base of barrier reefs exceeds the required depth for the development of coral polyps i.e. 300 feet (91m). Thus, the existence of barrier reefs at such greater depth (beyond the permissible depth of 6077m) poses the problem of their formation.
◇ Ocean currents and waves are favourable for corals because they bring necessary food supply for the polyps. It is obvious that corals grow in open seas and oceans but they die in lagoons and small enclosed seas because of lack of supply of food. Currents and waves also determine the shapes of coral reefs.
◇ There should be extensive submarine platforms for the formation of colonies by the coral polyps.
◇ Human economic activities viz. deforestation, industrialization etc. causing global warming adversely affect corals in their habitats. Corals are more susceptible to long-term climatic change. Corals are generally termed as rainforests of the oceans. These cannot survive in extreme warm environment. The scientists claim that about 10 per cent of the corals have died and become skeletons due to global warming caused by anthropogenic factors mainly industrialization.
• The increase in temperature causes bleaching in the corals wherein the corals lose their algae and become white in colour. This process is called coral bleaching, which causes death to corals.
• El Nino phenomenon has also been associated with coral bleaching (coral death).
◇ Besides global warming, human activities at local to regional levels such as pollution of oceanic water through excess flux of sediments and nutrients, industrial effluents, urban wastes, sewage; over fishing; clearance of maritime forest and filling of wetlands; mining of coral rocks; collection of rare species etc. cause fatal diseases to corals.
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