Subject - India & World Geography
Chapter : Atmosphere

Importance of Atmosphere

• Atmosphere contains life-giving gases, like oxygen for man and animal, and carbon dioxide for plants.

• It also acts like a greenhouse and thus keeps the Earth warmer than it would otherwise be.

• The atmosphere, thus, acts like a blanket.

• It regulates the heat balance of the Earth and also protects us from the harmful ultraviolet radiation of the sun.

• The atmosphere serves as a storehouse for water vapour, which leads to precipitation and hence facilitates the hydrological cycles.

Structure of the Atmosphere

• The atmosphere has a layered structure, because of density stratification as a result of which lighter gases move up and denser ones settle down.

• The lowermost part of the atmosphere in which we live, and which is the theatre for which we live, and which is the theatre for almost all the weather phenomena is known as the troposphere.

• The height of the troposphere at the poles is about 8 km, while at the equator it is about 16 km. This is because there is greater heating at the equator.

• Above troposphere is the stratosphere, which is important primarily because of the presence of zone.

• This layer of calm and clear air is preferred for high-speed jet flights because of the absence of air pockets.

• Above stratosphere is mesosphere, which is more of a transitional layer.

• Above mesosphere lies the ionosphere, which has electrically conducting layers that help in radio communication.

• There are two important layers in the ionosphere viz. E Layer of Kennelly Heavy side layer that reflects the medium radio waves, thus helping in short distance radio communication and F Layer or Appleton layer that reflects the short radio waves and helps in long distance radio communication.

• The outermost layer of the Earth’s atmosphere is known as the exosphere. It is so highly rarified a region that its boundary is not clear.

Heating and Cooling of the Atmosphere

Insolation

• The word ‘insolation’ is an acronym for Incoming solar radiation, which is received at the Earth’s surface at the rate of 1.94 calories per square centimeter per minute.

• Energy transfers in the atmosphere take place in three ways: radiation, conduction and convection.

• The vast amount of energy coming to and leaving the Earth is through radiation.

• Radiation from the Earth is called terrestrial radiation and it is in the form of long waves. The atmosphere is heated more by terrestrial radiation than the incoming solar radiation and this also explains why the atmosphere is heated from the ground up instead of vice versa, specially in the troposphere.

Heat Budget

• The average temperature of Earth remains rather constant. It has been possible because of the balance between the amount of incoming solar radiation and the amount of terrestrial radiation returned to space.
• This balance of incoming and outgoing radiation has been termed Earth’s heat budget.

Inversion of Temperature

• Occasionally the temperature in the lower layers of the air increases instead of decreasing with elevation.
• It occurs particularly on cold winter night, when the sky is clear, the air is very dry, and there is no wind.
• These conditions permit quick radiation of heat from the earth’s surface as well as from the lower layers of the atmosphere.

Temperature Anomaly

• Temperature varies even along the same parallel of latitude, land and water contrasts, prevailing winds and ocean currents.
• The difference between the mean temperature of any place and the mean temperature of its parallel is called the temperature anomaly or thermal anomaly.

Atmosphere Pressure

• Air is an extremely compressible gas, and as result, atmospheric pressure varies considerably with height.
• Since the pressure will vary according to the weight of the air above it, the lowest layers of air are densest.
• The modern metric unit of pressure measurement is the millibar (mb), one millibar being equal to the pressure necessary to support 0.75 mm of the mercury column.

Wind system

• Prevailing or Planetary winds blow throughout the year from one latitude to the other in response to the latitudinal differences in air pressure, for example, the trade winds and the westerly winds.
• Westerlies of the southern hemisphere are stronger and more constant in direction than those of the northern hemisphere because of the vast expanse of water.
• They are developed between 40º and 65º south latitudes. These latitudes are hence often called Roaring Forties, Furious Fifties and Shrieking Sixties dreaded terms for navigators.
• Periodic winds reverse their direction periodically with season. For example, monsoons, land and sea breeze and mountain and valley breeze.
• Local winds flow in comparatively small area and have special characteristics. A lot of them are found in the Mediterranean lands and their nomenclature derived from the regional language.

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Clouds

• A cloud is a mass of small water droplets or tiny ice crystals.

• Clouds form when moist air rises and cools.

• Heat from the sun turns, water in the oceans, rivers and moist soil, into water vapour.

• The water vapour expands as it rises and becomes cooler.

• Cool are cannot hold asmuchmoisture as warm air and soon reaches a saturation point.

• The water vapour condenses into tiny water droplets forming clouds.

• Their form, shape, height and movements tell us a great deal about the sky conditions and the likely weather.

• Clouds are classified according to their appearance, form and height. There are four groups.
  A  High clouds                     6000 to 12000 m
  B  Middle clouds                   2100 to 6000 m
  C  Low clouds below           2100 m
  D  Clouds of great                1500 to 9000 m vertical extent.

• The different types of cloud are given Latin names which are all combinations of the following words.
  1.Cirrus means looking like a feather, it is used to describe the very high clouds.
  2.Cumulus means looking like a heap. It is used to describe clouds which have flat bases and rounded tops. There are patches of blue sky between the clouds.
  3. Stratus means lying in level sheets. It is used for layer-type clouds. 
  4. Nimbus means rain cloud.
  5. Alto means high.

El Nino

• At interval of some 3 to 8 years there occurs a remarkable disturbance of ocean and atmosphere.
• It begins in the eastern Pacific Ocean and spreads its effects widely over the globe for more than a year’s time.
• It bring with it unseasonal weather patterns with abnormalities in the form of droughts. Heavy rain falls, severe spells of heat and cold, or a high incidence of cyclonic storms.
• This occurrence is referred to as EL NINO, meaning the ‘Christ Child’ in Spanish since this phenomenon occurs each year around Christmas time.
• With the onset of El Nino, upwelling ceases, the cool water is replaced by warm water from the west, and the plankton and their anchoveta predators disappear.
• Vast numbers of birds that feed on the anchoveta die of starvation.
• The El Nino (EN) condition influences climates on the two sides of the tropical Pacific Ocean.

Global Warming & Climate

Global warming refers to an average increase in the Earth’s temperature, which in turn, causes changes in climate. A warmer Earth may lead to changes in rainfall patterns, a rise in sea level, and a wide range of impacts on plants, wildlife, and humans. When scientists talk about the issue of climate change, their concern is about global warming caused by human activities.

The Impact of Climate Change

• It has been conclusively proved that climate change is attributable to human intervention. Carbon dioxide, emitted mainly by the burning of fossil fuels, and the emission of methane, nitrous oxide, CFC’s and other greenhouse gases thickens the blanket of greenhouse gases over the earth’s atmosphere, upsetting the natural flow of energy from the sun.

• Deforestation accelerates global warming by reducing the absorption of carbon dioxide in the atmosphere.

• Based on current emission levels, it is estimated that the global temperature will rise by between 1ºC and 3.5ºC by the year 2100; that even after emission levels are stabilized, climate change will continue to occur for hundreds of years

Greenhouse Effect

• The Greenhouse Effect is the result of the atmosphere’s absorption of long-wave radiation emitted by Earth.

• Among the atmospheric constituents, carbon dioxide absorbs the largest proportion of that radiation.

• Thus, when the carbon dioxide content of air changes, a corresponding increase or decrease in the greenhouse effect occurs.

Ozone Depletion

• The ozone layer is found in the atmosphere between 20-50 Km from the Earth’s surface.

• The ozone layer is a region of concentration of the allotrope of an oxygen molecule known as ozone (O3), which is produced by the action of solar radiation on ordinary oxygen atoms.

• It filters sunlight and prevents the harmful ultraviolet radiation from reaching the Earth’s surface by absorbing most of the ultraviolet radiation.

• If these ultraviolet rays were to reach the Earth’s surface in full intensity, all exposed bacteria would be destroyed and animal tissues damaged severely.

Acid Rain

• It is the precipitation charged with an excessive amount of acid droplets formed when oxides of Sulphur and Nitrogen, released by the burning of hydrocarbons, are converted to acids in the atmosphere.

• Acid rain leaches crucial minerals like calcium and magnesium, which are essential for plant growth, from the soils. A related problem is the continued leaching of heavy metals and other substances, providing a persistent source of toxicity to surrounding vegetation and aquatic life.

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