WORLD CLIMATE(part 3)

                                                   WORLD CLIMATE

 Group A: Tropical Humid Climates

 

• Tropical humid climates exist between Tropic of Cancer and Tropic of Capricorn.
• The sun being overhead throughout the year and the presence of Inter Tropical Convergence Zone (TTCZ) make the climate hot and humid.
• Annual range of temperature is very low and annual rainfall is high.
• The tropical group is divided into three types, namely:
• Af - Tropical wet climate;
• Am - Tropical monsoon climate;
• Aw - Tropical wet and dry climate.
• Tropical Wet Climate (Af)
• Tropical wet climate is round near the equator the major areas are the Amazon Basin in South America, western equatorial Africa and the islands of East Indies.
• Significant amount of rainfall occurs in every month of the year as thunder showers in the afternoon.
• The temperature is uniformly high and the annual range of temperature is negligible.
• The maximum temperature on any day is around 30°C while the minimum temperature is around 20°C.
• Tropical evergreen forests with dense canopy cover and large biodiversity are found in mis climate.

 

Tropical Monsoon Climate (Am)

• Tropical monsoon climate (Am) is found over me Indian sub-continent. North Eastern part of South America and Northern Australia. Heavy rainfall occurs mostly in summer. Winter is dry.                                

 

Tropical Wet and Dry Climate (Aw)

• Tropical wet and dry climate occurs north and south of Af type climate regions.
• It borders with dry climate on me western part of me continent and Cf or Cw on me eastern part.                                
• Extensive Aw climate is found to the north and soum of the Amazon forest in Brazil and adjoining parts of Bolivia and Paraguay in South America, Sudan and south of Central Africa.                                
• The annual rainfall in this climate is corisiderably less than that in Af and Am climate types and is variable also.                               
• The wet season is shorter and me dry season is longer with me drought being more severe.                  
• Temperature is high throughout the year and diurnal ranges of temperature are the greatest in the dry season.
• Deciduous forest and tree-shredded grasslands occur in this climate.

 

 

Dry Climates : (B)

 

• Dry climates are characterized by very low rainfall that is not adequate for the growth of plants.
• These climates cover a very large area of the planet extending over large latitudes from 15°-60° north and south of the equator.
• At low latitudes, from 15°-30°, they occur in the area of subtropical high where subsidence and inversion of temperature do not produce rainfall.
• On me western margin of the continents, adjoining the cold current, particularly over the west coast of South America, they extend more equatorward and occur on the coast land.
• In middle latitudes, from 35°-60° north and south of equator, they are confined to the interior of continents where maritime-humid winds do not reach and to areas often surrounded by mountains.
• Dry climates are divided into steppe or semi-arid climate (BS) and desert climate (BW).
• They are further subdivided as subtropical steppe (BSh) and subtropical desert (BWh) at latitudes from 15°-35° and mid-latitude steppe (BSk) and mid-latitude desert (BWk) at latitudes between 35°-60°.

 

Subtropical Steppe (BSh) and Subtropical Desert (BWh) Climates

• Subtropical steppe (BSh) and subtropical desert (BWh) have common precipitation and temperature characteristics.
• Located in the transition zone between humid and dry climates, subtropical steppe receives slightly more rainfall than the desert, adequate enough for the growth of sparse grasslands.
• The rainfall in both the climates is highly variable. The variability in the rainfall affects me life in the steppe much more than in the desert, more often causing famine.
• Rain occurs in short intense thundershowers in deserts and is ineffective in building soil moisture.
• Fog is common in coastal deserts bordering cold currents.
• Maximum temperature in the summer is very high.
• The highest shade temperature of 58°C was recorded at Al Aziziyah, Libya on 13 September, 1922.
• The annual and diurnal ranges of temperature are also high.

 

Warm Temperate (Mid-Latitude) Climates-<C)

• Warm temperate (mid-latitude) climates extend from 30°-50° of latitude mainly on me eastern and western margins of continents.
• These climates generally have warm summers with mild winters.
• They are grouped into four types :
• Humid subtropical, i.e. dry in winter and hot in summer (Cwa);
• Mediterranean (Cs);
• Humid subtropical, i.e. no dry season and mild winter (Cfa);
• Marine west coast climate (Cfb).

 

Humid Subtropical Climate (Cwa)

• Humid subtropical climate occurs poleward of Tropic of Cancer and Capricorn, mainly in North Indian plains and South China interior plains.
• The climate is similar to Aw climate except that the temperature in winter is warm.

 

Mediterranean Climate (Cs)

• Mediterranean climate occurs around Mediterranean sea, along the west coast of continents in subtropical latitudes between 30°-40° latitudes e.g. - Central California, Central Chile, along the coast in south eastern and south western Australia.
• These areas come under the influence of sub tropical high in summer and westerly wind in winter.
• Hence, the climate is characterized by hot, dry summer and mild, rainy winter.
• Monthly average temperature in summer is around 25°C and in winter below 10°C. ,
• The annual precipitation ranges between 35-90 cm.

 

Humid Subtropical (Cfa) Climate

• Humid subtropical climate lies on the eastern parts of the continent in subtropical latitudes.
• In this region the air masses are generally unstable and cause rainfall throughout the year.
• They occur in eastern United States of America, southern and eastern China, southern Japan, northeastern Argentina, coastal south Africa and eastern coast of Australia.
• The annual averages of precipitation vary from 75-150 cm.
• Thunderstorms in summer and frontal precipitation in winter are common.
• Mean monthly temperature in summer is around 27°C, and in winter it varies from 5°-12°C. The daily range of temperature is small.

 

Marine West Coast Climate (Cfb)

• Marine west coast climate is located poleward from the Mediterranean climate on the west coast of the continents.
• The main areas are : Northwestern Europe, west coast of North America, north of California, southern Chile, southeastern Australia and New Zealand.
• Due to marine influence, the temperature is moderate and in winter, it is warmer than I for its latitude.                   
• The mean temperature in summer months ranges from 15°-20°C and in winter 4°-10°C. The annual and daily ranges of temperature are small.
• Precipitation occurs throughout the year. Precipitation varies greatly from 50-250 cm.

 

Cold Snow Forest Climates (D)

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• Cold snow forest climates occur in the large continental area in the northern hemisphere between 40°-70° north latitudes in Europe, Asia and North America.
• Cold snow forest climates are divided into two types:
• Df - cold climate with humid winter;
• Dw - cold climate with dry winter.
• The severity of winter is more pronounced in higher latitudes.

 

Cold Climate with Humid Winters (Df)

• Cold climate with humid winter occurs poleward of marine west coast climate and mid latitude steppe.
• The winters are cold and snowy. The frost free season is short.
• The annual ranges of temperature are large. The weather changes are abrupt and short. Poleward, the winters are more severe.

 

Cold Climate with Dry Winters (Dw)

• Cold climate with dry winter occurs mainly over Northeastern Asia.
• The development of pronounced winter anti cyclone and its weakening in summer sets in monsoon like reversal of wind in this region.
• Poleward summer temperatures are lower and winter temperatures are extremely low with many locations experiencing below freezing point temperatures for up to seven months in a year.
• Precipitation occurs in summer. The annual precipitation is low from 12-15 cm.

 

Polar Climates (E)

 

• Polar climates exist poleward beyond 70° latitude. Polar climates consist of two types :
• Tundra (ET);
• Ice Cap (EF).

 

Tundra Climate (ET)

• The tundra climate (ET) is so called after the types of vegetation, like low growing mosses, lichens and flowering plants.
• This is the region of permafrost where the sub soil is permanently frozen.
• The short growing season and water logging support only low growing plants.
• During summer, the tundra regions have very long duration of day light.

 

Ice Cap Climate (EF)

• The ice cap climate (EF) occurs over interior Greenland and Antarctica.
• Even in summer, the temperature is below freezing point. This area receives very little precipitation.
• The snow and ice get accumulated and the mounting pressure causes the deformation of the ice sheets and they break.
• They move as icebergs that float in the Arctic and Antarctic waters. Plateau Station, Antarctica, 79°S, portray this climate.

 

Highland Climates (H)

 

• Highland climates are governed by topography. In high mountains, large changes in mean temperature occur over short distances.
• Precipitation types and intensity also vary spatially across high lands.
• There is vertical zonation of layering of climatic types with elevation in the mountain environment.

 

Climate Change

 

• The earlier chapters on climate summarized our understanding of climate as it prevails now
• The type of climate we experience now might be prevailing over the last 10,000 years with minor and occasionally wide fluctuations.
• The planet earth has witnessed many variations in climate since the beginning.
• Geological records show alteration of glacial and inter-glacial periods.
• The geomorphological features, especially in high altitudes and high latitudes, exhibit traces of advances and retreats of glaciers.
• The sediment deposits in glacial lakes also reveal the occurrence of warm and cold periods.
• The rings in the trees provide clues about wet and dry periods.
• Historical records describe the vagaries in climate. All these evidences indicate that change in climate is a natural and continuous process.
• India also witnessed alternate wet and dry periods.
• Archaeological findings show that the Rajasthan desert experienced wet and cool climate around 8000 B.C.
• The period 3000-1700 B.C. had higher raimall.
• From about 2000-1700 B.C., mis region was the center of me Harappan civilization. Dry conditions accentuated since men.
• In the geological past, the earth was warm some 500-300 million years ago, through me Cambrian, Ordovician and Silurian periods.
• During the Pleistocene epoch, glacial and inter-glacial periods occurred, the last major peak glacial period was about 18,000 years ago.
• The present inter-glacial period started 10,000 years ago.

 

Climate m the recent past

• A number of times Europe witnessed warm, wet, cold and dry periods, me significant episodes were me warm and dry conditions in the tenth and eleventh centimes, when me Vikings settled in Greenland.
• Europe witnessed "Utile Ice Age" from 1550 to about 1850.
• From about 1885-1940 world temperature showed an upward trend. After 19W,tne rate of increase in temperature slowed down.

 

Causes of Climate Change .

• The causes for climate change are many. They can be grouped into astronomical and terrestrial causes. The astronomical causes are the changes in solar output associated with sunspot activities.                                                             
• Sunspots are dark and cooler patches on the sun which increase and decrease cyclical manner.                                         
• According to some meteorologists, when the number of sunspots increase, cooler and wetter weather and greater stoniness occur.
• A decrease in sunspot numbers is associated with warm and drier conditions.
• Yet, these findings are not statistically significant.
• An another astronomical theory is Milankovitch oscillations, which infer cycles in the variations in the earth's orbital characteristics around the sun, the wobbling of the earth and the changes in the earth's axial tilt.
• All these alter the amount of insolation received from the sun, which in turn, might have a bearing on the climate.
• Volcanism is considered as another cause for climate change.
• Volcanic eruption throws up lots of aerosols into me atmosphere.
• These aerosols remain in the atmosphere for a considerable period of time reducing the sun's radiation reaching the Earth's surface.
• After the recent Pinatubo and El Cion volcanic eruptions, the average temperature of the earth fell to some extent for some years.
• The most important anthropogenic effect on the climate is me increasing trend in the concentration of greenhouse gases in the atmosphere which is likely to cause global waning.

 

Global Warming

 

• Due to me presence of greenhouse gases, the atmosphere is behaving like a greenhouse.
• The atmosphere also transmits me incoming solar radiation but absorbs the vast majority of long wave radiation emitted upwards by me earth's surface.
• The gases mat absorb long wave radiation are called greenhouse gases.
• The processes that warm the atmosphere are often collectively referred to as the greenhouse effect.

 

Greenhouse Gases (GHGs)

• The primary GHGs of concern today are carbon dioxide 
  (CO2),
   Chlorofluorocarbons (CFCs), methane 
  (CH4),
   nitrous oxide 
  (N2O)
   and ozone 
  (O3).
• Some other gases such as nitric oxide (NO) and carbon monoxide (CO) easily react with GHGs and affect their concentration in me atmosphere.
• The effectiveness of any given GHG molecule will depend on the magnitude of the increase in its concentration, its life time in the atmosphere and the wavelength of radiation that it absorbs.
• The Chlorofluorocarbons (CFCs) are highly effective.
• Ozone which absorbs ultra violet radiation in the stratosphere is very effective in absorbing terrestrial radiation when it is present in the lower troposphere.
• Another important point to be noted is mat the more time the GHG molecule remains in me atmosphere, me longer it will take for earth's atmospheric system to recover from any change brought about by me latter.
• The largest concentration of GHGs in me atmosphere is carbon dioxide.
• The emission of 
  CO2
  comes mainly from fossil fuel combustion (oil, gas and coal).
• Forests and oceans are the sinks for the carbon dioxide.
• Forests use 
  CO2
  in their growth. So, deforestation due to changes in land use, also increases the concentration of 
  CO2.
• The time taken for atmospheric 
  CO2
  to adjust to changes in sources to sinks is 20-50 years.
• It is rising at about 0.5 per cent annually.
• Doubling of concentration of 
  CO2
  over pre-industrial level is used as an index for estimating the changes in climate in climatic models.
• Chlorofluorocarbons (CFCs) are products of human activity.
• Ozone occurs in the stratosphere where ultra-violet rays convert oxygen into ozone.
• Thus, ultra violet rays do not reach the earth's surface. The CFCs which drift into the stratosphere destroy the ozone.
• Large depletion of ozone occurs over Antarctica.
• The depletion of ozone concentration in the stratosphere is called the ozone hole.
• This allows the ultra violet rays to pass through the troposphere.
• International efforts have been initiated for reducing the emission of GHGs into the atmosphere.
• The most important one is the Kyoto protocol proclaimed in 1997.
• This protocol went into effect in 2005, ratified by 141 nations. Kyoto protocol bounds the 35 industrialized countries to reduce their emissions by the year 2012 to 5 per cent less than the levels prevalent in the year 1990.