Warm Temperate (Mid-Latitude) Climates-C
- Warm temperate (mid-latitude) climates extend from 30° - 50° of latitude mainly on the eastern and western margins of continents. These climates generally have warm summers with mild winters. They are grouped into four types:
(i) Humid subtropical, i.e. dry in winter and hot in summer (Cwa);
(ii) Mediterranean (Cs);
(iii) Humid subtropical, i.e. no dry season and mild winter (Cfa);
(iv) Marine west coast climate (Cfb).
Humid Subtropical Climate (Cwa)
- Humid subtropical climate occurs pole ward 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)
- As the name suggests, 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 characterised 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 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 50-250cm.
Cold Snow Forest Climates (D)
- 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: (i) Df- cold climate with humid winter; (ii) 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:
(i) Tundra (ET); (ii) Ice Cap (EF).
Tundra Climate (ET)(imp)
- 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 Antartica.
- 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.
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 in a cyclical manner. According to some meteorologists, when the number of sunspots increase, cooler and wetter weather and greater storminess 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 Millankovitch 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 the 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 Pinatoba 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 the increasing trend in the concentration of greenhouse gases in the atmosphere which is likely to cause global warming.
- Due to the presence of greenhouse gases, the atmosphere is behaving like a greenhouse.
- The atmosphere also transmits the incoming solar radiation but absorbs the vast majority of long wave radiation emitted upwards by the earth’s surface.
- The gases that absorb long wave radiation are called greenhouse gases.
- The processes that warm the atmosphere are often collectively referred to as the greenhouse effect.
- The term greenhouse is derived from the analogy to a greenhouse used in cold areas for preserving heat. A greenhouse is made up of glass. The glass which is transparent to incoming short wave solar radiation is opaque to outgoing long wave radiation. The glass, therefore, allows in more radiation and prevents the long wave radiation going outside the glass house, causing the temperature inside the glasshouse structure warmer than outside.
- 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 the 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 that the more time the GHG molecule remains in the atmosphere, the longer it will take for earth’s atmospheric system to recover from any change brought about by the latter.
- The largest concentration of GHGs in the 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.
- Doubling of concentration of CO2 over pre-industrial level is used as an index for estimating the changes in climate in climatic models.
- 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 industrialised countries to reduce their emissions by the year 2012 to 5 per cent less than the levels prevalent in the year 1990.