Question : Explain the basis of Koppen's classification of climates. Also mention its merits and limitations.
(1994)
Answer : The earliest classification of climatic types was made by the Greek geographers. However, of the several schemes of climatic classification in modern times, the one devised by Wladimir Koppen,a German botanist and climatologist, still remains the most widely known descriptive system. It is a quantitative as well as empirical classification of climates.
Koppen proposed his first classification in 1900, using the world vegetation map of de Candolle, French plant physiologist. He revised his scheme in the year 1918, wherein he paid more attention to monthly and annual averages of temperature and precipitation and their seasonal distribution. He again modified his scheme in 1931 and 1936. Koppen's original scheme was modified in 1953 by Geigger - Pohl and the revised scheme is known as Koppen Geigger Pohl's scheme of classification of world climates.
Following are the five principal biological groups (identified by Candolle and used by Koppen) that are largely controlled by temperature and moisture :
In his revised schemes, Koppen not only used extremes of temperature and precipitation like,
(i)Temperature of coldest and warmest months and precipitation of wettest and driest months; and
(ii)Vegetational correspondence.
But also their seasonal variations and vegetational correspondence. He firmly believed that vegetation of a region is the most visible index of the climate.
Based on these criteria, Koppen divided the world climates into 5 principal types and designated them by capital letters A,B,C,D and E.
(1)A: Humid Tropical Climates
(2)B: Dry Climates
(3)C: Warm Temperate Rainy Climates
(4)D: Cool Temperate (snow) Climates or Taiga
(5)E: Polar Climates
At the next level, Koppen uses seasonal variations in rainfall to sub-divide the regions -
A is divided into Af, Am, Aw and As
C is divided into Cf, Cw, and Cs
D is divided into Df, Dw and Ds
E is divided into Et and Ef; where f denotes rainfall throughout the year; M denotes monsoon climate with short dry season; W denotes winter dry season and S denotes summer dry season.
Boundary between Af, Am and Aw
(i)Af: Equatorial rainforest climate;
(ii)Am: Tropical Monsoon Climate;
(iii)Aw: Tropical Savanna climate;
(iv)As represents tropical climate with well defined summer dry season which is rare, for example India's Tamil Nadu coast.
Likewise further division of C climates are:
(i)Cf-West European Type Climate
(ii)Cw-China Type Climate
(iii)Cs-Mediterranean Type Climate
In the same way D climate is divided into:
(i)Df-Humid cold climate with no dry reason;
(ii)Dw-Humid cold climate with dry winters;
(iii)Ds-Cool East Coast Climate: At the 2nd level, E climates is divided into two types:
(a)E T climate: It denotes Tundra climate. Here average temperature of warmest month is between 00C and 10°C.
(b)E F climate: It denotes Polar Ice cap climate. Average temperature of warmest month is 0°C or below. It may be noted that ET and EF have little difference in rainfall.
Dry Climates (B Climates): It is to be noticed that four of the principal categories of climatic groups (A,C,D,E) are based on temperature characteristics while the B type of climate has precipitation as its fundamental criterion.In B climates, the evaporation exceeds precipitation and the precipitation is not sufficient to maintain permanent stable water table.
The dry humid boundary is defined by the following formula:
(i)RT +28 when 70% or more of rain fall is in warmer 6 months.
(ii)RT when 70% or more of rain is in cooler 6 months.
(iii)RT + 14 when neither half year has 70% or more of rain.
(Here R is average annual precipitation in cm and T is average annual temperature in °C.)
B Climates are divided into two types:
(i)BW-Dry Desert Climate;
(ii)BS-Semi-arid or steppe climate.
The boundary between BW and BS climates is 1/2 the dry /humid boundary.
Further, at the Third Level Koppen has divided
(1)B into
(i)h-hot, mean annual temperature 180 C or greater;
(ii)k-cold, mean annual temperature below 18°C.
(2)C into - a,b,c
where a,b and c are defined with respect to decreasing temperature.
(3)D into - a,b,c, and d
where a,b, c and d are defined with respect to decreasing temperature.
At the Fourth Level, Koppen has used supplementary symbols to accommodate smaller and newer climatic sub-divisions. For example Cwg-Ganga Type Climate.
Merits:
(i)Koppen's classification has universal application.
(ii)His scheme offers differentiation also.
(iii)The classification has enough flexibility.
(iv)His classification is empirical and descriptive one, hence it is more stable and less liable to frequent changes.
(v)Temperature and precipitation are two elements on which data are easily available everywhere.
(vi)Koppen was the first one to use letter symbols for classification. This helps in avoidingrepetition.
Limitations:
(i)Koppen is criticised for achieving inter-relationship of climatic types with soil and vegetation by using faulty methods.
(ii)He has used temperature and precipitation data and attempted vegetational correspondence but vegetation is not a function of annual rainfall or temperature rather it is a function of moisture availability.
(iii)This classification does not at all deal with causal mechanism.
(iv)The concept of precipitation effectiveness used by Koppen to distinguish humid and arid type of climates is faulty.
(v)For these reasons, the classification is not of much use for meteorologists or climatologists.
Question : Discuss the development of local winds, and their influence on local weather, giving three examples of the well-known local winds in the world.
(2007)
Answer : The weather of any place is always the result of interplay among many climatic and geographical factors. The local winds, though narrow in scope, bring considerable influence upon the local weather conditions. The major factors that play important role inthe development of local winds are temperature, rainfall, proximity to sea, relief, direction of winds, aspect of sun etc. The importance and role of these factors vary from place to place.
Development of Winds: Wind is the flow of air, more generally, it is the flow of the gases which compose an atmosphere.Simply it occurs as air is heated by the sun and thus rises. Cool air then rushes in to occupy the area the now hot air has moved from. It could be loosely classed as a convection current.Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the geographic regions in which they occur, or their effect.
There are global winds, such as the wind belts which exist between the atmospheric circulation cells. There are upper-level winds which typically include narrow belts of concentrated flow called jet streams. There are synoptic-scale winds that result from pressure differences in surface air masses in the middle latitudes, and there are winds that come about as a consequence of geographic features, such as the sea breezes on coastlines or canyon breezes near mountains. Mesoscale winds are those which act on a local scale, such as gust fronts. At the smallest scale are the microscale winds, which blow on a scale of only tens to hundreds of meters and are essentially unpredictable, such as dust devils and microbursts.
Forces which drive wind or affect it are the pressure gradient force, the Coriolis force, buoyancy forces, and friction forces. When a difference in pressure exists between two adjacent air masses, the air tends to flow from the region of high pressure to the region of low pressure. On a rotating planet, flows will be acted upon by the Coriolis force, in regions sufficiently far from the equator and sufficiently high above the surface.
The three major driving factors of large scale global winds are the differential heating between the equator and the poles and the rotation of the planet. Some local winds blow only under certain circumstances, i.e. they require a certain temperature distribution.
Land Breezes and Sea Breezes: Diffe-rential heating is the motive force behind land breezes and sea breezes (or, in the case of larger lakes, lake breezes), also known as on- or off-shore winds. Land absorbs and radiates heat faster than water, but water releases heat over a longer period of time. The result is that, in locations where sea and land meet, heat absorbed over the day will be radiated more quickly by the land at night, cooling the air. Over the sea, heat is still being released into the air at night, which rises. This convective motion draws the cool land air in to replace the rising air, resulting in a land breeze in the late night and early morning. During the day, the roles are reversed. Warm air over the land rises, pulling cool air in from the sea to replace it, giving a sea breeze during the afternoon and evening.
Mountain Breezes and Valley Breezes: Mountain breezes and valley breezes are due to a combination of differential heating and geometry. When the sun rises, it is the tops of the mountain peaks which receive first light, and as the day progresses, the mountainslopes take on a greater heat load than the valleys. This results in a temperature inequity between the two, and as warm air rises off the slopes, cool air moves up out of the valleys to replace it.
This upslope wind is called a valley breeze. The opposite effect takes place in the afternoon, as the valley radiates heat. The peaks, long since cooled, transport air into the valley in a process that is partly gravitational and partly convective and is called a mountain breeze.
Mountain breezes are one example of what is known more generally as a katabatic wind. These are winds driven by cold air flowing down a slope, and occur on the largest scale in Greenland and Antarctica. Most often, this term refers to winds which form when air which has cooled over a high, cold plateau is set in motion and descends under the influence of gravity. Winds of this type are common in regions of Mongolia and in glaciated locations.
Because katabatic refers specifically to the vertical motion of the wind, this group also includes winds which form on the lee side of mountains, and heat as a consequence of compression. Such winds may undergo a temperature increase of 20°C (36°F) or more, and many of the world’s “named” winds (see list below) belong to this group. Among the most well-known of these winds are the chinook of Western Canada and the American Northwest, the Swiss föhn, California’s infamous Santa Ana wind, and the French Mistral.
The opposite of the katabatic wind is an anabatic wind, or an upward-moving wind. The above-described valley breeze is an anabatic wind.A widely-used term, though one not formally recognised by meteorologists, is orographic wind. This refers to air which undergoes orographic lifting. Most often, this is in the context of winds such as the chinook or the föhn, which undergo lifting by mountain ranges before descending and warming on the lee side.
The influence of local winds, generally, has two major dimensions. They either bring extremities to normal weather conditions or moderates the influence of already existing extreme conditions.
The local winds are found almost in all major climatic regions. Their influence range from geographical to cultural aspects also. They considerably influence the occupational structure, way of living, settlement types and pattern etc.
The development and influence of three major local winds is as under:
(a) Chinook (North America): The Chinook winds originate over Northern Pacific Ocean near the
coast of North America. It brings considerable influence over prairie grasslands in Canada and USA.
These winds come in south-westerly direction and ascend the Rockies and they descend to prairies. While ascending Rockies, they bring significant rainfall and even, sometimes, snowfall over Rockies, as they are very humid.
While descending, they lose their moisture and their temperature rises considerably. Now these winds become hot and dry.
In plains, they melt the snow-covered pastures and animal rearing and agriculture becomes possible. For this very reason, they are widely welcomed and hence called snow-eater.
(b)Sirocco (Mediterranean): Sirocco originates over Northern Sahara desert and, hence, it is a hot, dry and dusty wind. It moves in northward direction. After reaching the Mediterrnean sea it gains moisture and its temperature also comes down. Still it remains hot. It’s scorching heat hampers the growth of vegetation and crops in Mediterranean region. It may also cause “Blood Rain” as it carries red dust from Sahara desert.
(c) Mistral (Europe): This is a cold wind which originates over snow laden Alps mountains. This wind moves in southerly direction towards Mediterranean Sea. This moves along Rhone valley as a violent, high speed wind. The shape of valley further intensifies its speed. This wind is most frequent during winter.
The impact of this wind is so profound that fields and houses have to be protected from this wind by constructing a wall in the north.
Thus, the local winds have always been an important part of weather system of any place. They definitely play an important role in each and every aspect of human life.
Question : Compare the structure and associated weather condition of tropical cyclone with that of the temperate cyclone.
(2006)
Answer : The cyclones are closed air circulation around a low pressure centre having steep barometric gradient and an associated revolving storm, accompanied by cyclonic rain. In a cyclone winds circulate blowing inwards in anti-clockwise direction in the Northern Hemisphere and in clockwise direction in the Southern Hemisphere. They are mainly of two types based on their latitudinal location - Tropical cyclone and Temperate cyclone. A comparative study is made under the following heads:
I. Origin And Development:
Tropical Cyclone: Tropical cyclone is one of the most destructive of the weather phenomena occuring in tropical latitudes. This is like a heat engine that is energised by the Latent heat of condensation. Since there is more heating in tropical area, more moisture is available for the latent heat of condensation to lower this more violent weather disturbance.
Necessary Conditions: Through the exact trigger mechanism is not well understood certain conditions must precede the formation of tropical cyclone:
Life cycle / Evolution: Tropical cyclones develop in four stages:
Temperate Cyclone: V. Bjerknes and J. Bjerknes developed a polar front theory of the temperate cyclone’s origin which is described and depicted in six stages below:
Thus the genesis and sources of energy for tropical & temperate cyclones are markedly different. The latter grow in distinct stages having larger life span (8 to 10 days) than the former (3 to 6 days normally).
Moreover, temperate cyclones are formed more frequently in winter than summer while tropical cyclone are formed only during summer. Temperate cyclones form, develop and dissipate gradually while tropical one also form and develop slowly but their death is more sudden.
II. Distribution: The favourite grounds of tropical & temperate cyclones & their tracks are vividly depicted in two figures.
III. Size and Shape: The tropical cyclones have symmetrical elliptical shape. The isobars are generally circular with steep pressure gradient. They cover comparatively smaller area having a diameter of 500 to 600 km.
The temperate cyclones are asymmetrical. The isobars are generally oval or elongated in shape with low pressure gradient, having a diameter of 300 to
1500 km.
IV. Orientation and Movement: Being in trade wind belt, tropical cyclones usually travel from east to west with wind velocity more than 120 km. per hour.
Since temperate cyclones move with the westerlies, they generally travel from west to east having wind velocity normally not exceeding 50 km./hour. Their surface track has more direct link with the jet streams.
V. Structure: A simplified idea of the structure of a tropical cyclone given in the figure below shows that right at its centre there is a warm central core or EYE which is surprisingly calm and cloudless despite excessive low pressure. This is the warmest part due to descending adiabatic winds. It is surrounded by great vertical wall of cumulonimbus clouds extending up to tropopause, having maximum wind velocity and heavy rain.
In temperate cyclones, North Western sector is the cold sector and North Eastern sector is the warm sector. Thus all the sectors have different temperatures. Winds and rains are active everywhere.
VI. Weather: In tropical cyclones weather changes are faster. Rainfall is heavy and more concentrated over space and time both, accompanied with thunderstorms and lightning.
Weather changes are more gradual in temperate cyclones. Rainfall is slow, continuous and evenly distributed.
VII. Other Distinctions: Tropical cyclones are more destructive than temperate ones for two reasons:
On the contrary, temperate cyclones are less destructive as velocity is less and predictability is high.
Question : Discuss the mechanism and significance of tri-cellular meridional circulation of atmosphere.
(2003)
Answer : Temperature differences produced by the varying amount of insolation received at the earth’s surface account for the density differences that drive the atmosphere in three-dimensional motion on global scale. The distribution pattern of temperature, pressure and the resultant winds depend basically on the distribution of insolation and the orbit of our planet around the sun. Besides the geography of earth and the constituent of atmosphere also determine the general circulation pattern.
The old concept of the mechanism of general circulation of the atmosphere envisages that the movement of air is temperature dependent, i.e. due to the imballance of heat energy between the equator and the poles. Consequently, there is transfer of heat through horizontal air circulation from the areas of high solar radiation (low latitutdes). In its simplest form, this should operate like a gigantic heat engine and produce a single-cell circulation in each hemisphere. But this is at variance with the observed pattern of global winds. The reason is that the old school considers only the horizontal component of the atmospheric circulation and also it does not take into account the rotation of the earth on its axis.
As long ago as the eighteenth century, a more sophisticated tri-cellular model was proposed to explain the then known facts of atmospheric circulation. Hadlee in 1935, argued logically that in each hemisphere there must be a low-latitude cell which relies for its driving mechanism on the rising of heated air at the equator, its outflow aloft polewards and sinking at higher latitudes before returning to the equator. We call this cell now a days a thermally direct cell or Hadlee cell. A very important aspect of Hadlee cell is the vertical exchange of heat transfer.
Similarly, in this three-cell scheme, another thermally direct cell exists at the pole, the contraction of the air column in these cold regions initiating inward flow at high level and outward divergence at low level. The mid-latitude cell is considered to be thermally indirect, largely maintained by the circulations in the other two.
This tri-cellular concept fits in reasonably well with the surface winds, but with increasing information about the upper winds, this concept seems to be inadequate, particularly for not accounting the great pre-ponderence of westerly winds at high level in middle latitudes.
The tri-cellular model of the meridional circulation of atmosphere, prepared by Palmen in 1951, is a good example of modern thinking about the circulation system. The model makes it clear that there are two possible ways of transporting heat and momentum:
(a) by circulation in the vertical plane
(b) by horizontal circulation
The figure given here depicts the vertical and horizontal circulation in three distinct meridional cells in the Northern Hemisphere.
It was also discovered that anti trade wind system is neither regular nor continuous. The continuity of the anti trades are found more over the eastern part of the oceans and in the southern hemisphere. Over the continental land masses, the anti trades are characterized by interrupted movements.
The subsidence zone of the poleward moving upper flow in the tropical cell is the site of the world’s tropical desert. Centre of this subsidence zone is popularly known as horse latitudes. From the equatorward margin of the horse latitudes the surface flow towards the equator is known as the trade winds, thus completing the cellular pattern of the tropical circulation.
Another factor of major significance in the tropical circulation is its reasonal migration. The poleward displacement of ITCZ and thermal equator has an important role to play in the monsoonal circulation and the surface equatorial westerlies.
Mid-latitude cell or polar front cell: This mid-latitude cell also called the Ferrel cell, is thermally indirect. The surface wind blow from subtropical high pressure belt to subpolar low but the winds become almost westerly due to cariolis force. The prevailing westerlies are frequently disturbed by the migratory temperate cyclones and anticyclones.
Contrary to the old view of upper air tropospheric easterly winds in the zone between 30°-60° latitudes, there exists upper air westerlies in this mid-latitude zone, the cause of which is said to be meridional temperature gradient. According to Rossby the westerly momentum is transferred to mid-latitudes from the upper branches of the cells in high. The mechanism of these three meridional cells is being discussed here separately.
(i) Tropical cell: The tropical cell, also called Hadlee cell, is the dominant feature of the tri-cellular circulation model most likely the only regular cell-like arrangement among the three cells. It is through this cell that the poleward heat transport in tropical and middle latitude is accomplished. This circulation cell is located between the equator and roughly 30° latitudes. In the equatorial zone the warm ascending air currents form cumulonimbus clouds which release latent heat that provides the required energy to drive the tropical cell.
The rising air from thermally-driven tropical cell moves poleward in the upper-troposhere and is called the ‘anti trades’. These air currents found at elevations of 8000 to 12000 metres near the equator begin to descent in a zone between 20° and 25° latitudes. While moving from low to higher latitudes, these friction-less upper tropospheric winds are deflected greatly by the increasing Coriolies force and become geostrophic westerlies.
As more upper air data were made available, it was discovered that tropical circulation is not as simple as it looks. Broadly speaking, circulation is not as simple as it looks. Broadly speaking, circulation takes place in a series of inclined planes, tilted upwards at their poleward ends. Air moves upward and away from the equator on the western side of these cells—that is, on the western side of the oceans in the northern hemisphere-and downwards towards the equator on their eastern limbs. Thus in the cross-sectional notion of the Hadlee cell is added the plan arrangement of several high pressure cells of the subtropics, as shown and low latitudes. The upper air westerlies play a very significant role in the transfer of both air and energy. According to Tre–wartha, the middle and upper tropospheric westerlies are characterised by long waves and Jet Streams.
In the mid-latitude cell, warm air is seen ascending the polar front and breaking through near the tropopause. Interestingly the polar front is more continuous and prominent in middle troposphere. Major heat exchange takes place at the surface and aloft.
(ii) Polar or Sub polar cell: The third circulation cell over the polar and sub polar regions between 60° latitudes and the poles is very modified and weak one. Subsidence near the poles produces a surface flow towards the equator which becomes polar easterly under the Coriolis force. The cold polar easterlies in their equatorward movement clash with the warmer westerlies of the temperate regions and form polar front which becomes the centre for the origin of temperate cyclones. This cell is characterised by considerable horizontal mixing at all levels. Here heat transport is accomplished by the waves in the westerlies.
In conclusion, even though the general circulation pattern is said to be principally zonal, yet the meridional component of the wind system is very significant in the latitudinal heat balance of the earth. Perhaps one of the most important advances in recent times has been that the mode of heat transport does not have to be explained solely in terms of vertical air movement, a great deal of heat exchange is also accomplished in a horizontal sense, particularly in middle latitudes. This means frontal systems are a major force in the maintenance of general circulation, and not merely transitory features.Question : Discuss the criteria which Thornthwaite adopted for his 1948 classification of world climates.
(2002)
Answer : Thornthwaite, an American climatologist, presented his first classification in 1931 in which, he considered natural vegetation of a region as the indicater of climate of that region. He accepted the concept that the amount of precipitation and temperature had paramount control on vegetation but he also pleaded for inclusion of evaporation as important factor. Hence, precipitation effectiveness and temperature effectiveness were the chosen criteria for demarcation of the climatic regions.
After making sizeable modification, Thornthwaite presented his modified scheme of climatic classification in 1948. Though, he again used previously devised three indices of precipitation effectiveness, thermal efficiency and seasonal distribution of precipitation in his second classification, but in a different way. Instead of vegetation, he based his new scheme of climatic classification on the concept of Potential Evapotranspiration which is infact an index of thermal efficiency and water loss because it represents the amount of transfer of both moisture and heat to the atmosphere from soils and vegetation (evaporation of liquid or solid water and transpiration from living plant leaves) and thus is a function of energy received from the sun.
It may be pointed out that potential evapotranspiration is calculated (and not directly measured) from the mean monthly temperature (in 0°c) with corrections for day length (i.e. 12 hours). The potential evapotranspiration for a 30 day month (a day having only the length of sunshine i.e. 12 hours) is calculated as follows—
PE (in cm) = 1.6 (10t/I)a
Where, PE = potential evapotranspiration
I = the sum for 12 months of (t/5)1.5/4
a = a further complex function of I
t = temperature in 0°C.
Thornthwaite developed four indices to determine bounderies of different climatic types e.g.
(i) Moisture index
(ii) PE or Thermal efficiency.
(iii) Aridity and Humidity indices.
(iv) Index of concentration of thermal efficiency.
TE or PE
(i) Moisture index (Im)— moisture index refers to moisture deficit or surplus and is calculated according to the following formula—
Im = (1005-60D)/PE
Where, Im = monthly moisture index.
S = monthly surplus of moisture.
D = monthly deficit of moisture.
The sum of the 12 month values of Im gives the annual moisture index.
Annual Moisture Index
(ii) Thermal efficiency index —TE is simply the potential evapotranspiration expressed in centimeter as expressed above. It is, thus, clear that TE is derived from PE value because PE itself a function of temperature. The method of the calculation of PE is written above.
(iii) Aridity and Humidity indices—These indices are used to determine the seasonal distribution of moisture adequacy. These are calculated as follows—
Aridity index—in moist climates annual water deficit taken as a percentage of annual PE becomes aridity index.
Humidity index—in dry climate, annual water surplus taken as a percentage of annual PE becomes humidity index.
(iv) Concentration of thermal efficiency—refers to the percentage of mean annual potential evapotranspiration accumulating in three summer months.
On the basis of moisture index, Thornthwaite identified 9 moisture or humidity provinces.
Im Humidity Province
100 & above — A Perhumid
80 to 100 — B4 Humid
60 to 80 — B3 Humid
40 to 60 — B2 Humid
20 to 40 — B1 Humid
0 to 20 — C2 Moist Sub-humid
– 33.3 to 0 — C1dry Sub-humid
– 66.7 to– 33.3 — D Semiarid
– 100 to – 66.7 — E Arid
On the basis of thermal efficiency nine thermal provinces were recognised :
Thus, according to Thornthwaite, climate of a place is determined by combining the aforesaid elements of the climatic classification e.g. Moisture index, thermal efficiency index, summer concetration of thermal efficiency and seasonal moisture adequacy.
Ironically, on the basis of above indices the classification system becomes so complex due to large number of climatic type that it becomes difficult to represent them cartographically.Question : Give an account of the types and distribution of precipitation on the surface of the earth.
(2000)
Answer : Precipitation has been defined as water in liquid or solid form falling to the earth. According to Foster, precipitation is deposition of atmospheric moisture and is perhaps the most important phase of the hydrologic cycle. The first step in precipitation is condensation. The process of condensation involves a change from water vapour to liquid, while the process of precipitation involves the falling of water as rain, snow, hale or some other form.
Precipitation Types: The three types of precipitation are: (1) Convectional type; (2) Orographic type; (3) Frontal type
1 Convectional Type: Convectional type of precipitation is associated with convectional movement of air. Air that is near the surface gets heated up, expands and ascends. When heated air moves up, the temperature decreases with height. This results in cooling of the air. If this rising air mass contains more water particles and is saturated, condensation occurs. This leads to precipitation. Since this type of precipitation involves the up-ward movement of air it is called the convectional type of precipitation.
Convectional type of precipitation generally occurs during summer. This is often local and is of short duration. Rain associated with convection current, falls down. Rainfall in the doldrum belt of the equatorial region is of a convectional nature. For example the Congo basin in Africa, the Amazon basin in South America and island of South-east Asia receive convectional rainfall.
2. Orographic Type: Orographic precipitation may be defined as precipitation which is caused by up thrust effect of winds on the highlands. In this type precipitation is formed when air rises and cools because of a topographic barrier, that is a mountain. The greatest annual total of rainfall in the world occurs where mountain barriers lie across the paths of moisture bearing winds. The moist air, that hits the steep slopes of a mountain at right angles is forced to rise along the slopes. Besides forcing moist air a loft, orographic barriers hinder the passage of low pressure areas and fronts, promote convection due to differential heating along the slopes and directly chill moist wind which come in contact with cold summits and snow fields. The cooling and saturation of the air leads to condensation and precipitation on the windward slope (the slope facing the wind). Such a type of precipitation is known as orographic precipitation. By this type of precipitation, the windward slope gets heavy rainfall and the leeward side (the slope on the other side) will get very low rainfall. The wind that has given abundant rainfall on the windward side descends on the leeward side and gets heated up. This air being a dry air can not lead to saturation, condensation or rainfall. Hence this leeward side is known as the rain-shadow region. For example, during the south-west monsoon period, winds blow from the Arabian sea towards India. These winds are obstructed by the Western Ghats located along the west coast of India. As a result, the west coastal region gets heavy rainfall. At the same time, the eastern slopes of the Western Ghats and the region located to the east of the Western Ghats receive very little rainfall and these are termed rain shadow regions. This explains the marked difference in annual rainfall between Mumbai (183 cm) and Pune (70 cm).
3. Frontal Type: Frontal precipitation is produced when air currents converge & rise; when air masses of different temperature & humidity converge, they cause turbulent storm conditions generally followed by precipitation, particulary along the front. This type of rainfall is known as frontal rainfall. In the middle latitudes frontal convergence, characterised by more gradual sloping ascent of warm air is found over cold air. However, convectional activity frequently occurs along fronts where the temperature of the air masses concerned is quite different. Mixing of the air along the front leads to condensation and thereafter to precipitation.
1. Cold air, 2. Warm upthrust air, 3. Clouds, 4. Precipitation, CF Cold Front
1. Cold air 2. Rising warm air
3. Clouds
4. Precipitation
5. WF, Warm Front
Apart from these three types of precipitation, there are two more types depending upon the type of wind causing precipitation. They are cyclonic type and monsoonal type.
Cyclonic Type: Cyclonic type of precipitation is associated with tropical and temperate cyclones. In cyclonic type of precipitation the essential mechanism is the ascent of air through horizontal convergence of air streams in low pressure cells. The tropical cyclonic rainfall is characterized by convectional precipitation as a result of air stream convergence. The temperate cyclonic rainfall is characterized by the frontal precipitation.
Monsoonal Type: Where the precipitation is associated with monsoon winds, for example, the south-west monsoon and north east monsoon over India, the precipitation is termed monsoonal precipitation.
Distribution: The mean annual precipitation for the entire earth is about 100 cm (40 inches), but amounts very greatly from place to place. Precipitation data for the oceans are extremely limited, and those for mountain ranges, forested regions, and wind swept coasts are large estimates. At least 30 years of records for a dense network of observing stations are desirable to establish reliable climatic pattern.
If we examine the Zonal distribution of mean annual precipitation, the maximum amount occurs in a belt about 10 to 20 degrees wide near the equator. The average precipitation is about 160 cm. However, in the Northern hemisphere the belt extending from the equator to 10º N latitude receives more precipitation than the corresponding belt in the northern hemisphere.
The principal reason for this is the dominant location of the doldrums in the Northern hemisphere. At latitudes about 20º to 30º north and south are found the belts of lower precipitation. The mean annual precipitation is about 80 to 90 cms. In this belt of subtropical anticyclones, when the air subsides it becomes warm and dry.
In the latitudinal zones, extending from 40º to 55º North and South, the mean annual precipitation varies from 80 to 120 cms. In this belt the maximum cyclonic activity accounts for the heavy amount of precipitation. There is an abrupt diminution in the amount of precipitation in both the hemisphere from 50º to 55º latitudes reaching the primary Zonal minimum of less than 15 cm in the polar regions.
The global latitudinal pattern of precipitation is modified by the distribution of land masses and oceans and by the direction of prevailing winds. On-shore winds which blow from the oceans to the land masses are moisture laden and such winds give rainfall to the coastal regions. Off-shore winds which blow from land masses to the oceans are generally dry winds and they do not give any rainfall. Winds which blow from high latitudes to low latitudes get warmed up and they do not give much rainfall. Winds which blow from low latitudes to high latitudes get cooled down and they give much rainfall.
In the belt of trade winds, the eastern margins of continents experience on-shore winds from the oceans. Therefore, eastern margins of the continents get heavy rainfall. Rainfall decreases from east to west and is a minimum on the western margins of continents. Tropical deserts are thus located on the western margins of continent in the trade wind belts.
In the belt of the westerlies, the winds blow on-shore on the western margins of continent. Therefore, western margins of continents in the belt of westerlies receive maximum rainfall and rainfall decreases from west to east. Eastern margins of continents in this belt get moderate, winds from on-shore winds in summer owing to the formation of low pressure on the land.
The land and sea distribution results in contrasts in the distribution of rainfall on the eastern and western margins of continents. The land and sea accentuated in places where high mountain ranges occur along the coast at right angles to the on-shore winds. The western slopes of the Rockies in North America and the Andes in South America in the belt of the westerlies receive heavy rainfall.
Regions of heavy precipitation are those receiving more than 100 cm per year. These are the equatorial regions such as the Amazon Basin in South America, West African coast and Zaire basin in Africa and islands of south-east Asia including Peninsular Malaysia. Another region of heavy rainfall is the Tropical Monsoon region covering parts of India, Bangladesh and South-east Asia. Another region of heavy rainfall in the eastern margins of continents is the Trade wind belt, Such as South China, South-Eastern United States and Central-America, eastern Brazil, east coast of South Africa including Madagascar and north-east coast of Australia. The western margins of continents in the westerly wind belt also receive annual rainfall of more than 100 cm. These are the north-west coast of North America, Southern Chile in South America, parts of Western Europe and Southern Island of New Zealand.
Question : Cs type of climate as per Koeppen's classification.
(1999)
Answer : Cs type of climate as per Koeppen’s classification:
Climate:
(i)A dry warm summer with off shore trades: Due to apparent movement of the sun there is seasonal shifting of world pressure belts. When the subtropical high pressure belt move poleward in summer, they enter the region of this climate, pushing the rain bearing westerlies further poleward out of this region. The region is then influenced by off-shore trade winds, making the summer almost dry.
(ii)A concentration of rainfall in winter with on-shore westerlies: This is the most out-standing feature of Mediterranean climate caused by the equatorward shift of on-shore westerlies in winter.
(iii)The annual precipitation totals generally fall within the range of 400 to 900 mm (less than 100 cm.). The mean monthly temperature does not generally exceed 27ºC, varying from 20°C to 27ºC in summer and 5 to 10ºC in winter (Average annual temperature ranging between 14º to 18ºC).
(iv)Some prominent local winds are hot dry Sirocco, (from Sahara to Mediterranean) Santa Ana (in California) and cold wind mistral (in Rhone valley) and Bora (Adriatic coast). These winds varying in intensity and duration affect the crops and activities of people of the region.
Vegetation: Well adapted to long summer drought, shrubs and trees are typically equipped with hard and thick leaves and are called Chaparral (Sclerophylls) in California, Maquis in France, Machhia in Italy and Malle scrub in Australia. Mediterranean climate has acquired a high reputation for abundance of citrus fruits and flowers and viticulture is most prominent occupation.
Question : Discuss the nature and composition of earth’s atmosphere.
(1998)
Answer : The air, sea, and land constitute the major portions of four great material realms or spheres, that comprise the total global environment. Three of these realms are inorganic, (i) Atmosphere, (ii) Hydrosphere and (iii) Lithosphere. The fourth realm the biosphere, encompasses all living organisms of the earth. All these realms are in the figure given below:
Of the three inorganic spheres the atmosphere is the gaseous realm. Humans live at the bottom of an ocean of air. They are air-breathers dependent on favourable conditions of pressure, temperature and chemical composition of the atmosphere that surrounds them.
The atmosphere is a mixture of numerous gases. It envelops the earth and extends as far as 9600 km above the earth’s surface. This gaseous cover of the earth is held around it by the gravitational attraction. The density of the atmosphere decreases rapidly with altitude. About 97% of the air is concentrated in the lower 29 K.m.
Actually the gaseous cover of atmosphere penetrates to a certain depth in land and water. Since there is unequal distribution of land and water on the earth’s surface, the atmosphere is present everywhere and every time.
The air is a colourless odourless and tasteless substance. Besides it is mobile, elastic and compressible. Although air is not as dense as land and water, it has weight, and the pressure it exerts on the surface is called the atmospheric pressure.
It is the atmosphere that provides most of the oxygen and carbon di oxide and maintains the requisite level of water and radiation in the earth’s system. It is the gaseous covering of the earth which maintains the temperature that suits us. The atmosphere also shields us from much of the sun’s ultraviolet radiation. The currents, motions and various other activities of the atmosphere combine together to produce the weather. We can say that all our activities are governed directly or indirectly by the conditions found in atmosphere.
Composition: The gases of the present atmosphere are not a direct residue of the earliest form of the planet, rather they are the evolutionary products of volcanic eruptions, hot springs, chemical break-down of solid matter and contributions from the biosphere including photosynthesis and human activities.
Interactions among land water, air and life forms constantly use and renew the atmosphere. For example, the weathering of rocks, burning of fuels, decay of plants and breathing by animals entail chemical exchanges of oxygen and carbon dioxide. Nitrogen follows a complex cycle through bacterial activity in soil, animal tissues, organic processes in decay and return to the air.
Four gases- Nitrogen, oxygen, Argon and Carbon di oxide account for more than 99% of the dry air. Nitrogen alone constitutes nearly four-fifths by volume and oxygen one-fifth.
Nitrogen is an important gas which does not combine readily with other elements but it is a constituent of many organic compounds. Another of its main effects in the atmosphere is to dilute oxygen and thus regulate combustion and oxidation.
Of all the gases oxygen happens to be the most important, for it is so essential to all life forms. Oxygen combines chemically with many other elements and is necessary for combustion and animal metabolism. Its availability in a free state is due to continuous replenishment by photosynthesis in plants. Oxygen combines with hydrogen to form water which is so precious for the existence of many life forms. Presence of oxygen also helps in the chemical weathering of the rocks as it reacts readily with many elements.
Carbon di oxide is a product of combustion and is exhaled by animals, as a source of carbon in plant photosynthesis it is essential for both vegetation and animal life. It is considered to be a very important factor in the heat energy budget. It absorbs part of the long wave radiation from the earth and emits about half of the absorbed heat back to the earth. It is estimated that from 1890 to 1970, the carbon di oxide gas content of the atmosphere has increased more than ten times as a result of the burning of fossil fuels. Many scientists apprehend that this increase in carbon di oxide will ultimately lead to a warming of the lower atmosphere which may cause a large scale climatic change.
Another important gas in the atmosphere is Ozone (O3) which is a type of oxygen molecule formed of three atoms. It is produced by the recombination of oxygen by the lightning discharges or under the influence of ultraviolet radiations at high altitudes. Its maximum concentration is found between about 20 and 25 k.m., although it is formed at higher levels. Its capacity to absorb ultra violet radiation limits the amount which reaches the earth’s surface. Chemical reactions between ozone and nitrogen oxides, chlorofluorocarbons are suspected causes of ozone depletion. Ozone is a powerful oxidizing agent and therefore a threat to life and property when its content exceeds safe limit.
Argon is an inactive gas of little importance in natural processes. Also present in extremely small amounts are neon, helium, krypton, xenon, hydrogen, methane and nitrous oxide. Carbon monoxide, sulphur dioxide and nitrogen dioxide have polluting effects for out of proportion to their minute percentage.
Water Vapour: So far we have considered major gases of dry air, of more direct importance in weather and climate is water vapour. Variations in the proportion of water vapour in air are a major concern of meteorology and climatology.
Like carbon di oxide water vapour plays a significant role in the insulating action of the atmosphere. It is the source of all clouds and precipitation. Through the condensation of water vapour vast amount of energy is released in the form of latent heat of condensation, the ultimate driving force for most of the violent weather disturbances.
The most significant aspect is that about 90 percent of water vapour lies below 6 kilometers of the atmosphere. Water droplets account for several optical phenomena like rainbows, halos, etc.
Dust Particles: The term dust particles includes all the solid particles present in air except the gases and water vapour. The amount of atmospheric dust varies greatly over the earth, but even over oceans hundreds of particles per millimeter have been counted. They include sea salts from breaking sea waves, pollen and various organisms lifted by the wind, smoke and soot from fires and certain tiny particles raised from active volcanoes and disintegration of meteors.
Dust absorbs part of the incoming solar radiation as well as outgoing long-wave radiation and also is an agent of reflection and scattering. The blue colour of the sky and the red of sunsets and sunrise are due to selective scattering of the visible solar spectrum by gas molecules and dust. These dust particles are hygroscopic in character and therefore, act as nuclei of condensation. Thus they are a major contributory factor in the formation of clouds and fogs.Question : Examine critically the drawbacks of Koppen's classification of climates. Explain how Thornthwaite attempted to overcome Kopper's limitations.
(1996)
Answer : C.W.Thornthwaite, an American climatologist, induced two climatic classifications. One in 1931 and other in 1948. His most significant concept is that of “Potential evapo-tranpiration” on which he based his 1948 classification. This concept has been applied in practical studies of water balance to solve the problems of water-use. Let us explain some of the basic criteria used in his climatic classification.
Criteria:
Moisture and Thermal provinces have been further subdivided in accordance with seasonal variation of effective moisture.
Thus, on the basis of potential-evapo-transpiration, average annual thermal efficiency, seasonal variation of effective moisture, precipitation, summer concentration of the thermal efficiency-Thornthwait based his Climatic Classification.
Comparision with 1931: Compared to 1931 classification, 1948 classification is an improved version. Both of them use latter combinations to designate climatic types and both of them use thermal efficiency as an important basis. In 1931, Thornthwaite adopted precipitation effectiveness and temperature efficiency indices for his climatic classification- and his delimitation of climatic boundaries becomes difficult and vague. In 1931, there was lack of climatic data and that presented a handicap. He tried to define climate boundaries quantitatively and also based it on vegetation. The 1948 classification is based on potential evapo-transpiration [PE], average aunnal thermal efficiency [ET], moisture with its sesasonal variation, precipitation among others.
Criticism: Thorrnthwaite classification is not without limitations. It has been applied to many regions but it falls short of widely acceptable generalisation. His major criticisms are:
All told, ‘Thornthwaite’s most significant contribution is the concept of ‘evapo-transpiration’ and ‘soil-moisture balance’. It involves a lot of complicated calculations- and therefore, his system is seldom used.
The earliest classification of climatic types was made by the Greek geographers. However, of the several schemes of climatic classification in modern times, the one devised by Wladimir Koppen,a German botanist and climatologist, still remains the most widely known descriptive system. It is a quantitative as well as empirical classification of climates.
Koppen proposed his first classification in 1900, using the world vegetation map of de Candolle, French plant physiologist. He revised his scheme in the year 1918, wherein he paid more attention to monthly and annual averages of temperature and precipitation and their seasonal distribution. He again modified his scheme in 1931 and 1936. Koppen's original scheme was modified in 1953 by Geigger Pohl and the revised scheme is known as Koppen Geigger Pohl's scheme of classification of world climates.
Following are the five principal biological groups (identified by Candolle and used by Koppen) that are largely controlled by temperature and moisture:
In his revised schemes, Koppen not only used extremes of temperature & precipitation like,
(i)Temperature of coldest and warmest months and precipitation of wettest and driest months; and
(ii)Vegetational correspondence.
But also their seasonal variations and vegetational correspondence. He firmly believed that vegetation of a region is the most visible index of climate. Based on these criteria, Koppen divided the world climates into 5 principal types & designated them by capital letters A,B,C,D and E.
At the next level, Koppen uses seasonal variations in rainfall to sub-divide the regions -
A is divided into Af, Am, Aw and As
C is divided into Cf, Cw, and Cs
D is divided into Df, Dw and Ds
E is divided into Et and Ef; where f denotes rainfall throughout the year; M denotes monsoon climate with short dry season; W denotes winter dry season and S denotes summer dry season.
Boundary between Af, Am and Aw
Likewise further division of C climates are:
In the same way D climate is divided into:
The dry humid boundary is defined by the following formula:
(Here R is average annual precipitation in cm and T is average annual temperature in °C.)
B Climates are divided into two types:
The boundary between BW and BS climates is 1/2 the dry /humid boundary.
Further, at the Third Level Koppen has divided
(1) B into
(2)C into - a, b, c
where a, b and c are defined with respect to decreasing temperature.
(3)D into - a,b,c, and d
where a, b, c and d are defined with respect to decreasing temperature.
At the Fourth Level, Koppen has used supplementary symbols to accommodate smaller and newer climatic sub-divisions. For example, Cwg-Ganga Type Climate.
Merits
Limitations
