Chapter II CHAWIB-H PHYSICAL ENVIRONMENT OF THE STDDY AREAS
2.1 INTRODUCTION The Indian sub continent has a peculiar geographical location with unique characteristics, physically and economically dominated by the most important monsoon system of the world. The summer monsoon being the principal source of water and the winter monsoon meeting the water need of southern peninsular region particularly Tamil Nadu. The whole year supply of water over the major part of the country is realized in just three to four months in summer monsoon season which make the people critically dependent on the monsoon activity. India's complex variety of physiographic pattems with extensive mountain ranges- Himalayas, Eastern and Western Ghats, large river basins, plateaus and long coastline flanked by Arabian Sea, Bay of Bengal and north Indian Ocean control the climate of India. The role of Himalayas helps in the maintenance and control of the monsoon system. During the northern Hemisphere winter, the Himalaya ranges serve as an effective barrier to the intensely cold continental air blowing southwards into the country while during the monsoon season the Himalayas forces the rain bearing winds up the slope to release most of their moisture on the southem side of the mountains. The most important effect of Himalayas on the monsoon of the coxmtry is the location of the heat low over the north western part of the country and the monsoon troughs across northern India The location of these low pressure centers is determined by the mountain and hill configuration. The mountains in general influence the atmospheric flow as result of mechanical lifting and also through heating due to condensation which is in turn caused by air rising over the mountain barriers. The Western Ghats also play a major role in orographic lifting of the moisture laden monsoon winds leading to heavy precipitation towards the windward side the Ghats. The Tibetan plateau with the elevation of 5 km is strongly heated by the insolation and built up strong Tibetan high in the upper troposphere leading to easterlies over the country at that level.
15 The plateau gives heat to the atmosphere with the maximum in late spring and early summer. The interannual variability of the strength of this heat source considerably influences the monsoon activity and far reaching impact on the circulation pattern. Thus, the uniqueness of the climate of hidia is basically derived from the leading role played by its physiographic feature in determining the climate. Therefore, a general perception about the physiographic environment of a region is important for understanding its climate as well as variability. With these in view, an attempt is made in this chapter to know the physiography and its role in governing the climate of the study region. This section, therefore, discusses the physiographic and induced climatic features, such as mountain barrier, topography and climate over the two areas viz., northeast and west coast regions of hidia.
2.2 NORTHEAST REGION 2.2.1 LOCATION AND EXTEND North East India comprises of eight states that are straddled by eastem Himalayas and are co-joint to the Indian mainland by the 33 km 'Chicken neck' north of Bangladesh (Fig 2.1). The eastem Himalayan mountain range forms a 1500 miles long barrier that separates the low lands of Indian sub continent from the dry, high Tibetan plateau. The ice mass over this region is the third largest on the earth after the arctic/Greenland and Antarctic regions hence is often referred to as the 'third pole', hi climate terms this region is also known as the 'white spot' given the relative absence of research and data. As one of the 25 mega biodiversity hotspots of the world, the NEI region is home to over 10000 plant, 240 mammal and 750 bird species. The region is a rich cultural mosaic of Buddhist, Hindus, Christians and animists, all of whom have lived closely with nature for centuries. Many communities hve in isolation and their livelihoods and traditions deeply depend on natural resources, making conservation an integral part of their lives. NEI has been identified as a carbon sink for Europe. This too has its inherent pitfeUs. Although this region has been romanticized as a mythical paradise, it is a fragile land facing many challenges. Apart from harboring rich cultural and biological diversity, the NEI region is a major supplier of timber, medicines, fibre, oils, spices and condiments, firewood, organic manure, fodder and hydropower. It is reckoned as 'gene-
16 bank' and continues to remain an important centre for the origin of the crop diversity and numerous under-utilized and potential future crops. North-East Indian region under consideration is the area spread between 20°- 29°N as latitude and 88°-98°E as longitude. Its northern and eastern boundaries forming natural barriers are represented by the high Himalayan Mountains and Patkai Hill ranges respectively. However, its southern and western boundaries are more political than natural. To the north, lie countries of Nepal, Bhutan and China while to the east lies Myanmar and to its southwest lies Bangladesh. To the west the India state of West Bengal lies. Out of all directions, it is only towards the west, the region is bordered by plains. Northeast region (NER) is almost isolated fiiom the rest of the country, except for a narrow Siliguri corridor of about 21 km width. The region, therefore, assumes a great strategic importance. The total area of the region is 2,62, 184.69 km^, representing about 7% of the country.
2.2.2 PHYSIOGRAPHY Physiographically, the NER can be divided into three divisions namely; the plateau region, the hills and mountain and the plains (Fig. 2.2).
2.2.2.1 The plateau region The hard crystalline massif forms the core of the region. This plateau is in reality the extension of Deccan plateau. The Deccan plateau extends underground from Rajmahal hills of Chotanagpur plateau below malda district of West Bengal and Rajshahi, Dinapur and Rangpur district of Bangladesh and appears in the northeast above the surface as Meghalaya plateau and Karbi hills. It is believe that the downwards created the Malda Rangpur gap which was subsequently filled up the alluvial deposit of Brahmaputra. These two plateaus extend about 402.2 km from Singimari river in the west to Dhansiri river in the east. The unit's average width is 80 km and covers and area of 32, 829 sq km. The plateaus are high in the middle and low towards west and east. The Meghalaya plateau (Plate 2.1) comprises of Garo, Khasi and Jaintia hills while the Karbi plateau encompasses the Karbi and Rengma hills. This plateau is isolated from Meghalaya plateau by age long erosion of the heads stream of Kopili and Dhansiri rivers.
17 o« nI.
UJCLQJJHOQOCOO.".. O^CMM o o o o o o o o o o o o in t -d O ri M The Meghalaya-Karbi groups of Plateaus are topographically different from the Deccan in that they present more a hilly character with hills, ranges, deep valleys and gorges. The Meghalaya Plateau is divided into Garo, Khasi and Jaintia Hills. Garo Hills in the west is a relatively low and more dissected part covering about 7769.9 km^ with and average height is 900 m. The Garo plateau rises to a distinct east west range known as Tura range with an average height of 1,100 meters reaching its highest peak Mount Nokrek (1529 m). The whole Garo plateau is criss-crossed by numerous streams of which Singang, Krishnai, Dhudnoi are important. To the east of the Garo Hills lie the Khasi Hills. This happens to be the highest part of the Meghalaya Plateau with an average height of about 1066 m. The central upland covering an area of 1250 km^ is however still higher reaching a general height of about 1400 meter. This highland is characterized by Shillong range ruiming in east-west direction with its highest Shillong Peak reaching 1961m. As one moves from Shillong to Guwahati the Khasi hills descends through three discemable steps to the Brahmaputra plain in the north. In the present study, two stations namely Shillong (1500 m) and Cherrapimjee (1313m) falling in Khasi hills regions have been considered for studying the climate change in this region. The easternmost part of the Meghalaya Plateau is the Jaintia Hills, this part of the Plateau slopes down from Khasi Hills from 1200 m to about 500 m in the east, the average height being 900 m. It covers an area about 3790 km^. The Karbi Plateau, as stated above, lies almost detached from Meghalaya Plateau due to headward erosion of the Kapili and Dhansiri rivers. It covers an area of about 7000 km^. The plateau has developed a radial drainage pattern however it central part still remain high with the highest peak Dambukso (1363 m). 2.2.2.2 The hills and mountains In the first part of discussions on physiography of the study area, the regions of plateau were considered. Now in the second part, details on the regions comprising of hills and moimtains are presented by dividing into two sections with (i) Northern mountainous part of Sikkim and (ii) Arunachal Himalayas and the eastem and southern Patkai -Purvanchal hills. 18 2.2.2.2.1 NORTHERN MOUNTAINOUS REGION: 2.2.2.2.1.1 Sikkim Himalaya This region comprising eastern Himalayas covers the state of Sikkim (Plate 2.2). The moimtain ranges runs in east-west direction with the predominance of length over the breath. The Sikkim Himalayas includes the 84 km stretch of mountains which lie between the Mechi river in the west and Chumbi Valley at the westem border of Bhutan in the east. The mountains rise in elevation northward and it is deeply cut into steep escarpments, longitudinal valleys, high peaks and have no open valley and plains. This region can be subdivided into three divisions namely lower Himalaya/Shiwalik, middle Himalayas and higher Himalayas. The Shiwalik hills stand between altitude from 300 m to 1800 m and has hilly topography with flat cultivated land in patches. Middle Himalayas varies from 1800 to 3000 m comprises of major forest areas while the Higher Himalayas lies between 3000 m^500 m covers with shrubs and grassland and the area above 4500 m is perpetually snow covered and is without any vegetation. Kanchenjunga (8,598 m), the world's third highest peak, is located in the northwestern part of the Sikkim on the boundary with Nepal. Sikkim Himalaya poses twenty-eight mountain peaks and nine mountain passes, in the east the ridges more often tum north-south. Gangtok (1,812 m) situated in the east of the Sikkim Himalayas bordering with Bhutan has been considered to represent this region. 2.2.2.1.2 The Arunachal Himalaya The portion of eastem Himalayas which stretches between Bhutan-Arunachal border between Jia Dhansiri river and Arkhala range in the west to the Siang river in the east is known as Arunachal Himalayas (Plate 2.3). The Arunachal Himalayas rises steeply from Brahmaputra plain attaining higher altitude northwards and ultimately cuhninating in the snow clad greater Himalayas range along the Indo- China border. Within Arunachal Himalayas, Kangto (7, 590 m) is the highest peak. There are few glacier and glacial lakes in this area. The Arunachal Himalayas can be broadly divided into four distinct physiographic regions: the greater Himalayan range with snow-capped mountains with altitudes rising to 5,500 m, the lower Himalayan ranges up to an altitude 19 6 ffi I^^^^H^ ^v^^^^^^^^^^^^l ^^^^^^^^^H 1 11 f iR| 00 1 /• o d J3 O o o- u o o 00 .••^-^^PT, '^-n , » ^ 1 ^^^^^K^jK^^^^^^ rX I ^1 « ^^^H^HHH^I c3 1 "3 1 ^^^^^^^^^^^ft ' ^^B^^ ^1 * o d 3 00 1 ou o of 3,500 m; the Sub-Himalayan belt including Siwalik hills with altitude up to 1,500 m. The number of streams comes down from Great Himalaya with torrential force and thereby creating numerous deep gorges and narrow valleys. It is only in the middle part that limited flat portion like Apatani plateau and Tenga valley can be identified. Pasighat (157 m) which is located in the eastern side of Arunachal Himalayas has been considered as representative station of this sub-region. 2.2.2.2.2 Patkai-Purvanchal hills The Eastern mountains is characterized by north-south aligned ranges defined by narrow and parallel valleys, spread into ranges with irregular serrated ridges with tapering cliffs and deep gorge and it bulges towards the west (Pate 2.4). The portion of the Arunachal Pradesh, east of the Siang River is hke a mountain knot (Sikang-Luhit knot), extensive and high, giving out ranges to the west, south and north-east. This is the extension of the Great Himalaya which continues toward north east at a height of about 4000 m to 5000 m. In this range, there are several peaks, passes, glaciers and glacial lakes. From this knot, the ranges mn through Changlang and Tirap district of Arunachal Pradesh, Nagaland (Patkai Bum) bordering Myanmar and continue southward towards Manipur (Bhuban, Barail and Letha range), Mizoram ( northern Arakan Yoma) and Tripura and extend up to North Cachar Hills of Assam. Although this region is above 950 m of mean sea level, some of its ranges attain a height of more then 3000 m. The ranges generally slope down steeply to the westem side and gently to the eastern side. The highest range, i.e. Patkai Bum attains 2000 m -3000 m with the highest peak Saramati (3826 m) lying at Nagaland-Myaimiar border. In between the parallel ranges of Manipur there lies the pearl-shaped Manipur Valley which is about 700 m a.s.l. There are deep gorges between the parallel ranges in Mizoram whereas in Tripura, the ranges are generally low with intervening wide valleys and isolated hillocks known as Tilas. 2.2.2.3. THE PLAINS Apart fi-om plateaus, hills and mountains, NER has four plains sub regions. Of these the Brahmaputra Plain (54315 km.^) is the largest and the other three namely, Barak Plain (6962 km^) Manipur Plain (1843 km^) and Tripura Plain (3500 km^) are small. But 20 each of them is of immense human importance as each of these plains is physically separate, and they may be discussed individually as follows: 2.2.2.3.1. The Brahmqjutraplain This plain (Plate 2.5) is surroimded by the Arunachal Himalaya in the north, Patkai Bum and Arunachal hills in the east and Naga Hills, Karbi plateau and Meghalaya plateau hills in the south while it is open and joined with the Ganga plain in the west. The length of the valley is 720 km and the width varies from 45 to 90 km. The sedimentary deposits in the plain are on the average 1500 m thick. These are, however, overlain by the alluvial deposits of the Recent and Sub- Recent times. Thus, the present shapes of the Brahmaputra Plain are the results of both aggradational and degradational activities. An interesting feature of the plain is the presence of isolated hillocks on either bank of the Brahm^utra; these are the outliers of the Meghalaya-Karbi Plateau and made of hard pre-Cambrian granite and granitic-gneiss. The gradient of the plain is extremely low ranging from 28 m at Dhubri in the west to 130 m at Sadiya in the east. Thus, average gradient of the plain is only 14 cm per km. These facts clearly indicate the possibility of flood hazards in the plain. The extremely low gradient of the plain from east to west, its flatness and hills along with the hills and plateau in the north and south make the plain ill drained. The tributaries coming down from the Himalayas, deposit enormous quantities of sediments, often raise their own beds and consequently during the rainy season the water overflow the banks. All this contribute to presence of marshes and lakes in the plains, hi most of the cases, the north-bank and south-bank rivers give rise to the ox-bow in their abandoned courses. In the mid-eastern part, there is a riverine island called MajuU (800 sq km) which is the largest riverine island in the world. Five meteorological stations namely Dibrugarh (111m), North Lakhimpur (102 m), Tezpur (79m), Guwahati (54 m) and Dhubri (35 m) are considered for which long series of data are available have been considered from this region to study the cUmatic change. 21 ^' i < .# )'iSr'9 •-'\ i& / • BJ • \. % 1. '' if L (ii i • • %^: J, f 1 ^ ^ ^^ •p^ •f \ V.1 ^ jj ^^^B Mj^S^^**-: N > Vr o * 1 en u o C/3 2.2.2.3.2. The Barak plain The Barak Plain (Plate 2.6) is located in the southern part of Assam encircled on the north by the North Cachar hills, on the east by the Manipur hills and on the south by the Mizoram hills. To the west the plain merges with Sylhet Plains of Bangladesh. The Plain is horse-shoe shaped with 85 km east-west and 70 km north-south extension near Bangladesh border. The plain also includes the Jiribam Sub-division of Manipur which lies at its apex in the east. The total area of the Barak plain is 6962 km^. The plain is low lying, its height being 75m at Jiribam which slopes down to 51 m near Karimganj. The plain is so low that it assumes a bowl shape having numerous ox -bow lakes and swamps on either side of the Barak River. Silchar (29 m) which lies in the northern part is considered as the representative stations from this region. 2.2.2.3.3 The Manipur plain Amidst the extensive hills and ranges of Manipur, there lays a wheat shaped plain (Plate 2.7). This intermontane plain is about 70 km long from north to south and 40 km from east-west covering an area of about 1,843 km^, it is said to be of lacustrine origin. The plain slopes gradually down from 838 m in the north to 793.5 m in the south at a gradient of 75 cm/km. The southern part of the valley has numerous lakes and marshes with the largest freshwater Loktak Lake surrounded by isolated hillocks scattered here and there. Imphal (781 m) is situated in this valley was selected for studying the chmatic change. 2,2.2.3.4. The Tripura plain The Tripura Plain (Plate 2.8) is a piedmont one, lying at the foot of the north- south frending hill ranges extended from Mizoram. The plain is in the western part of Tripura cover an area of 3,500 km^ as a result of erosion caused by the headstreams of Manu, Khowai, Titas, Gumti, Muhari and Feni rivers. This piedmont plain merges with the Bangladesh plain in the west while the eastern part of the state is covered by parallel ranges with relatively narrow valley plains in between. The Tripura Plain slopes down west and southward from the Atharimura Range at a fairly high gradient providing little 22 scope for flood to occur. Agartala (16 m) and Kailashahar (29 m) are considered as representative stations for studying the climatic change in this area. 2.23 CLIMATE OF THE NORTHEAST REGION Northeastern region extends between the 20°-29°N latitude and 88°-98°E longitude, the tropic of cancer passes along the southern part through Tripura and Mizoram. It also lies within the monsoon belt of south and Southeast Asia and therefore witness tropical monsoon climate. However, its location and the topogr^hy-encircled on three sides by high mountain ranges and the presence of precipitous Meghalaya plateau athwarting the course of incoming south west monsoon winds have rendered the climate some what different from that of the other parts of India. The Himalayas in the north, the Patkai and other moimtains and hills in the east and the Meghalaya plateau in the centre have affected the general tropical warm climate of the region. Many of the hills and mountains are high enough (sometime one km to 5 km) rendering the climate cool and bracing. The Himalayan mountains chain, the Patkai and high hills ranges along Manipur and Mizoram border with Myanmar prevent the rain bearing monsoon winds from escaping from this region. Also at the same time these ranges do not allow the dry and cold winds of cenfral Asia to enter the northeast region. The Meghalaya plateau standing diagonally the course of the southwest monsoon winds make them rise orographically causing the heaviest rainfall in flie world in its southern margin. Another very important effect of the encircling hills and mountains on the climate of the region is that in summer while the plains become hot the air over the hills and mountain remain cool. Thus, local low pressure systems are buih up over the valley and these low pressure systems over Brahmaputra and Barak valleys modify the climate in this region. The climate of the region following Koppen climatic classification can be classified under two main types; tropical rainy climate and warm temperature rainy climate. 23 2.2.3.1 TROPICAL RAINY CLIMATE The annual average temperature of the region varies between 15.6° to 24.8° C. It is lower at Gangtok (15.6° C) both because of its latitude and altitude (1,812 m). January is the coldest month for this region. The mean temperature of this month is less than 18° C for almost all the stations except Tezpur and Silchar where temperature is more than 18° C. These two stations located on Ihe plains experience tropical wet and dry climate. Both of these stations belong to Am category where the dry season is very short however the total aimual rainfall is so great that ground remain sufficiently wet to support rainforest. This type of climate is immediate between Af and Aw, resembling Af in the amount of precipitation and Aw in seasonal distribution. 2.2.3.2 WARM TEMPERATURE RAINY CLIMATE For majority of the places, the average temperature of the coldest months is less than 18°C and therefore they belong to the warm temperate rainy climate category. This climate can be fiirther classified into three sub groups depending on the rainfeU regimes. Majority of the stations with the exception of Shillong and Cherrapunjee belong to Cwa type, a denoting hot summers and dry winter; w-denoting dry winter. While Shillong and Cherrapunjee, they assimilate in Cwb category having dry winters and cool summers. Most of the stations are elevated sites where altitude reduces the temperature of tropical climate which prevail in the adjacent lowland. For the remaining 11 stations, the temperature of the coldest month is below 18°C but above -3°C, with average temperature of the warmest month is over 10°C. These stations, therefore, witness warm temperate rainy climate. Most of these stations are elevated sites where altitude reduces the temperature of tropical climate which prevail in the adjacent lowland and experiences winter dry climate. In general, the year is divided into four seasons. 2.2.3.1.1 Winter season After the autumn equinox, the temperature starts felling; winter sets in towards the later part m November and continues up to the end of February. The weather during this period is influenced by high pressure systems of central Asia and the sub- tropical jet 24 stream. The temperature falls down over the hills and mountains often to 0°C. In fact, the high areas of Arunachal Pradesh experience temperature below 0° C at night. The mean minimum temperature in January varies between 4° C at Gantok, Imphal and 12.3° C at Passighat. The hills of Nagaland, Manipur and Mizoram experience minimum temperature around 4°C while Shillong from Meghalaya remains at still lower mean minimum temperature about 3.6° C in January (Fig 2.3 and Fig 2.4). Tripura, because of its southerly location and lower height, experiences its mean minimum temperature to be around 9° C in January. Mean maximum temperature varies from 13.5° C at Gangtok in the hills to 25.7° C at Agartala and Kailashahar over the plains. The average temperature during winter ranges between 8.9° C at Gangtok and 18.3° C at Silchar. December is the month when the region receives the lowest rainfall in a year. Dhubri receives around 0.7 mm while North Lakhimpur gets around 24 mm of rain. During the winter season, a local low pressure sets in the Brahmaputra valley and along with this the Mediterranean lows bring cloudy and rainy weather. Besides this, the cold northeast frade winds occasionally blow over eastern Arunachal Pradesh, Nagaland, Manipur, Meghalaya and Mizoram hills, brings down the temperature of these areas. An important feature of the weather during this season is the presence of thick fogs all over the region. The hills have thick fogs in the evening while the plains have it early in the mornings. Brahmaputra valley witness maximum foggy days in December-Guwahati recording the highest (16.5) days i.e., in general every altemate day is a foggy day. In the Brahmaputta valley foggy days are more frequent in the south bank than on the north bank. Dhubri (2.6), Tezpur (2.8) and Lakhimpur (5.8) located on the north bank record less foggy days than those of Guwahati (29) and Dibrugarh (22.4) on the south bank (Fig 2.5). This may be due to the mountain winds coming down from the Himalayas slope drive the fog to the south bank. Across the hills, Cherrapunjee and Gangtok reported 8.5 and 16 days respectively during this season. In the Barak valley, Manipur valley and Tripura plains fogs do form during this season. It is interesting to note that Tezpur located in Brahmaputra valley records few foggy days throughout the year while Guwahati located on the south bank of Brahmaputra records the highest foggy days (16.5) during December. This may be because of nearness of wetlands (Deepor Beel) during the period when moisture persists 25 1 1 o O X o •«— s - - P? _ 0) o! - /_J \ • • 1 ^7 ^~\ Q. i E MLi^^jaa B^a 1 1—> E s - E !:£ Z-S!^ i- TO 3 c: to o- Z SU u s s ^ *^ oS - H 1 1 1 1 1 1 1 1 2 1=> S .3 -c cvj tb - £ A •^ V 2 i-.^ i 4 s- 1 - s j^^^L s 2 -^^^^^H • J Ml r * * to - 0} ^-1 _ "TO Q. E >5^ E ) 1 E '8 - X (O ro CD o — Z lU as - > > 5 t c X i 5 i 0) 0 c « •n to ^ 9. £ t ca -J 1 S S ^ o S > B O M It W around. As compared to Barak and Tripura plains, foggy days are more in Manipur valley where Imphal reported 18.6 during this season. The rainfall is normally scanty during this period (Fig 2.4 and Fig 2.6). The northeastem part of the region that is upper Brahmaputra plain Lakhimpur, Dibrugarh and Pasighat and Arunachal foothills so also Gantok from the northwestem part receive an average rainfell of more than 10 cm. Rest of the northeast India receive around 5 cm while the extreme westem part of Assam records the least amoimt of rainfall. Normally, the sky is clear cool and gentle breeze and morning fog together make the winter weather pleasant during this season. 2.2.3.1.2 Summer season With the end of February, temperature begins to rise in the region. March, April and May become sufficiently hot and rains are yet to come in their full strength. Thus, this season is a transitional season between the dry, cool winter and warm, rainy monsoon season. The main characteristics of this season are the rapid increase in temperature, disappearance of fogs and occurrence of frequent thunderstorms and hailstorms. The frequency of thunderstorms is highest at Kailashahar (36.8) followed by Guwahati (36.1) and Gantok (34.1) while it is lowest at Tezpur (13.7) and Dhubri (11.9). Tezpur and Dhubri located on the north bank of Brahmaputra registered least thunderstorm while Guwahati on the southern bank records the highest (Fig 2.7). With the northward migration of the sun, the temperature increases rapidly in the region especially over the plains. Thus, the pressure decreases rapidly and the gradient of pressure between the hills and the plain becomes steep. This facilitates the incoming of the Mediterranean lows from the west and fropical cyclones from the south giving rise to local cyclonic conditions. All these lead to storms and rains. Since these storms appear to come from the west, they are called "Norwester". These storms are said to be the result of Mediterranean lows progressing from the west and fropical cyclones from the south. The eastward progressing Mediterranean lows in eastem India atfract the moisture laden cyclonic current from the Bay of Bengal. When these two currents meet, full-blown cyclone occurs carrying hail storms and thunder showers. Sometimes these storms move at a velocity of 50-65 km. In northeast, they occur with a slightly lesser intensity. 26 o o ' o o o in i i O o o g t s o o S • mo" CM CO A • • . 5 • • 3 ^^K. •1 o o o mom >- CM CM o , . . 'S^' § o o o jr^^ " o o o 5 ••^B o m o E ,•-•—J S • ^^^^^^^^H % V r- t- CM * S' "11! t ! a 5 «^^ it . mk o o o o o o 'T o o o mom mom r^ o cNi CO t •^ >- CM CM 1 1 . O o o o 55 s s- o o o S O CD O S o m o ^ CO en ^ A st- tle- C8M sA ^•^••••B >^V 1 ill o (A 2.2.3.1.3 Monsoon season The monsoon season prevails over northeast during the months from Jxme to September. With the northward migration of the sun the landmass gets heated and a low pressure system is established over India. The mean maximum temperature over northeast India varies between 32.2° C at Tezpur at Brahm^utra plain to 22.6° C at Gantok. The mean maximum temperature is 30° C over the plains and is generally governed by the altitude and latitude for e.g., both the mean minimum and maximum 27 1 1 1 - • ,> y v""ii^ ^^^^H - « "S- • ^ 0) a> e s .s g-1 1— s - E 13 E Z-!!< 8- ro L o J Z UJ H 1 1 1 1 1 1 1 1 1 O s X o '^ 2 •o <^ to OJ A (N V .;-"tV » - s g >i^ s • 'IS s • • s - 9S ri tit 4i • • - s- 2) <*— :3 1 1 2 a. E / 1 g- s - E J E / 5^ Z-^ 8- X ^J ?< to L— - s ^^^ •—- Z UJ fe! S - 1 1 temperatures are more at Imphal (700 m) than Aizawl and Kohima which are located at the ahitudes of above 1000 m. By the first week (around 3^'' June), generally, southwest monsoon sets in over the northeast region - winds are fi"om the south west direction. This season is thus, characterized by heavy rainfall, high relative humidity, cloudy sky with moderate temperature and light surface wind. As the relative humidity goes on increasing with the increasing rainfall, the month of August becomes most sultry and uncomfortable. The temperature falls on account of heavy rain and thick monsoon cloud, much of the annual precipitations fall during this season. The average range of temperature varies from 5° to T C. The mean minimum temperature of 16.8° C in the month of July is recorded at Gantok in the hills while over the plains it is 25.5 " C at Guwahati and Dhubri (Fig 2.4 and Fig 2.9). The mean maximum of 32.2° C is witnessed at Tezpur over the plain whereas over the hills it is 22.6° C at Gangtok and Cherrapunjee (Fig 2.4 and Fig 2.8). During this season, the mean temperature ranges from 20° C over the hills and mountain while over the plains and valleys it is 28° C. The south-west monsoon winds enter the northeast region through two routes, viz., first, the Bay of Bengal current being obstructed by Arakan Yoma moves northward and causes rainfell in Tripura, Mizoram and Barak plain. The second, it strikes against the steep slope of the Meghalaya Plateau causing orographic rainfall. In fact, the effect of orography is strikingly brought out in this season. Hills and mountain ranges get more rainfall than the neighboring plains however, variation from day to day, with elevation and in different areas bring out the complexity of this problem. Cherrapunjee located in Khasi hills of Meghalaya is the rainiest place in India with annual rainfall of 11,465 mm (1,146 cm) and monsoon rainfall of 8431 mm (843 cm). The highest rainfall in a day at this station is 104 cm (latest). In 1876 the aimual rainfall was 2299 cm, the highest so far. The Garo Khasi hills are 300 km in length and 70 km in width running west to east. The highest peaks are below 2 km where Cherrapunjee is 1,300 m high on the southern slopes. The slope in this area is about 1:8 starting from the plains about 10 km to the south. The mean air flow is from south up to 3 km becoming easterly aloft. The Barail range to the east prevents air flow around. Rainfall in the monsoon season in the plains at Mymensingh 50 km to the south of land rise but to the west of Cherrapunjee longitude is 28 1 If) O ^ CM CM 0) ir> • ' •O CM 2_ •=, C - C3 l" — A CM •«- - ^ 8 - • • • *> S- • • - 0} 1—> a. > 1 •S E Ui / 8-1 fe - E 5 E Z-^ 8- 1) c: 1 3 H —> 1 1 1 1 2 1 —l- 1 1 e > „ CO CN1 (1) C3 „• • •g f^ lb c1 e c A CM C\ » - s 3 'x J ?"' :i^^H • ^^7 s - ^ Sl- a> "(3 CL E s- E E X 5 >. 3 —5 as - • 1 160 cm. Shillong (1,500 m) just to the north of the crest of the hill range 35 km north of Cherrapunjee with a rainfall of only 159 cm. Tura on the on the westem slope of Garo hills is 247 cm of rain more than Shillong. Mawphlang (25°27' N, 9r46' E, height-1500 m) gets 249 cm. The rainfall reports noticed above as observations of the meteorologist who visited the areas during rains, suggest that the phenomenal rains at Cherrapunjee prevail over a short range, a small portion of the slope. The fiumel shape catchment opening to south on either side of which Cherrapunjee and Mausyram area located seems to increase the convergence in the air rushing from south (Rao Y.P, 1976). The north flowing masses descend along the northem slope of Meghalaya, spread over the Brahmaputra Valley and strike against the foothills of Arunachal Pradesh. The descendence on the northem slopes creates a rain shadow zone in the leeward side of Meghalaya and over Lumding and Nowgong in Assam, where average annual rainfall is only 1233.2 (123.3 cm) and 1068.3 mm (106.8 cm) respectively. This is the driest part in north east region. In a nutshell, it can be said that the amount of monsoon rainfall in the northeast varies from 800 mm (80 cm) in the leeward side of hills and mountains to over 8000 mm (800 cm) in the wind ward side of Khasi-Jaintia hills (Fig.2.4 and Fig 2.6). The southern part of the northeast region receives higher rainfall in the month of June while for the northem, July is the rainiest month. Northeast is one of the wettest regions in the country. 69% of the annual rainfall is received during the monsoon season. The rainfall regime is characterized with high spatial and temporal variability. While temporal variability is determined largely by fluctuations in the monsoon and pre-monsoon circulations, spatial variabiUty is controlled overwhelmingly by the physiography of the region (Das and Goswami, 2003). The highest rainfall in a day is 104 cm (Rakhecha and Pisharoty, 1996) and Khasi-Jaintia hills are noted for heavy rainfall of over 85cm in one day. The mean rainy days varies from 8.9 days at Imphal in the month of September to 27.9 days at Cherrapunjee during the month of July. It is important to note that high rainfall can occur during the break monsoon when the seasonal monsoon trough shifts to the foothills of the Himalayas. Influence of eastward moving trough in the westerly seems to cause this heavy rain. In this region, this above normal rainfall giving the breaks extends to the plain regions also. 29 The monsoon distuAances that originate in the Bay of Bengal and the remnants of Typhoons from pacific that reach Myanmar approach the foothills gives heavy rainfall resulting in floods. The gradual decrease in rainfall marks the end of rainy season where the south west monsoon weakens with an abrupt retreat followed by the post-monsoon season. 2.2.3.1.4 Post-Monsoon season Towards the latter part of Septeinb«^,1i^l^/Sun crosses the equator, by 15 October; the southwest monsoon gets withdrawn from the northeast region. The temperature begins to decrease (Fig 2.4 and Fig 2.10), the low pressure system developed over northern India loosens its grip and the south west monsoon winds ceases. The sky clears out with only the isolated patches of cirrus and cumulus cloud. As the relative humidity remains very high, even a slight fall of temperature is enough to make haze and mist appear in the distant horizons, especially in the mornings and evenings. The mean minimum temperature over the hills at Gangtok is 12.3" C whereas parts of Tripura and Barak valley at also Tezpur in the Brahmaputra plains it is more than 20 C in the month of October. The mean maximum varies from 21.7° C to 31.2° C over Gangtok and Kailashahar respectively. Rainfall occurs during this season but not as much as monsoon season (Fig 2.4 and Fig 2.6). Amongst the stations, Cherrapunjee reported 527 mm (52 cm) of rainfall during the month of October. Along with rainfall, the rainy days also decrease. The average rainy days varies between 5 and 11 days. As the sea get warmer than the land, cyclones and depression begins to appear in the Bay of Bengal influencing the northeast region by giving rainfall. Apart from this description about the seasonal characteristics of rainfall and temperature distribution, wind also plays a major role in bringing about the weather the following section therefore discuss about the wind pattems during the summer and winter season. 2.2.3.1.5 Wind Wind, an important climatic element when blow with moderately to high speed creates a problem to soil and growing crops, it also evaporates moisture from the soil. 30 o O oj o 'in •o <^ ?« •^ £ A !^ V ^^K - P? ^^BP^^s ^^^^^^^H vi 3 ^—^ ^^^^^^^H IS d) o a. o E O % e c / g-l .s 3 \ i^——^_/ S - u E z^^ §- 1 S N 1 t3 U-^ o -J # • • ^ UD ^ i ^BHB • P4 (£< 03 \ y-«-v. 3 / J T^^* •.( E s E S-] E I 5L_,—-^^j S - X re oj /•' «—'V- -oQ \ r - IN •5 c w •""' "Sj e a. It /^ 1 E E I,. ^-''*^ 5 ^,-^^ t ^f • \ ^ft-ff'tif-5'. . ^- « £ » t c \^ \ c z-«^ i "I \ ^1 %. •^ J ^ < ? o a CM II E E '. \' -r^'* r^ Pi 2^ •^ 3 SI en U (A o 1 ; ^ ^> I' ^ ^— OH c 2<< 1 o y Generally, the wind velocity increases during the summer south west monsoon season. While in winter the winds are light and the occasions of calm increase. In order to understand the variation in the wind directions during different seasons the data related to percentage number of days wind coming from the different directions in January and July have been collected for 15 stations in the northeast region. It is observed that during January (Fig 2.11) the predominant wind direction particularly in the Brahmaputra plain is NE with the exception of Passighat where wind approaches from NW direction. The stations Agartala and Kailashahar records southerly wind (Jan). In the evening, the predominate direction in the Brahmaputra plain continues to be NE while Aizawl there is a reversal in wind direction. It is also observed that the number of calm days is more in the evening as compared to the morning conditions. Shillong Silchar and Kailashahar record more than 90% of calm days. While Cherrapunjee, Aizawl and Kohima, the no of cahn days are less with Aizawl receding the lowest (4%). It is surprising that since during the winter seasons the air is generally dense and still with higher number of calm days. For Aizawl the morning and evening calm days practically remains the same. In July (Fig 2.12), the stations in the Brahmz^utra plains experienced again northeasterly as predominate wind direction while for the southerly located stations like Agartala Kailashahar Cherrapunjee Guwahati the predominant wind direction is South and Southwesterly. There is not much variation in the predominate direction during the evening period. However, it is surprising that clam days are more than 70% at Gantok and Shillong during the morning and evening hours. It is possible only if the winds do not reach higher altitude i.e. above 1300 meters indicating the role of physiography on weather. 23 WESTCOAST REGION 2.3.1 LOCATION AND EXTEND Between Sahyadris and the Arabian Sea Ues the west coast plain. It is 48-64 km wide and 1,515 km from north to south and the elevation extends up to 60 m above the mean sea level. The west coast is situated between 9°N-20°N and 73° E-77°E covering 4 states and lies ahnost at the sea level. It covers an area of about 63,694 sq km (Fig 2.13). 31 The west coast can be divided into Konkan, Goa, Coastal Kamataka (Kanara) and the Kerala Coast (Malabar Coast). In the west coast region, majority of the cities are densely populated and among them Mumbai is one of the most populated cities followed by other cites. All the cities in the west coast are situated in the swampy coastal plains surrounded by bays, creeks, lagoons, lakes, delta plains and interrupted by low hills/ hillock. 2.3.2 PHYSIOGRAPHY Physiographically, this region is a distinct strip of lowland, interspersed by hills, rising in elevation from the sea level to 60 m; however, in some places it reaches more than 300 m. It is composed of one or more of the following features; sandy beach, coastal sand dunes, mud flats, alluvial tracts along the rivers, lagoons/estuary, laterite platforms and the residual hills. The west coast region occupies a narrow strip of land between the crest of the Western Ghats on the east, which is 1,200 m high, and the Arabian Sea on the west running parallel to the coast (Fig 2.14). Several peaks exceed 2000 m in the south (e.g. Anai Mudi 2695 m and Nilgiri hills 2636 m). The Western Ghats are a continuous mass running from south to north that slopes towards the Arabian Sea which is access only through gaps or passes namely; Thai, Bhor and Palghat. The west coast has gentler slopes, widening valleys but is broken by isolated hills and gap where hot wind rushes from the east (Deccan plateau) during the summer season. The west coast region lies on the windward side of the Western Ghats gets heavy orographic rainfall. Often spurs of the hills that reach up to the coast interrupt it. The coastal lowland is divided into three sub-regions namely: the Konkan, the Kanara (Coastal Kamataka) and the Malabar (Kerala) coast. 2.3.2.1 Konkan coast The Konkan (Plate 2.9) is the most well defined physiographic units of Maharashtra state. Its western limit is defined by the Arabian Sea and the eastem one is marked by the steep rise of the Western Ghats scarp. With high rise scarps of the Westem Ghats maintaining an altitude of 1000 m on the east and the Arabian Sea defining its westem margin, the general slope of the land is from east to west. 32 c o M o Cd •*-• O o Ml=l •^^ = = a E £ c i £ = s • 0 U c- »• N rK B e N « 9 ^ ^ O ^ o r 0 0 h mo c o , N o 2S. £ 2Soo o 7' Dl^ A <- in O J? I The Konkan is 500 km long coastline and a narrow coastal plain stretching from north to south consist of plateau, plains and hills. It is separated from upland Maharashtra by a west - facing escarpment of Sahyadrian Mountains. It extends from Damanganga River in the north to the Terekhol River in the south. The width of the coast is not imiform and varies from 40 to 50 km. The coastal strip is wider in the north than in the south. All along the region, the shoreline is broken by frequent headlands and outcrops, which are the sites of steep sea cliffe, sandy beaches, drowned river valleys, small tidal inlets and major river creeks. Narrow, flat and low shoreline terraces covered with a thin apron of coastal alluvium border the tidal inlet. These land facets have contributed immensely to distinctiveness of the Konkan coast. The hill range, although not continuous, forms a watershed between coastal streams and the scarp foot plains to its east. Based on the lithology, geomorphic configuration and nature of hinterland, Konkan region can be divided into North Konkan, Middle Konkan and South Konkan. 2.3.2.1.1 North Konkan It starts from Dahanu to Karanja and is characterized by forested hills and plateau to a height of 350 m. Narrow plain invariably borders the river channels. Ulhas plain is covered by Brown silt; it is a marine planation surface forms in the early quaternary. North to south oriented coastal ranges is an important feature in this sub region. The entire region is an area covered by Deccan Trap rocks with intertrappean beds. Data for Mumbai can be considered as a representative of this region. 2.3.2.1.2 Middle Konkan The coastal belt from Uran to Shrivardhan can be identified as Middle Konkan. This region is relatively more flat. The central part of the east - west beh of this area is hilly with height ranging from 300 to 500 m. Most of these hiUs as residual hills and a plain surface at about 550 m. Basah is the main lithological formation of this region. Alibag and Hamai are taken as representative of this zone. 33 2.3.2.1.3 South Konkan This is the longest stretch of Konkan covered by thick laterites. Barren Lateritic plateaus; deeply entrenched river channels and the piedmont plains at the foot of the Sahyadrian escarpment are the significant land facets, which reflect the impact of lithology. The plateau is 150 to 200 m high and is covered by laterite which is 8 to 12 meter thick. The granitic and gneissic formation influences the landform development around Kankavali, Kudal, Sawantwadi, Malvan and Vengurla (Diddee et al, 2002). Ratnagiri Panjim and Marmagoa have been selected as representative of this region. 2.3.2.2 COASTAL KARNATAKA (KANAKA COAST) 2.3.2.2.1 Coastal plain This extends all along a length of about 400 km with varying width, maximum in the east of Mangalore (around 70 km). The coastline is broken at places by headlands made up of Precambrian rocks jutting into the sea (Plate 2.10). There are a few rocky islands as well. Sand dunes, beach ridges, mudflats and a few lagoons met with coastal plain. The plain rarely rises to more than 30 m and is almost flat. The spit and lagoons shoreline extends from the border with Kerala in the south up to Karwar in the north. Below are given locations of some of the prominent features in the coastal in the coastal plain of the state. Headlands: These are the landmasses along the coast jutting out into the sea of considerable height with a steep clifif face. These are generally made up of granite and gneiss. Some of the important headlands are: - North West of Baindur, South of Bhatkal and near Haldipur. Lagoons: These are shallow stretches of sea water between the mainland enclosing offshore bars, communication with the sea, often extending parallel to the coast. Most of them are similar to Kayals of Kerala, usually narrow and long, occupying an earlier flat or mudflat or widened tidal creek. A few are seen close to Kumta and Malpe. Ancient Beach Ridges: These usually linear sandy ridges usually parallel or oblique to the coast are made up of consolidated sands and generally represent ancient 34 beaches or spits. Prominent ones are from Kap via Mulur to Mulki, west of Udupi close to the coast and northwest of Kanvar, north of Kahnadi River. Tidal creeks: There are many tidal creeks emanating from the sea and intruding into the coastal plain over a distance of 5 to 20 km but generally parallel to the coast. Kodi Hole, Pavanji creek are some of the important ones. Mudflats: Mudflats are usually found along the tidal creeks, between two creeks and close to the estuaries, they are mostly made up of clay. These tidal flats are foimd conspicuously along sections of Kollura, Gangolli and Kali rivers. Sand dunes: Sand dunes are generally very close to the coast behind the beach, with a linear extension rising to 2 to 3 meters. There is a prominent dune between Kundapur to Pamapalli, extending over 8 km. Spits: These are long narrow accumulation of sand and or gravel, with one end attached to the land and tiie other projecting into the sea, deposited generally by the action of the long shore currents. Generally, they are almost parallel to slightly oblique to the coastline but a little separated from it. Some of the spits are found near Mangalore, Kundapur, near KoUuru river, Honavar, near Tadri creek, near Udupi. Beaches: They are relatively linear in the soudiem part of the coast, but are made up of discontinuous crescent shaped forms in the northern part. Island: These are a few islands off the west coast. Some of them are made up of granite in the south and volcanic in the north. The important islands are St. Mary islands, Coconut Island, North Island and Darya Bahadur Ghar (Radhakrishna and Vaidyanadhan, 1997). This region is represented by three stations namely Karwar, Honavar and Mangalore 2.3.2.2.2 Coastal plateau The coastal plain rises to a region made up of small plateau covered mainly by laterite, in a long NW - SE belt with a width around 25 km. These range in altitude from 45 to 120 meters. Further, the east of this belt is isolated by cluster of hills varying in altitude from 90 to 300 meters. These may or may not have a cover of laterite and end up close to the foothills of the next succeeding Ghats to the east. Within tiiis belt can be seen 35 steep - sided valleys and low erosional platforms rimmed by cliffe of varying heights. Extensive laterite plateaus occur between Udupi in the west and Ghats in the east. In a few places the coastal plateaus extends quite close to the coast as; near Bhatkal, west of Karkala, NW of Baindur, SW of Kasargod and Kumta. 2.3.2.2 KERALA COAST (MALABAR COAST) The Malabar Coast (Plate 2.11) is 550 km long and 20 to 100 km wide. It is narrow in the north and south and wider in the middle section. Its maximum extension is found in the valleys of Beypore, Ponnani, Periyar and Pamba-Achankovil rivers. It is only on the Malabar Coast that there are seen a number of lakes, lagoons or back waters which form a noteworthy feature of that coast. This back water, e.g., the Kayals of Kerala is shallow lagoons or inlets of the sea lying parallel to the coastline. They form an important physical as well as economic feature of the Malabar Coast, affording facihties for mland water communication. The silt brought by the recurring monsoon floods support large forests and plantations along their shores. At some places, especially along the tidal estuaries, deltaic fronts, or salt marshes, there are the remarkable mangrove swamps lining the coasts. The whole sea board is surrounded by a narrow submarine ledge or platform, the "plain of marine denudation", where the sea is very shallow, the surrounding being much less than 100 fathoms. This shelf is of greater breadth on the Malabar Coast than on the Coromandal coast. 2.3.2.3 WESTERN GHATS The Western Ghats (Plate 2.12), as the name Ghat denotes, are, down to Malabar, steep sided, terraced, flat -topped hills or cUffe facing the Arabian Sea coast and running with a general parallelism to it. Their mean elevation is some 900 m. Some of the peaks rise to the altitude of 2,652 m (Dodabetta peak). From the Nigiris the Western Ghats extend through tiie Anaimalai hills, to the extreme south of the peninsula. The horizontal bedded lavas of which they are wholly composed have, on weathering, given to them a characteristics "landing-stair" aspect. The physical aspect of the Western Ghats south of Malabar-that is, the portion comprising the Nilgiris, Anaimalai, etc-is quite different from these squares-cut, steep-sided hills of the Deccan proper. The difference in scenery 36 arises from difference in geological structure and composition of the two portions of the Western Ghats. Beyond Malabar they are composed of the most ancient massive crystalline rocks, and not of horizontal layers of lava flows. 2.3.3 CLIMATE OF THE WEST COAST REGION Its coastal location and the high mountains of the Western Ghats have a great influencefl'on the climate of the west coast. The west coast is influenced by Arabian Sea, therefore gets the beneficial of sea breezes throughout the year. This region is characterized by heavy southwest monsoon rainfall along with low aimual temperature range (Fig 2.15). The region enjoys an equable cUmate with high temperatures almost throughout the year. Its mean monthly temperature ranges between 23°C and 30°C. The maximum temperature rarely exceeds 34° C and the minimum rarely falls below 18''C. The highest temperature ever recorded in the west coast was 41 ° C at Mumbai (Santa Cruz) on 12* May, 1979. The daily range in temperature is about 10 to 14° C in winter and about 3 to 6° C in monsoon season. The annual range initially decreases southward from 7° C at Mumbai to 5° C at Hamai and again increases to 6° C at Trivandrum. April and May are the hottest months of the year. High humidity's and refreshing on shore breezes, particularly in the aftemoon and in the evening are typical of this region. The seasons in the west coast are discussed below. The rainfall over the west coast varies between 215 cms at Mumbai and 372 cms at Honavar. The rainfall initially increases from Mumbai to Panjim. There is a decrease of about 35 cms from Panjim to Marmagoa. Rainfall then increases as one proceeds further southward where Honavar receives about 372 cms of rain. The rainfall in Kerala decreases from 300 cms in the north to 182 cms at Trivandrum and 100 cms at Kanyakumari. About 80% of the annual rainfall is received during four months from June to September in Kokan and north Kamataka divisions. The length of the rainy season increases towards the south from four to five months in Kokan and north Kamataka to seven months near Mangalore and eight to nine months in Kerala. A double maximum of rainfall- a primary maximum in June, July and secondary one in October, November is therefore typical of Kerala coast. 37 2.3.3.1 Winter season The cool season extends from December to February. During this season, it is warmer during the daytime and moderate during the nighttime with clear sky and low humidity. The mean temperature is in January ranges between 23° C (Alibag) and 27.3 " C (Alleppey). With appreciable decrease in night temperature, the daily range in January is at its maximum (11 to 12° C) (Fig 2.15). The mean temperature decreases in February. This decrease is largely due to the fall in mean maximum daily temperature. As the night temperature remains the same the daily range is reduced to 9° C in February. During this season, the total amount of rainfeU varies from 0.4 cms at Mumbai to 13 cms at Calicut (Fig 2.15 and Fig 2.16). Winds are easterlies at most of the stations with calm days varying between 0 at Alleppey and 31 at Trivandrum while at coastal Kamataka and Konkan calm days range between 0 and 27 and 4 and 23 respectively (Fig 2.17). During winter evenings, the winds are westerlies and south westeriies over coastal Kamataka and Kerala while for Konkan the winds are westerlies and north westerlies. Over the entire region the calm days vary between 2 and 4 (Fig 2.17). 2.3.3.2 Sunmier season An increase in the mean temperature in March indicates the approach of the hot season. The season is characterized by clear anticyclonic air, steady increase in temperature where the gradient of the temperature is steep till May. The highest temperature is recorded in May for the stations of Konkan and coastal Kamataka however for Kerala April happens to be the hottest month. During this period, the maximum temperature varies between 29.2 to 33.2° C while the minimum temperature ranges between 21 and 27° C (Fig 2.15). During this season, Konkan and coastal Kamataka receive rainfell between 1 and 11 mm. Kerala on the other hand receives about 20 to 40 mm (Fig 2.15 and Fig 2.16). This rainfall is mostly associated with the thunder activity. The thimder days during this season vary between 2 and 40. The thunder days increase in number as one proceeds from Mumbai to Trivandmm (Fig 2.18). Due to the prevailing high humidity the weather during this season is very oppressive. The end of May marks the arrival of the monsoon which gives the way for rainy season. 38 iiil. : Mi!. ! U u - I 3 m IT) ,^i 2" X « , 1- rv ifi tn o iri o m S K o o ° o o 2 S o o o o o S " o m o ^< i il^ K t^m J^K^ •• N S ^ 1 .-•J > f i M s .V^ i I 1.^^ ^ ^ I Pi o I 00 ri 2.3.3.3 Monsoon season The onset of the southwest monsoon at the end of May in Kerala and in the first week of June in Konkan is associated with the development of cyclonic storms in the Arabian Sea. A fall in the mean temperature of 2 to 3° C takes place with the outbreak of monsoon. July and August receives rainfall between 60 and 120 cm are the rainiest month for most of the stations except for Trivandrum where rainfall varies 13 to 22 cm (Fig 2.15). There are at least 14 to 26 rainy days in each of the month from each of the months from June to September. At Mumbai, a maximum ramfall of 94.4 cm has been recorded at one single day i.e. 26* July 2005. This rainfall is due to the development of low pressure over northwest Bay of Bengal, intensification of monsoon trough, presence of convective vortices over central India accompanied with strong monsoon current from Arabian Sea. However, in spite of the heavy rainfall the variability of rainfall in the region is as great as in the interior of the peninsula. The rainfall over the west coast region is influenced by orography and the synoptic patterns of the region. Apart from this; rainfall seems to depend only on the ground contour in the vicinity, so that any interpretation in terms of broad orogr^hic pattern has to be attempted with greatest care. Colaba and Santa craz differ in rainfall amount by 25% though within 30 kms. A hillock of 300 meters within 5 kms of Santa cruz apparently accounts for this increase (Y. P. Rao 1976). Ramakrishnan et al (1958) studied the rainfeU distribution pattem for the west coast station and they show a maximum at 14 degrees north and a secondary maximum at 18 degrees north. Raghavan (1964) found the relationship between coastal rainfall and the distance of 150 meters contour firomth e place, giving the correlation of 0.6. In nutshell, it can be concluded that the west coast region experienced heavy rainfall during this season (Fig 2.16) from the synoptic system associated with southwest monsoon. The formation of off-shore troughs which are shallow zones of convergence causes enhanced rainfell activity along the regions. The Arabian Sea Branch of southwest monsoon strike against the Western Ghats thereby giving heavy amount of rainfall. Nearly half of the active to vigorous monsoon situation in Konkan and three-quarters of such events in coastal Kamataka are associated with the off shore trough (Rao, 1976). 39 The monsoon season brings heavy rainfalls as compared to other season and even the circulation pattern for weak convection is able to produce rainfalls. During this season maximum temperature falls on accoimt of extensive cloudiness due to heavy rain but the air remains hot and sultry. The winds are from westerly direction for majority of the stations however over Trivandrum and Alleppey the North West component pre dominates. The calm days in the morning vary between two at Marmagoa and 34 at Cochin (Fig 2.19). 2.3.3.4 Post -Monsoon season As the hiter-Tropical Convergence Zone (ITCZ) moves to lower latitudes, marks the autumn season. In the west coast region, the retreat of the monsoon begins by 1^ October from the Konkan coast, day temperature begins to increase and a secondary maximum in day temperature is reached in November with progressively cold night and the clouds begin to decrease, the volume of rain and number of showers decrease. It may be mentioned that while the arrival of monsoon is sudden and abrupt, its withdrawal is rather gradual. With the withdrawal of south west monsoon, the temperature rises forming a second maximum. During the month of October, the mean maximum temperature varies from 29.5° C at Trivandrum to 30.1° C at Mumbai while the mean minimum temperature ranges between 23.3° C at Karwar and 24.8° C at Mumbai (Fig 2.15). The rainfall during this season ranges between 7 cms at Mumbai and 57 cms at Alleppey (Fig 2.16). The rainfall increases from north to south. Kerala coast records more rainfall during post monsoon season when compared with the summer season. Good amount of rainfall, mostly as thunder showers occurs in this season in Kerala due to the influence of north east monsoon. Rainfall associated with northeast monsoon ceases by December in Kerala. 2.4 SIMILARITIES AND CONTRAST The topography of the northeast and west coast have similarities; moimtain, plain, valleys, rivers and lakes. The west coast faces the Arabian Sea while the northeast is located inland. In the northeast, the Himalayas block the cold air mass that blows from the north during the winter season whereas in the west coast region, the hot wind from 40 the east (Deccan plateau) blow toward the coast tiirough gaps and passes of the Western Ghats during the summer season. North east extends northward of 22 N while the west coast region extends from almost 8° N in the South to 20° N in the North. As such, the effect of the latitude in the temperature and diurnal range is different in these two areas. The west coast of course shows a different range of temperature in annual and diumal range as compared to those over the northeast due to the effect of continentality. Minimum daily range of temperature is the characteristic features of marine climate while extremely high daily range of temperature characterized the continental climate, hi northeast, the maximum temperature is in May whereas in west coast it is May and November. Also there is one maximum rainMl in the northeast in July and west coast region. Temperature is moderate along the west coast due to the maritime influence and is highly uniform through the year, except during the rainy season where as the temperature in the northeast region is modified by its high elevation and its proximity to the Himalayas. There is a general increase in precipitation with altitude in northeast region while precipitation in west coast region increases from the coast toward the interior. The mountain ranges of the northeast (Himalayas, Meghalaya plateau and Purvanchal ranges) and the west coast regions receive heavy rainfall due to orographic effect. Both the regions receive mean annual rainfalls of more than 100 cm, 68% of the aimual rainfell occurs in the monsoon seasons. The Arabian Sea and Bay of Bengal branch of southwest monsoon arrives simultaneously in the west coast and West Bengal/northeast region. Northeast India presents the worst flood prone sites, extremes in temperature, heavy rainfall, violent thunderstorms, and hailstorms, occurrence of thick fogs and cold waves but in the west coast, moderate temperature and heavy rainfall, cyclone, changing urban climate and thundershowers are the main features. The common features are heavy rainfall, thxmderstorms, floods, infrequent cyclone and low intensity droughts. Apart from this, the circulation features prevailing over the west coast and northeast region of India influence onset, duration and progress in monsoon. The interesting features of contrast and comparative patterns over the two regions of India (northeast and the west coast) in terms of climatology of various associated weather parameters, such as temperatures, rainfall along with their extremes, actually 41 merit this study and hence their aspects are discussed after analyzing by taking a sufficiently large time series. The next chapter therefore intents to analyze the temperature patterns over the two study areas to understand the trends and their impacts. 42