Climatic condition of West Bengal -

Transcription

1 CHAPTER 3: Climatic condition of West Bengal - The climatic condition of any place is directly related with three terms - Weather, Climate and Climatic regions. Weather1 refers to the state of atmosphere at any given time denoting the short-term variations of atmosphere in-terms of temperature, pressure, wind, moisture, cloudiness, precipitation and visibility. It is highly variable. It is constantly changing, sometimes from hour to hour and at other times day to day. Climate is defined as average weather*. It is the sum total of the variety of weather conditions of an area or a place from day to day. The World Meteorological Organization has suggested a standard period of 31 years for calculating the climatic averages of different weather elements. The result of close observation of the climate of various parts of the globe shows that the same pattern of climate does not prevail everywhere. This happens due to variation in the constituent elements of climate. When in a particular region uniform climatic characteristics prevail, that region is known as climate region. 57

2 WEST BENGAL IN A NUTSHELL:- 4. Uttar Ojrsajpirf 5. DafsgWim F.: ilrtetoun Madia 11. PllflaRa IS 14. NtaShi PssstiEm Wdlifkipore 10, HSswaJh IF. 18. Sewii S4 'Faigaftas 19. Piute E^fudfrnlpcse Map of West Bengal The area of West Bengal is 88,752 sq.km. Capital of West Benagal is Kolkata. The number of districts of West Bengal is

3 West Bengal is the only state in India, where snowy mountains exist in the north, sea at the south, with both plains and plateau covering the remaining region. Geography of West Bengal is diverse, consisting of high peaks of Himalaya in the northern extremes to coastal regions down south, with regions such as plateau and Ganga delta intervening in between. West Bengal was created as a constituent state of the Indian union on 15th August, 1947 as a result of partition of the undivided British Indian province of Bengal into West Bengal. It is bounded on the north by Sikkim and Bhutan, on the east by Assam and Bangladesh, on the south by Bay of Bengal and on the West by Orissa, Jharkhand, Bihar and Nepal. The state lies between and " north latitudes and u and " east longitudes. physiographic divisions. - The state of West Bengal can be divided into eight natural 1) Darjeeling Himalayan hill region covering the whole Darjeeling district except Siliguri division and a narrow part in the northern part of the district Jalpaiguri. This region belongs to the eastern Himalaya. 2) Terai region covering the plains of Darjeeling district, whole of Jalpaiguri and upper region of Coochbehar district in West Bengal. The slope of the land goes from north to south. The entire region is made up of sand, gravel and pebbles laid down by the Himalayan rivers. 59

4 3) North Bengal plains : This region starts from the south of Terai region and continues up to the left bank of the river Ganges. The geographical *egion covers the southern parts of the district Jalpaiguri, North Dinajpur covering some extreme northern regions South Dinajpur, Malda and southern part of Coochbehar district. 4) Rarh region : It is the regon that intervenes between the western plateau and high lands and the Ganges delta covering a portion of the districts Murshidabad, Birbhum, Bankura, Bardhaman and Paschim Midnapore. 5) Coastal plain : A small coastal region exists on the extreme south of West Bengal. A portion of the district of Purba Midnapore along the Bay of Bengal constitutes the coastal plain. 6) Sundarban area : The Sundarban delta is the largest mangrove forest in the world situated in the south 24- parganas district. It lies at the mouth of the Ganges. 7) Western plateau and high lands : The western part of Purulia, Bankura, Birbhum, Bardhaman and Paschim Midnapore district constitutes this area. 8) Ganges delta : This region covers the whole of districts Murshidabad, Nadia, Kolkata, Hooghly, Howrah and the 60

5 northern part of north and south 24-parganas. The northern portion of south 24-parganas district is known to be the active delta of the river Ganga, because the formation of delta is a continuing process Climate of West Bengal - Although traditionally, the climate of West Bengal is divided into six seasons, but in recent yearn, the Meteorological Department of Government of India recognized four seasons. These four seasons are - a) The hot season, b) The advance of the monsoon, c) The retreat of the monsoon and d) The cold season. The six traditional seasons are a) Summer, b) Rainy, c) Autumn, d) Hemanta (pre-winter), e) Winter and f) Spring. The climate of West Bengal varies due to it s location. West Bengal is situated in a strategic position in eastern India and the tropic of cancer passes through this state. The state extends from the snow-clad Himalayas in the north to the Bay of Bengal in the south. The state is meteorologically divided into two sub-divisions. One is Sub-Himalayan West Bengal (SHWB) in the north and the other one is 61

6 Gangetic West Bengal (GWB) in the south. The northern part of the meteorological sub-division SHWB is bounded by the hills (Himalayan ranges) which play an important role in rainfall distribution in pre-monsoon and southwest monsoon seasons. The six districts viz, Darjeeling, Jalpaiguri, Coochbehar, North Dinajpur, South Dinajpur and Malda belong to the SHWB division. Floods are very common in some districts of this region almost in every monsoon for their geographical positions and geomorphic character of the region. In meteorological sub-division GWB, the northern parts are combination of hilly and plain areas; whereas the southern part areas are mainly plain lands. The districts - Murshidabad, Birbhum, Nadia, Purulia, Bankura, Burdwan, Hooghly, Howrah, Midnapore, North 24-parganas and South 24-parganas fall under GWB division. Amongst them three districts viz, Midnapore, North 24-parganas and South 24-parganas are the coastal districts Factors affecting climate of West Bengal_- variables. The climatic condition of any place depends upon some climatic The followings are the main variable factors that affect the climate of West Bengal from many angles - 1) Temperature It is the degree of hotness or coldness of an environment. Air temperature or surface temperature in meteorology is 62

7 defined as the ambient temperature indicated by a thermometer exposed to the air but sheltered from direct solar radiation. 2) Atmospheric Pressure Pressure is the force per unit area. Atmospheric pressure is the pressure exerted by the atmosphere on any surface by virtue of its weight, it is equivalent to the weight of a vertical column of air extending above a surface of unit area to the outer limit of the pressure. 3) Rainfall - Broadly, it is a shower or fall of rain. Specifically, it is the quantity of water, expressed in milimetre, precipitated as rain, snow, hail or sleet in a specified area and time interval. Water falling in drops from vapour condensed in the atmosphere. 4) Cloud amount - Cloud is defined as a visible body of very fine water droplets or ice particles suspended in the atmosphere at altitudes ranging up to several miles above sea level. It is a mass of dust, smoke or steam, suspended in the atmosphere or in outer space. Cloud amount that defines as portion of the sky cover, which is attributed to clouds. The unit of measurement is the okta. There are many other different factors that affect climate around the world. The most important factors are - 1) Distance from the sea. 2) Ocean current. 3) Direction of prevailing winds. 4) Relief 5) Proximity to the equator. 6) Recently, 'human activity1 is considered as factor affecting climate. 63

8 1. Distance from the sea - The sea affects the climate of a place. Coastal areas are cooler and wetter than inland areas. Clouds form when warm air from inland areas meet cool air from the sea. 2. Ocean current - Horizontal and vertical system of ocean water produced by gravity, wind friction and water density variation in different parts of the ocean. 3. Direction of prevailing winds - Winds that blow from the sea often bring rain to the coast and dry weather to inland areas. 4. Relief - Climate can be affected by mountains. Mountains receive more rainfall than low lying areas because the temperature on top of the mountains is lower than the temperature at sea level. The higher place which is situated above sea level is colder than plain area. This happens as altitude increases. Air becomes thinner and is less able to absorb and retain heat. 5. Proximity to the equator - The proximity to the equator affects the climate of a place. The equator receives the more sunlight than anywhere else on earth. This is due to its position in relation to the sun. 6. Human Influence - The above climatic factors are natural factors. However, we also affect the climate in many ways. The Industrial Revolution, starting at the end of the 19th century, has had a huge effect on climate. The invention of motor engine, the increased burning of fossil fuel and the cut down of trees has increased the amount of carbon-dioxide and other harmful gases in the atmosphere. 64

9 3.3. Specific climatic Zones - The areas or places where the climatic nature or climatic characters are same, that regions fall under one climatic zone. The researcher has tried to divide West Bengal into specific climatic zones, consulting the literature available in this context. West Bengal is divided into different climatic zones based on some fundamental elements, arranging in different orders and combining them for evolving a satisfactory and useful climatic zonal division of this state.1 This fundamental characters are briefly described below - a) Rainfall - West Bengal is dominated by the major climatic element rainfall round the year, for this reason, this state can be considered as monsoon land. But, the amount.of rainfall varies widely depending on a number of basic geographical and meteorological factors. The state West Bengal is divided into four main types and a few sub-types considering the amount of mean annual rainfall (Table - A). Table -A Climatic zones 1. Super humid 1a. Super humid 1b. Super humid 2. Humid 3.Semihumid 4. Subhumid montane so. slope terai Mean Annual >3000 > <1500 Rainfall In mm MISHRA (Swadesh). Climatic regions of West Bengal: a meso climatic classification. Indian Journal of Landscape Systems and Ecological Studies. 15,1; 1892;

10 Zone 1 (Super humid) It has been further sub-divided into two groups- Super-humid montane southern slope (>3500 mm) and Super-humid terai ( mm) based on the relief and slope which are very significant in the distribution of rainfall. The amount of rainfall is heavier in Himalayan range than the adjoining plains due to the pattern of relief intercepting the moisture bearing winds from the south. Zone - 2 (Humid) - Here, the annual rainfall is about ( ) mm. Zone - 3 (Semi-humid) - In this zone, the annual rainfall is about ( ) mm. Zone - 4 (Sub-humid) - The annual rainfall is less than 1500 mm. b) Temperature - After rainfall, the second most important climatic parameter is temperature. In a study of this nature, the mean annual temperature could have been considered as a guiding criterion, but, except the hilly northern region, the variation in mean annual temperature in the rest of the state is insignificant. This, therefore, cannot be considered as a guiding parameter for dividing the state into different climatic zones. Then, the 'mean diurnal range of temperature has been taken into consideration. The mean diurnal range of temperature is also found unsuitable as there are only two regions, one along the coastal belt of Midnapore and 24-parganas (S) and other in the northern mountainous region of Darjeeling and Jalpaiguri district, where the diurnal range is less than 10 c. The rest of the state shows very little variation. Hence, this (mean diurnal range) also does not provide a suitable data to divide the state into climatic zones. 66

11 The most suitable thermal factor to divide the state may \ be the mean annual range' of temperature, i.e., the difference between the mean temperature of the hottest and the coldest month. Map showing the climatic division based on mean diurnal range temperature and mean annual range of temperature are attached in this thesis (fig - 1 & fig - 2)1 (appendix-7). The mapping presentation indicates the followings : i) The coastal areas show very low mean annual range of temperature (below 10 c). ii) An area over the terai and Himalay also has a very low annual range, slightly above that of the coastal region. iii) The mean annual range' gradually increases as one proceeds westward and is maximum at Purulia (14.6 c). ii) The 'mean annual range changes gradually, and this change is in conformity with the change of humidity and precipitation parameters. Mean Annual Range of Temperature, therefore, can be successfully used to divide the state into distinct climatic zones and is hence adopted as a secondary parameter. 1. MISHRA (Swadesh). Climatic regions ofwest Bengal: a meso climatic classification. Indian Journal of Landscape Systems and Ecological Studies. 15,1; 1992;

12 Potential Evapotranspiration - Evapotranspiration occurs when plants secrete or transpire water through pores in their leaves. Evapotranspiration is the summation of Evaporation and Plant Transpiration, that means, the combined loss of water from soil and plants to atmosphere in the form of evaporation (from soil or surface of earth) and transpiration (from plants) is called Evapotranspiration. Evapotranspiration depends upon two factors - i) Water available from evaporation and transpiration. ii) Availability of energy (sunshine and hence temperature) to evaporate this water. It is clear from above that a change in any of the above two factors will affect the parameter Evapotranspiration. For example, if water supply to desert (where water supply and growth of vegetation - both are less) were increased, the quantum of vegetation and the rate of transpiration (evapotranspiration as well) would increase. If sufficient water is made available, evapotranspiration rises to a maximum. This maximum evapotranspiration possible is called potential evapotranspiration. Potential evapotranspiration is an important climatic parameter, because, the quantum of rainfall or precipitation alone in a zone does not determine, whether a climate is moist or dry. It is also important to determine whether this precipitation is enough to cater the needs of 68

13 evaporation and transpiration. Precipitation (P) i.e., rainfall in excess of the need of Potential evapotranspiration makes a climate moist and when it is lesser, the climate is dry. When both are equal, the climate is neither humid nor dry. The mean annual P-PE values are calculated and plotted on the map (fig -3)1 (appendix - 7)' which is attached in this thesis and the following conclusions are obtained - i) The mean annual precipitation of the western plateau area covering the districts - Purulia, Bankura, Birbhum, Bardhaman, north western Midnapore and western Murshidabad is less than potential evapotranspiration. ii) The mean annual rainfall over the coastal areas of Midnapore and 24- parganas exceeds mean annual potential evapotranspiration by more than 200 mm. Another zone - the whole area of south and north Dinajpur have also the same amount of excess moisture. iii) There are a zone of transition covering central part of Midnapore, Howrah, Hooghly, Northern 24- Parganas, Nadia, Murshidabad and Malda between zones (i) and (ii), where the precipitation is almost equal to or slightly in excess of (below 200 mm) potential evapotranspirtion. iv) Towards further north, rainfall greatly exceeds potential evapotranspiration. 1. MISHRA (Swadesh). Climatic regions of West Bengal: a meso climatic classification. Indian Journal of Landscape Systems and Ecofoglcal Studies. 15,1; 1992;

14 This picture shows a satisfactory and reliable picture of the moisture factor of the state. So, this has been taken as the third criteria for the climatic classification. Combination - To divide West Bengal into specific climatic zones, the above three criteria and mean annual relative humidity are combined. In addition to these major climatic elements, geographical nomenclature has also been applied to specify climatic zones specially the two identical zones are seen in different areas. The combination of all the above aspects are mapped in the fig. no. 4 and attached in this thesis. TABLE - B also shows the combination of three climatic criteria (mean annual rainfall, mean annual range of temperature and mean annual (precipitation - potential evapotranspiration)). 1.MISHRA {Swadesh). Climatic regions of West Bengal: a meso climatic classification. Indian Journal of Landscape Systems and Ecological Studies. 15,1; 1992;

15 Table - B No. Specification of Mean annual Main annual range of Mean zones r.f. In mm temperature in c annual P- (difference in the mean PE In mm temperatures of hottest and coldest month) (I) (II) (III) (IV) 1. SUPER HUMID Above 3000 Above a/S1 Super humid More than montane oo o 1b/S2 southern slope 10 c- 13 c Super humid Terai 2. HUMID a/H1 Humid montane c- 13 c northern slope 2b/H2 Humid interior c - 13 c 2c/H3 Humid coastal below 10 c 3(S3 & SEMI HUMID dc-13 c S6) (North & South) 4. SUB HUMID Less than P=PE or oco slightly> PE 4a/S4 Sub humid east Less than 10 c - 13 c but no more 1500 than 60 4b/S5 Less than mm. Sub humid west 1500 above 13 c P<PE a u a M 1.MISHRA (Swadesh). Climatic regions of West Bengal: a meso climatic classification. Indian Journal of Landscape Systems and Ecological Studies. 15,1; 1992;

16 On an analysis of the map and the above table (TABLE - B) the following features are revealed - i) If one moves from north to southwest, there is a gradual sequence of climate from the super-humid to sub-humid type. ii) The same sequence is seen from south to northwest i.e., from the coastal to western plateau area. iii) The super-humid type is further subdivided into two sub-types depending on their location in two distinctly separate physiographic regions i.e., (a) Super humid montane southern slope covering the south facing Himalayan slope of Darjeeling and Jalpaiguri district and (b) Super humid terai covering the plain section of Darjeeling and Jalpaiguri and almost whole of Coochbehar districts. The former is responsible for much higher rainfall (>3500 mm) than the later ( ). iv) The humid type is divided into three sub-groups viz., (a) humid montane northern slope, (b) humid interior and (c) humid coastal, which are located in three different parts of the state. These three zones differ from one another not only in their location, but also due to difference in both the range of temperature and annual means due to altitude and aspects. v) The semi-humid type is also located in two separate zones, one to the south, bordering the humid coastal zone to the south and the subhumid zone to the north, covering central part of Midnapore, almost the whole of Howrah excluding its southern top and Hooghly, southern part of Nadia and the northern part of 24-parganas. The other one is to be 72

17 north bordering the humid zone to its north and sub-humid zone to its south and covering the northern part of Malda and southern part of North and South Dinajpur districts. This is a zone of transition between the humid and sub-humid group. vi) The sub-humid type is divided into east and west by the extremity of temperature and aridity, as one proceeds from the east to the west: (a) The sub-humid east covers northern half of Nadia, eastern part of Bardhaman, whole of Murshidabad and southern part of Malda district, (b) the sub-humid west covers rest of the state including north-western Midnapore, whole of Purulia, Bankura, Birbhum districts and western part of the district of Bardhaman. vii) The geographical nomenclature in the particular background of the state is meaningful in explaining the general climatic environment. Therefore, it may be stated that the above scheme of classification (as outlines in fig.- 4) (appendix - 8) may be taken as more satisfactory to give a rational climatic division of West Bengal considering its size and varied climatic features prevailing over it. The name of the places vis-^-vis the climatic zones they belongs to has been represented in the following tabular form (Table -C). 73

18 Table - C Name of the climatic zones Name of the places falling under each specific zone. Climatic location 2A/H1 Darjeeling, Kalimpong Humid montane northern slope a'ea 1A/S1 Sonada, Kurseong, Super-humid montane southern sope. 1B/S2 Siliguri, Jalpaiguri, Birpara, Falakata, Super-humid terai area. Malbazar, Alipurduar, Cooch-behar, Dinhata, Mathabhanga. 2B/H2 Islampur, Balurghat Humid-interior area. 3A/S3 Kaliyaganj, Raiganj, Samsi, Malda Semi-humid north area. 4A/S4 Kaliachak, Jangipur, Jiaganj, Berhampur, Beldanga, Kandi, Karimpur, Krishnanagar, Nabadwip, Burdwan 4B/S5 Rampurhat, Saithia, Suri, Bolpur, Dubrajpur, Chrttaranjan, Asansol, Raniganj, Durgapur, Jhalda, Purulia, Raghunathpur, Bankura, Sonamukhi, Bishnupur, Panchmura, Garbeta, Ghatal. 3B/S6 Bagula, Shantipur, Ranaghat, Midnapore, Kharagpur, Belda, Dantan, Panskura, Tamluk, Nandigram, Belur, Howrah, Amta, Uluberia, Hooghly, Chuchura, Chandarnagar, Srirampur, Rishra, Uttarpara, Arambagh, Dakshin Barasat, Garia, Narendrapur, Champahati, Vidyanagar, Thakurpukur, New Barrackpur, Jhargram, Barasat, Bidhannagar, Barrackpur, Basirhat, Gobardanga, Palta, Naihati, Habra, Taki. Sub-humid east area. Sub-humid west area. Semi-humid south area. 2C/H3 Haldia, Contai, Diamond Harbour, Kakdwip Humid coastal area. 74

19 To arrive at conclusion regarding division of the state West Bengal in a number of climatic zones, the researcher has consulted the records of the Meteorological Department at Alipore. The Meteorological Department has 23 recording stations all over West Bengal, wherein the data regarding (i) Temperature, (ii) Rainfall, (iii) Relative Humidity and (iv) Cloud amount are collected and recorded. However, the meteorological department does not sub-divide the state into distinct climatological zones. Being not a climatologist, it is not possible for the researcher to divide the state West Bengal into specific climatic zones. This division is based on the article Climatic regions of West Bengal: a meso clinatic classification".1 In West Bengal, the season of south-west monsoon is the principal rainy season. The amount of raimall of the season varies from the southern part of Malda to the south facing slopes of the Himalayan region in Darjeeling and Jalpaiguri districts of West Bengal and ranging from the west central part of Gangetic West Bengal to the coastal belt of south 24- parganas. The amount of rainfall is not unrorm throughout the year, but, its distribution varies from year to year and from one region to the other region in the same year. This variation depends on the following factors - a) location of the place with respect to moisture bearing monsoon air current, b) position of land and water, c) break in the monsoon and its duration, d) general strength of the monsoon, e) frequency and movement of depressions, and f) formation of other low pressure systems. 1. MISHRA ( Swadesh ). Climatic regions of West Bengal : a meso climatic classification. Indian Journal of Landscape Systems and Ecological Studies. 15,1; 1992;

20 With the arrival of the monsoon rains, the maximum temperatures fall rapidly from (36-38) c in May to (32-33) c in July. The data on average annual rainfall of districts of West Bengal of different climatic zones and mean annual range of temperature of different climatic stations or places are given below in Table - D and Table - E.1 Table - D SI. No. District Predominant Climatic Zone 1 Daijeeling 1A/S1 (Super-humid: montane southern slope) and 2A / H1 (Humid : montane northern slope) 2 Jalpalguri 1B / S2 (Super humid : terai) Average Annual Rainfall In mm Coochbehar 1B/S2 (Superhumid: terai) Dakshin Dlnajpur 2B / H2 (Humid : interior) Uttar Dlnajpur 3A / S3 (Semi-Humid : north) Malda 3A / S3 (Semi-humid : north)

21 7 Murshidabad 4A / S4 ( Sub-humid : east) Nadia 4A / S4 (Sub-humid : east) Burdwan 4B / S5 (Sub-humid :west) (east part) Purulia 4B / S5 (Sub-humid : west) Bankura 4B / S5 (Sub-humid :west) Birbhum 4B / S5 (Sub-humid : west) Hooghly 3B / S6 (Semi-humid : south) Purba Midnapore 3B / S6 (Semi-humid : south) Paschlm Midnapore 3B / S6 (Semi-humid :south) (west part) 16 Kolkata 3B / S6 (Semi-humid : south) 17 Howrah 3B / S6 (Semi-humid : south) Parganas (N) 3B / S6 (Semi-humid : south) (east part) Parganas (S) 2C / H3 (Humid : coastal)

22 Table - E No. Station Zone to which the station belongs Mean Annual Range of Temperature In *C 1 Sonada (Daijeeling) 1A /S1 (Super-Humid montane southern slope) 2 Kalimpong or 2A/H1 (Humid: Daijeeling montane northern slope) 3 Coochbehar 1B / S2 (Super humid : terai) B/H2 (Humid: Balurghat interior) 5 3A/S3 (Semi-humid Wlalda : north) 6 Berhampur 4A / S4 (Sub-humid: east) 7 Suri 4B / S5 (Sub-humid : west) 8 Midnapore 3B / S6 (Semi-humid : south) g Sagar Island 2C/ H3 (Humid: coastal) MISHRA (Swadesh). Weather and climate of West Bengal. West Bengal: Colours of climate. Xlvitl, 6; 2006; IT

23 The above data of TABLE - D and TABLE - E shows that there exists some distinct climatic divisions, viz, super humid, humid, semihumid and sub-humid zones. In West Bengal, if we traverse from northern part to southern parts of the state, the significant difference in data in different climatic zones has been noticed. From a comparative study of the above tables (TABLE - A, TABLE -D and TABLE - E), it is noticed that the overall trend is the same in all the tables. The significant differences between the two tables are quantitative rather than qualitative. It may be noted from the tables that the rainfall is maximum in the southern slopes and terai of the Himalayas, slightly lesser than the northern slopes and gradually decreases, as one proceeds from north to south and from east to west. However, the quantum of rainfall again increases in the coastal regions of Midnapore and 24- Parganas(S). It is also to be noted that in recent years temperature has increased and rainfall has decreased gradually and this may be attributed to the phenomenon of global warming. 79