SEASON OF RETREATING MONSOON

Transcription

1 CHAPTER V SEASON OF RETREATING MONSOON The season of retreating monsoon (October and November) is a period of transition between the warm rainy monsoon and the dry cool winter. The change begins in early October and is usually completed by December. V.l. PRESSURE : During October and November and sun gradually moves towards south and the place of maximum heat gradually shifts to the southern hemisphere. As a consequence, the landmass of the northern hemisphere becomes relatively colder than the seas of the south. The low pressure conditions prevailing in north India in the monsoon season are obliterated by October. The monsoon trough becomes weak, open and moves to the south. They are transferred to the centre of the Bay of Bengal by the beginning of November, and to the south of the Bay early in December. The high pressure gradually and slowly establishes itself over the north-west India. Since it is a period of transition, the variation of air pressure all over the country is low. Fig.5.1 shows the mean pressure condition and wind direction of October. V.2. SURFACE WIND : As the monsoon withdraws from most part of the country in October, an anticyclonic condition is established over northwestern India. The westerly and south-westerly winds of June to September weaken and become variable in October, while in November north-westerly winds gradually become active. Fig. 5.2 shows the mean surface wind direction and speed (mean

2 MEAN PRESSURE AND WIND-OCTOBER F IG.5.

3 WEST BENGAL FIG.5.2

4 59 ' of October and November) of the season. It is evident from thefig. that the wind is mainly easterly over North Bengal which comes down the Brahmaputra valley. Over other areas of the State the surface wind is mainly northerly and north westerly over the coastal and the hilly region the wind direction is slightly different and is shown is Figs:5.3a and 5.3b. Due to lesser pressure difference, the wind speed is also low everywhere below the annual average. V.3. UPPER WIND : The upper wind in October at a height of 2 Km. is north westerly which gradually becomes westerly with increasing height. The upper wind at 8 Km. is mainly westerly with speeds varying from 48 to 64 Km. per hour. V.4. TEMPERATURE : The clear autummn weather following the termination of the monsoon rains is accompained by a rapid fall of tempera ture.the fall of temperature is well marked all over the State. Minimum temperature drops more steadily than the maximum creating larger diurnal range (fig:5.4). The mean temperature of October is about 2 C lower than the mean temperature of September but the change from October to November is still larger.the average temperature of November is 3.5 C (Sagar Island) to 5.3 C (Maldah) lower than the average temperature of October. The mean daily maximum temperature of October varies from 32.7 C (Bankura) to 30.0 C (Sagar Island) over the greater part of the State Excepting the northern mountainous beld where the mean daily maximum is 18.6 C at Darjiling and 23.3 C at Kalimpong.. The mean temperature of the season varies from 25.9 C in the south (Kanthi, Sagar Island) to 23.4 C to the end of the northern plain (Bagdogra). Western part of the State (Puruliya,

5 AT HRS.t.S.T. winu LiiHtci (On DURING RETREATING MONSOON - COASTAL STATION SAGAFl ISLAND AT HRS. I.S.T. > / k \ KAN THI r1 > * FIG. 5.3a FIG. No. 5.3b

6 MARCH OF RAINFALL TEMPERATURE & HUMID! TY IW7.Wc -I IK) BAGDOGRA 100X- la Lhj PURULIYA In n n n n n, MED1NIPUR ALIPUP ratytyoc nnnnnnnn^n. SAGAR Is. n n n, KANTHI PEN TAD No. ** **»* «! II <> I* IS 44 «JS» SJ J* 59 < «l j) n «15 P 47 FIG. 5.4 * OCT NOV > * OCT NOV TEMPERATURE RELATIVE HUMIDITY MEAN DAILY MAXIMUM MEAN DAILY MINIMUM AT 8*30 HRS.I.S.T AT I7 30 HRS. LS.T...

7 60 Asansol, Siuri, Sri Niketan) records relatively higher, temperature (of at least 1 C) than the-eastern part (Baharampur & Krishnanagar). Fig. 5.5 shows the distribution of mean temperature of the season. The mountainous tract of the north records mean temperature 5 C to 10 C lower than the adjoining plains due to elevation. Due to the cessation of monsoon rain, clear sky, low humidity and lesser day time, the diurnal range of temperature also gradually and steadily increases. Fig. 5.6 shows the mean diurnal range of temperature of the season. Along the coastal belt (Sagar Island 6.3 C) and in the mountainous zone of the north (Darjiling 7.5 C) the range is minimum. In the central and western part of the State the range is much higher which reaches its highest at Bankura (12.3 C) during the season. V.5. RAINFALL : The monsoon withdraws from West Bengal after some intermittent action in the first half of October. Rainfall gradually ceases to occur over most part of the State except the coastal areas-, where the amount of cyclonic rain is fairly high. During October the coastal stations receive the highest amount of rain in the State. The amount of rain varies from 80.1 mm at Puruliya to mm at Kanthi. The amount reduces remarkably in November. In the Himalayan and sub-himalayan West Bengal, the amount varies between 6 mm (Bagdogra) and 35 mm (Buxa Duar). In the central Bengal, the amount in general is higher and the increase continues towards the coast reaching maximum of 38.3 mm at Kanthi. Figs.3.7 and 5.8 show the distribution of rainfall and number of rainy days during the season. Along with the rainfall the number of rainy days also reduces gradually. It is seen from the above Fig. that the number of rainy days varies from 3.8 at Bankura to 10.5 at Kanthi and 11.6 at Buxa Duar.

8 WEST BENGAL FIG. No. 5.5

9 WEST BENGAL FIG.No. 5.6 I

10 I WEST BENGAL FIG.5.7

11 WEST BENGAL Mean Number of Rainy days Retreating Mansoon (Oct.-Nov.) V LEGEND ] Above io days below a 23, 30 jv _ IOC 120 Km [1111 FIG. 5.8

12 61 The source of rain in the season is primarily cyclonic storms and depressions which frequently form and invade the coastal areas. Quite a few thunderstorms of much less intensity also occur at various parts of the State which contribute some amount of rain of isolated nature. V.6. HUMIDITY : During the season of retreating monsoon the relative humidity decreases gradually as the season progresses from October to November due to the gradual establishment of continental air from the north. The relative humidity all over the State remains moderately high - everywhere above 67%. Fig. 5.9 shows the distribution of mean relative humidity of the season (average of 0830 and 1730 Hrs. I.S.T.). Towards the western plateau (are') the relative humidity is below 70% but to the east it is a bit higher (above 70%) due to the presence of water bodies, rivers and marshy lands. Along the coastal areas of the south and in the mountainous zone of the north the relative humidity is still higher reaching maximum at Kalimpong (81.2%). V.7. CLOUD : By the 2nd week of October monsoon leaves West Bengal. Since then there is marked decrease in the cloud cover as the season progresses. The sky becomes gradually clear with some white patches of floating cumulus clouds with occasional development of cumulonimbus., Thunders and lightning appear subsequently and the cloud gives some amount of rain in isolated localities. During cyclonic storms and depressions the sky remains overcast with low cloud which persists for days together specially in the coastal areas. The proportion of cloud cover is higher in the coastal areas which gradually decreases to the north. Low clouds almost disappear from practically the entire State only to reappear along the southern coasted belt during cyclonic storms. Cumulus

13 WEST BENGAL ' Relative Humidity Retreating Monsoon (Mean of OB3o & hrs. I.S/Q?. V v V-"' 26' LEGEND Above 80 percent below y i -* * / A ** <?. \... -r fr V )} W yt b yi "i ) S ^ V h'i j ^ V o J. \ Y-*. * 'T^^r +! +A ) /, * *..., *,/ y>v' '*i'b v ^ C 40 eo 80 toe (20 Km 86 88* so e FIG. 5.9

14 62 is commonly found to occur almost all over the State. Stratus is seen during the afternoon and evening. Cirrus and cirrocumulus are seen as the season advances. This is the season when all the major types of clouds are found. V.8. THE WEATHER CHARACTERISTICS : In early October the weather in West Bengal improves. The monsoon starts retreating from northern India in the last week of September and by the middle of October it leaves West Bengal also. The days of discomfort are soon over. The last month of the rainy season according to the Bengali calender is 'Bhadra* which lasts roughly from 15th August to 15th September. It is called "Pacha" or rotten by the Bangalees as it is very sweaty, due to high humidity coupled with rising temperature. There is an amelioration of these discomfortable conditions in October. The night starts becoming cooler as the day's heat is radiated out fast through the clear, night skies. There is much dew in the morning, which is caused by the sudden drop in temperature at night. Morning fogs are also frequent. Table 6 (shows the occurrences of different weather phenomena during this season. Fig. 5.4 shows the daily temperature, rainfall, humidity etc. for some selected stations.

15 63 TABLE - 6 WEATHER PHENOMENA - RETREATING MONSOON SEASON (Mean no. of days with) SI. No. Stations Hail Thunder Fog Dust storm Squall 1. Kalimpong Darjiling Oalpaiguri Maldah Baharampur 0* Asansol Krishnanagar Bardhaman Calcutta (Dum Dum) Calcutta (Aiipur) Medinipur Sagar Island Bagdogra Sand heads Koch Bihar Puruliya Kanthi Source : Climatic Tables of Observatories in India ( ) I.M.D

16 64 The most important feature of weather of the season is the formation of fierce cycionic storms in the Bay of Bengal and their subsequent travel across the coastal region. Details about the storms have been discussed in the Chapter VI A.