Thursday, April 05, 2012
Day temp. over S. Karnataka, S. Andhra, Tamilnadu and Kerala to be on milder side from Saturday, 7-Apr with some T.showers around.
KAAL-BAISHAKHI OR NOR’WESTER
Phenomena of curse and blessings
The dramatic appearance of nor’wester in the afternoon or evening of pre-monsoon months over Bengal, Bihar, Assam and adjoining areas has been a matter of great interest not only to the meteorologists but to all sections of people and has a bright existence in Bengali literature. Famous Bengali poet Mohit Lal Majumder described nor’wester so nicely in his poem “KalBaisakhi” that a meteorologist can hardly do. However, Nor’wester is subject of universal interest for a number of reasons. It not only gives relief after mid-day heat but pours well its crops. Its nature is unique and the causes are really interesting.
During the hot weather period i.e from March to May the eastern and North-eastern states of the subcontinent like West Bengal, Bihar, Assam, Orissa (parts) and Bangladesh experience dramatic appearance of a special type of violent thunderstorm know as Nor’wester.
In Bengal it is known as ‘KalBaisakhi’ or calamity of the month of Baisakh (April,15-May,15). Apart from its destructive effects like sudden rise in wind speed, lightning, thunder and hail the rainfall associated with the storm although small in amount, is extremely helpful for the pre-Kharif crops like jute, Aus paddy, summer till and a large number of vegetables and fruits and the sudden drop in temperature gives relief after unbearable mid-day heat.
The first sign of nor‘wester is a low bank of dark cloud in the north-west, the upper outline of which has the appearance of an arch. It approaches at first slowly and then more and more rapidly with a strong gust or squall. There is frequent thunder and lightning followed by down pour of rain and sometimes accompanied with hail, which is driven by the strong wind. On some occasions the wind blows with almost hurricane force. Nor’easter in Bengal starts as early as February. The number gradually increases reaching maximum in April and May and decreases thereafter along with the onset of monsoon. On an average the hot weather season experiences about 15 such thunder squalls (Violent wind) of which March records 3, April and May 5 to 6 each. But the frequency is highly variable from one year to the next and one region to the other. Usually these thunder squalls occur in the afternoon and evening but they are also observed at other times of the day and night. A large number of these thunder squalls strike from north-west for which they are called nor’wester. In a ‘KalBaisakhi’ wind speed from 55 to 80 km per hour is common but in some cases the speed may exceed 140 km per hour. On the approach of the squall, in general, the temperature drops and pressure rises suddenly due to descending cold air from aloft. Fall of temperature from 20-40c is very common but drop of temperature more then 120c has also been recorded. The amounts of rainfall from such squalls are highly variable. The amount varies from place to place and between individual squalls. Over the Gangetic West Bengal the amount of rainfall from a thunder squall varies from less than a millimeter to more than 80 millimeter and increases from west-north-west to east-south direction. Over North Bengal the amount increases from west to east depending on the source of moisture supply. Hail from the towering umulonimbus cloud occurs during the early part of the season and decreases gradually along with the progress of the season due to gradual rise of the freezing level as a result of increasing surface heating. All such thunder storms occur in association with cumulonimbus (CB) clouds. The bases of these clouds are as low as 450 to 600 meters from the ground surface while their tops frequently go above 12 to 15 kilometers. Duration of ‘KalBaisaki’ over a station varies from a few minutes to about an hour.
A pendant-shaped cloud resembling an elephants’ trunk some times descends downwards from the base of CB cloud and may ultimately touch the ground with a great suction-effect, tremendous pressure fall added to a violent whirl wind. These whirl winds are extremely destructive in nature and are called tornadoes, same as ‘twister’ in USA. The tornadoes have a small diameter of a few hundred meters only while the length of their track may extend to only from 25 to 35 kilometers or even less. In case of a tornado the wind speed associated with the storm is much more. It may often go as high as 300 km per hour or more. Instances are on record where heavy materials like a passenger bus had been lifted sodily from the road and thrown into an open field at a distance near Diamond Harbour on 21st March, 1969. Due to the suction effect the whole quantity of water from a pond along with the fish is known to have been lifted up and was dropped at a distance showering fish and snake along with the rainwater in Cooch Behar on 19th April of 1963. On April 15th, 1977 a tornado lashed two villages ‘Bhaitgarh’ and ‘Bagbheria near Kanthi in Purba Medinipur. At least 10 people were killed, 50 injured and a large number of cattle heads destroyed. In the evening of 12th April, 1983, about 21 villages were lashed and 27 persons were killed in the Gaighata block of North 24- arganas district due to tornado hit. In a tornado which occurred in the afternoon of 9th April, 1993, five villages were completely destroyed, 57 persons were killed, about 300 persons were injured and thousands of people rendered homeless in Kandi and Khargram Police station of Murshidabad district; on 24th March 1998 by 15-00 hours a violent tornado lashed 26 mouzas of Dantan and Egra Police Stations of Paschim and Purba Medinipur districts of West Bengal.
What causes a nor’wester?
For the occurrence of a nor’wester, the lower level moisture flow from the Bay of Bengal is essential. In the summer months the atmosphere becomes unstable due to intense insolation. On such days if there is a suitable synoptic situation such as a surface low pressure area over Jharkhand and adjoining districts of Gangetic West Bengal, the cyclonic vorticity reaching up to 1.5 kilometers above sea level with dry air aloft and presence of a jet stream between the levels 300 mb and 200 mb in the vicinity of Gangetic West Bengal are extremely helpful for the occurrence of nor’westers..
Any mechanism suggested to explain the occurrence of nor’westers in Bengal during the summer months should satisfy the following important facts associated with them:
(i) Thunderstorms approach most frequently from west to north-west and hence they are called nor’westers. The number of thunderstorms approaching from other directions is small.
(ii) The majority of nor‘westers occur between 17-00 and 21-00 hrs, IST.
(iii) Nor‘westers usually occur later over south-east than over southern West Bengal.
(iv) Nor’westers is usually more severe in south-east than in southern West Bengal.
(v) In the majority of nor’westers air pressure rises and temperature falls.
(vi) Nor’westers starts simultaneously or at different times over different places. Time sequence for some nor’westers occurring on the same day at different places is also noticed.
Meteorological studies reveal that during the summer months, there is normally moist southerly air in the lower levels over Bengal and dry westerly or northwesterly air in the upper levels. The depth of moist current is greater in the southeast than in the south west Bengal. Between the moist and dry air mass there is an isothermal region or region of contrast. This distribution of air masses gives rise to latent instability conditions over Bengal in summer months. From the upper air observation it is seen that this instability is always manifested but may exist in a latent state and this only requires a further impulse for the energy to be released sometimes with great violence. This impulse is known ‘trigger action’.
This ‘trigger’ or the impulse is supplied by one or more of the followings which help to rise the moist underlying air up to form cloud, rain and hail:
(a) Insolation-which reaches its maximum in the afternoon and cause large convectional updraught of air from the lower to the higher levels of the atmosphere. As a result of this process, the nor’wester or thunder squalls of Bengal mostly occur.
(b) Out-flow of relatively cold air in different directions from a parent thunderstorm.
(c) East ward passage of the cold front of a western disturbance generally in an occluded form.
(d) Convergence of air due to katabatic flow from the hills to the north and east. As a result of this process the thundersqualls of North Bengal i.e. Jalpaiguri and Coochbehar mostly occur during night.
(e) Inflow of moist wind from the Bay of Bengal due to the formation of a cyclonic storm.
Thunderstorms over Bengal have been classified as follows:
(i) Those which occur with the passage of a cold front in connection with the western disturbances moving east wards. In this case the nor’westers occurs along a line and one finds a time sequence. Nor’westers due to this mechanism can occur at any time of the day depending upon the time of the passage of the cold front.
(ii) Those which occur in connection with cyclonic storms or depressions in the Bay of Bengal approaching the coast. These
Thunder storms can approach a place from any direction and can occur at any time of the day.
(iii) Those which occur in the morning (after mid-night): These occur in the North Bengal plains and adjoining Assam when there is a flow of cold air from the adjoining hills. Cold air from these thunderstorms can start chains of fresh thunderstorms.
(iv) Those which occur simultaneously over a large area and generally in the
afternoon, evening or early part of night. These are associated with either the approach of the western disturbances or with the accentuation of the east west pressure gradient over Bengal. About 70% to 80% of the nor’westers are of this type. The initial trigger for the release of energy of latent instability and starting the primary or parent thunderstorm is insolation. The released heated moist air goes higher up to form thick CB or thunder cloud. The descending air is generally from higher levels i.e from regions where there is westerly to north westerly wind; being dry, it gets humidified and cooled due to evaporation of rain through which it passes and reaches the surface as cold air. The cold air rushed down with great violence gives rise to squall. Once a thunderstorm starts due to insolation acting as trigger, the cold air from primary or parent thunderstorm can spread easily in the direction of the descending current, i.e generally from north westerly to south easterly direction, in the form of fingers spreading from the palm of the hand. Secondary thunderstorms or nor’westers can start at the tip of each finger as the cold air moves forward. Tertiary, quarternary etc. thunderstorms can also start due to the same mechanism as cold air spreading from the secondary or tertiary etc. thunderstorms can act as trigger. Thus a number of thunderstorms can take place simultaneously and there will be a time sequence in the occurrence of nor’westers. Nor’westers are generally more severe in south east than in south west Bengal as the moist current is deeper there and hence more energy of latent instability can be released to increase the violence of the storm.
(v) Besides the above four types, one should also consider the thunderstorms which can start late in the night as a result of radiative cooling at the top of clouds, giving rise to sufficient instability of strong descending current.
Very specific forecasting of nor’wester well ahead is not a very easy task. Apart from the analysis of surface synoptic charts, upper air charts, other charts and diagrams x-band weather radars are capable of detecting these violent storms in their formative stage and subsequent maturity and regeneration. With judicious use of these informations received from such weather radars, the meteorologists can issue warning against such violent storms about three hours before the nor’wester hits. Using doplar radar, mobile van equipped with radar, data from automatic weather stations and satellite picture at one hour interval, attempts are being made to issue specific forecast about the time and area of occurrence of Kalbaisakhi considerably earlier.
Nor’wester and agriculture in Bengal
When success of the cultivation of Rabicrops in the State during winter depend on favourable temperature regime and abundant sunshine, cultivation of pre-kharif crops depend on adequate summer rain or nor’wester rain. Broadly speaking entire rabi cultivation is solely dependent on irrigation while pre-kharif crops largely depend on the amount and distribution of nor’wester rain in the state. During the three summer months, from March to May the state receives an average rainfall varying from 135 mm at kaliachak in the southern end to 805 mm at Buxa Duar in the north-east corner of the sub-Himalayan West Bengal and 95 mm at Para in the western part to 280 mm at Tehatta in the eastern border of the Gangetic West Bengal which constitute 7 to 17 per cent of the average annual rainfall. The main source of rainfall of the season is nor’wester. Cultivation of Boro paddy, jute, Aush paddy, summer til, ground nut, maize, summer pulses and a large number of summer vegetables and fruits depend directly upon the amount and distribution of nor’wester rain. Area production and productivity of all the summer crops has a direct and positive co-relation with the nor’wester rain. Prolonged absence of nor’wester rain is also reflected on the quality of fruits of the season like mango, lichi, jack fruit etc. The rain also helps to cover the dry and empty fields with a thick carpet of grass, the major source of food for the cattle population of rural Bengal. Mohit Lal Majumder rightly said in his poem ‘Kalbaisakhi’, it not only destroys the old but creates new and contributes a lot.
The author is agricultural meteorologist & rainfall registration authority of West Bengal.
Searched BY: Mahendra Rai, At Hitech Computers and Services, 2010
Bottom end of a trough extending from Uttarpradesh to S. tip Tamilnadu.. will give rise to a local circulation over S. Tamilnadu, S. Kerala
RT @rajugana: @weatherofindia Baroda 3.40pm, Sunny, hot, temp 40-24C range, windy at times.