vagaries© of the weather.blog written by rajesh kapadia.concentrating on meteorology of the Indian sub continent and extreme world weather since 55 years For Any Information taken from here, due credit must be given to Vagaries.
Thursday, April 16, 2020
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4th June Night: Uncertainty remains over the formation and track of the first low pressure system of Arabian Sea (AS-1)! A cyclonic circulat...
The low, has literally "whizzed" past,and thru the central regions, and by saturday night, is already over south Rajasthan/Gujarat...
Is it possible that there is a difference in the dates because the onset criteria used then were less stringent as compared with the more expansive/ definitive criteria used by the IMD today. For e.g., could it be possible that the occurrence of pre-monsoon thunderstorms served as an indicator of the monsoon's arrival in certain parts of peninsular India (Bombay for e.g.)- that could explain why the onset date is earlier and the withdrawal date is later. The only flaw with this reasoning is that it cannot be applied to KA, KL and the north-eastern states, where thunderstorms start much earlier.
Could this be because less stringent criteria were used to define monsoon onset and withdrawal at that period, because not much was known about it at that time. For e.g., could pre-/post-monsoon thunderstorms have been used to indicate the onset/ withdrawal in Bombay? This however cannot be applied to KA, KL and the northeastern states.
Is it the withdrawal dates of Bengal very unusual. Nowadays monsoon withdraws during end of September but here it’s showing almost end of October. Any specific reason for that sir??
Although not much big difference in onset dates but surely huge change in withdrawal dates
I feel it was more realistic as pre & post SWM Cyclone or TS were also covered as part of monsoon rain.
Ex: Take Mumbai here. As we all known regular Pre monsoon shower also begins in Mumbai just 4-5 days prior to SWM onset so if 10 June is onset then Pre Monsoon rains from 5th June are associated due to change in weather dynamics only as Kerala onset gives wind shear feature above 15 N which is beginning of TS period till monsoon lines climbs the latitude
Same in October as we have Withdrawal TS due to ITCZ line moving south so that was also considering as monsoon though both gives Easterlies Rain but are due to presence of monsoon dynamic.
Also Bengal withdrawal date 15-30 Oct, had wider window frame. Seems post SWM cyclone activity was also kept in picture here.. Means those year having cyclone hitting Bengal that year may be withdrawal would have been postponed
But there is variation in dates of SWM for Mumbai. See under existing column of IMD which is of period 1901-1940(pre independence period) there onset date given 10th June whereas Rajesh sir article is also of pre independence period there date given 5th June. Similarly for monsoon ending also there are variations in date. Why is that so??
I feel it would be inappropriate to compare these monsoon onset and withdrawal dates. Particularly because while comparing the current Monsoon onset and withdrawal dates and pre independence dates for the areas mentioned in the posted image, one will have to take into account the changed geographical extents of the areas mentioned. The aforesaid image mentions Bombay, Bengal, NW Province and Punjab from pre independence India, whose geographical extent was vastly different from those referred to by these names in modern India. For example pre independence Bombay Presidency had its southern extent comprising of North West Karnataka. Hence early monsoon onset dates and late withdrawal dates would correspond to probably southern border of the then Bombay Presidency in Karnataka. Similar must be the case with other provinces of Bengal, NW Province and Punjab!
good points put forward.
The pre monsoon and post monsoon showers, can they be considered as monsoon, even if monsoon parameters like 200, winds and humidity show otherwise ?
and Bombay presidency was from North Kar and included Gujarat and till Karachi...so as a whole region one date was given ? i dont think so..it was Bombay city (Now Mumbai).
Good points and discussion on broader platform
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