Perhaps the most well known feature of orographic rain in northeast India is the extremely large amount of rainfall at Cherrapunjee. Cherrapunjee is located at 25º 15′ N, 91º 44′ E and Mawsynram at 25º 18′ N, 91º 35′ E. The rainfall at Cherrapunjee is believed to be the highest ever recorded in the world, although some claim that the record should now go to another nearby village Mawsynram. The Mawsynram records are not so reliable as those of Cherrapunjee. The figures for Mawsynram are still being disputed.
Both Cherrapunjee and Mawsynram are located on the southern slopes of the Khasi Hills, the mean height of which is 1.5 Km. There is little doubt that the major part of the rainfall recorded at these two places can be attributed to the orographic features. Whereas Cherrapunjee is located at the northern end of a deep valley running from the south to the north, Mawsynram is at the top of the hill in the middle of the valley. One notable difference between the orographical features of Cherrapunjee and Mawsynram is that whereas Cherrapunjee is situated on the confluence of many gorges converging on it, Mawsynram has no such feature. Mawsynram is located in the top middle of a range parallel to Cherrapunjee’s. To its south between the Bangladesh plains and the valley is Laitkynsew Hill. This valley runs from east to west. When the monsoon winds blow from the south, they hit Laitkynsew hill, which has an elevation of 3000 to 3700 feet and then Mawsynram. The clouds also are channeled through the valley and pushed up the slopes of Mawsynram. As for Cherrapunjee when the clouds are blown over the hills from the south they are funneled through the valley between Laitkynsew hill and Mawsynram hill, through the Umwai valley, the Wahlong valley, the Mawsmai valley, and the Kutmadan valley. The clouds do strike Cherrapunjee or Mawsynram in a perpendicular direction and the low flying clouds are pushed up the steep slopes. It is not surprising to find that the heaviest rainfalls occur when the winds blow directly on the Khasi Hills. Most of Cherrapunjee’s rain is the consequence of air being lifted as a large body of water vapour.
A curious feature of monsoon rain at Cherrapunjee is that most of it falls during the morning hours of the day. It has been suggested that this is partly caused by two different air masses coming together. During the monsoon months the prevailing winds along the Brahmaputra valley generally blow from the east or the northeast. On the other hand, the winds over Meghalaya are from the south. The confluence of these two winds systems is usually located in the vicinity of the Khasi Hills, but why should this particular feature result in more rain in the morning hours is not well understood. It seems likely that the winds that are trapped in the valley at night begin their upward ascent only after they are warmed during the day. This explains partially the observed preference for morning rainfall. Apart from orographic features, atmospheric convection plays an important role during the monsoon and the period just preceding it.
In the pre-monsoon months of April and May for example, parts of northeast India, especially Meghalaya, Assam, West Bengal and Bihar experience severe pre-monsoon thunderstorms. They are given the picturesque name of ‘Nor’Westers’ because they appear to come from a northwesterly direction. In Bengal., they are known as a ‘Kal Baisakhi’, meaning a mass of dark clouds in the month of Baisakh. In Assam, it is known as ‘Bordoi Sila’. The rainfall associated with these thunderstorms is of a transient nature. The intensity of precipitation is high – often as much as 5 cm (50mm) of rain are recorded in one hour – but the rainfall is of short duration. Monsoon rain is of a different genre. It is in the nature of continuous rain spread over days, and the intensity of precipitation is not as high as that of convective rain. ‘But there are occasions of ‘cloud bursts’ within a spell of monsoon rain. As mentioned earlier, on the 16th June 1995, Cherrapunjee recorded the highest rainfall received in one day (24 hour period) aggregating 1563 mm.
Excerpt from “The Monsoons” by Dr.P.K.Das, Pages 67-71 (besides local observations.) Published by National Book Trust, India. Price Rs.75.00.