Causes of High Rainfall
Cherrapunjee receives rains from the Bay of Bengal arm of the Indian Summer Monsoon. The monsoon clouds fly unhindered over the plains of Bangladesh for about 400 km. Thereafter, they hit Khasi hills which abruptly erupt out of the plains to reach a height of about 1370 m above MSL within a short distance of 2 to 5 km. The orography of the hills with many deep valleys channels the low flying (150-300 m) moisture laden clouds from a wide area to converge over Cherrapunjee which falls in the middle of the path of this stream. The winds push the rain clouds through these gorges and up the steep slopes. The rapid ascendance of the clouds into the upper atmosphere hastens the cooling and helps vapours to condense. Most of Cherrapunjee's rain is the consequence of air being lifted as a large body of water vapour. Extremely large amount of rainfall at Cherrapunjee is perhaps the most well known feature of orographic rain in northeast India.
Occasionally, cloudbursts can occur in one part of Cherrapunjee whereas other areas may be totally or relatively dry depicting high spatial variability of rainfall. Atmospheric humidity is extremely high during the peak monsoon period.
The major part of the rainfall at Cherrapunjee can be attributed to the orographic features. When the clouds are blown over the hills from the south, they are funneled through the valley. The clouds strike Cherrapunjee 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.
A notable feature of monsoon rain at Cherrapunjee is that most of it falls during the morning hours. This could be partly due to 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 usually takes place in the vicinity of the Khasi Hills. Apparently the winds that are trapped in the valley at night begin upward ascent only after they are warmed during the day. This explain8is partially the frequency of morning rainfall. Apart from orographic features, atmospheric convection plays an important role during the monsoon and the period just preceding it.
