The micro-climatic conditions in and around Cherrapunjee, that is estimated to cover 100 to 200 square km, has been classified by meteorologists as follows:
|Pre-monsoon months||March, April, May|
|Monsoon months||June, July, August, September|
|Post-monsoon months||October, November, December|
|Winter months||January, February|
Cherrapunjee gets the Bay of Bengal arm of the Indian Summer Monsoon. The monsoon clouds fly unhindered over the plains of Bangladesh after they cross over to land. After about 400 km of traveling over land they are confronted by fortress wall like Khasi hills, which abruptly erupt out of the plains to reach a height of about 4500 feet above MSL within a short distance of 2 to 5 km. The orography of the hills with many deep valleys, acting like funnels to channel the low flying moisture laden clouds (500-1000 feet) helps clouds from a wide area to converge over Cherrapunjee. Cherrapunjee falls directly on the middle of the path of this stream of southwest monsoon. The monsoon 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 water vapour to condense into water. Most of Cherrapunjee’s rain is the consequence of air being lifted as a large body of water vapour.
In Cherrapunjee you get to see rain coming in a slant with such velocity that one can be drenched in no time. Occasionally cloudbursts can occur in one part of Cherrapunjee whereas other areas may be totally or relatively dry. Atmospheric humidity is so high during the peak monsoon period that water vapour condenses into water droplets on the hair, eyebrows and moustache. The high humidity during the monsoon months causes clothes, books etc. to become damp and get molded. Books come off their binding. Clothes get black spots due to moulds. Films get stuck to each other. Moisture creeps into framed photographs and soon the photos start fading from the corners. It becomes difficult to dry clothes due to lack of sunshine. At present only a few cloth dryers are around here. Common people put a charcoal fire under an upturned bamboo basket (called polo) and spread the damp and wet clothes on the top of the basket to dry.
An analysis of the rainfall data for the last 25 years reveals that the first pre-monsoon showers arrive at Cherrapunjee by 10th March every year. These pre-monsoon showers are called Nor’Westers because they appear to come from the northwest direction. The gap between rainy days start shrinking as one proceeds from March to May. The days preceding rains progressively become warmer and the temperature takes a dip after the showers. By May the rains become frequent. These pre-monsoon showers are a severe type of thunderstorm that occurs around this region, accompanied by strong squalls. They are called ‘Kal Baisakhi’ in Bengal and ‘Bordoi shila’ in Assam. They are mainly experienced during the hot months from March to May, but they are sometimes experienced in the latter half of February and in the first half of June. They cease to occur after the Southwest monsoon sets in by June.
June and July can have continuous spells of rain lasting a few days to a couple of weeks or more. Sometimes you cannot see the sun for a week or more. However, in these two months one can have continuously no rain days for a week too. At Cherrapunjee one can have a few days every year having more than 300 mm of rainfall in 24-hour period. Sometimes it rains upwards of 500 mm. That is to say that quite a few days every year at Cherrapunjee get 50% to 80% of the annual rainfall of London or Paris in just 24 hours. In 1995, it rained 1563 mm on 16th June. The internationally accepted rainfall-reporting format in four digits including one decimal digit proved inadequate on that day. On 19th July 2004 it rained 793.2 mm in 24 hours.