A Monsoon is a moisture laden wind that brings dark clouds, heavy rain and cool breeze. It is welcomed in South and South East Asia as the months preceding it are very hot; especially on the places situated in the Indian Subcontinent. A Monsoon is a mixed blessing. On one hand, it brings relief from the oppressive heat of the Indian summer but on the on the other hand it raises the heat index to uncomfortable levels due an usual rise in relative humidity it brings along with it.
For the present case, we will discuss the Indian monsoon where the effects of discomfort can more easily be noticed than anywhere else in the world. If we take a general look on the air temperature prevailing in the North west and North Central Indian plains in the months preceding the monsoon, we find out that mean maximum temperature figures between 105-115 F are generally the norm. If we glance over the figures of the relative humidity, it is observed that stations located in these areas record 15 percent or less relative humidity in the afternoons. Now as human body cools itself with evaporation and drier the air, the rapid the evaporation rate. Consequently such high air temperatures are bearable because the apparent temperature or heat index is not more than 104 F anywhere in North Central India.
When the monsoon comes, there is a sudden drop in temperature but a corresponding surge in relative humidity. The temperatures typically drop 10 F and are around 95 F in central India but the relative humidity is typically 60 percent in the afternoon; when the monsoon is fully established. The combined effect causes the apparent temperature or heat index to cross 120 F. This means a rise, not less than 15 degrees in heat index upon arrival of a monsoon. The air becomes warm and humid. The rate of evaporation from human body drops considerably and the people begun to feel suffocated. Indeed if proper ventilation or artificial measures like air conditioners are not used, the chances of heat stroke combined with extreme exhaustion are very high.
The hill stations of Northern India are, however, very lucky in this regard, Due to their height, the temperature, on arrival of a monsoon drops to such a low level that heat index does not rise to uncomfortable levels. That is why the summer seasons at the hill stations all over India ends upon arrival of monsoon and very pleasant weather prevails over these stations.
In short, a monsoon is a mixed blessing. On one hand it brings cool air and much awaited rain but on the other hand it raises the heat index to dangerous levels. Only hill stations far north are spared from this phenomenon due to their high elevation which makes them safe from the harmful effects of the heat index.