The weather during a monsoon in Islamabad is of mixed nature. When it rains, it is cool and pleasant; otherwise hot and sultry.
The monsoon season sets in Islamabad, normally, in the first week of July and continues up to the middle of September. The cool winds bring dark clouds along with them and in the onset dates, it rains heavily in the city accompanied by cool, pleasant wind. The temperatures drop 10-15 degrees F during a strong wet spell.
After an initial period of heavy rain for a week or two, the rain suddenly stops and for several days there is no rain at all. The weather becomes hot and sultry and the heat index rises sharply. This is called break in the monsoon.
Since Islamabad is located at the tail end of the monsoon, coming from India, so these breaks can, at times be quite long. The humans perspire profusely and lack of any wind increases the torture. After about a week or two of this trying weather, monsoon rains resume with normal intensity and duration.
Another peculiar thing about the monsoon rain of Islamabad is that since it is located pretty close to the outer Himalayan stations like Murree, the monsoon winds exert full force on the hills and since they cannot cross the high ranges so condensation takes place giving copious rainfall. In Islamabad, in recent years, as much as 48 inches of rain has been recorded in 24 hours; thus causing urban flooding.
The months of July and August get the heaviest rain so much so that the average rain of these months even exceeds that of Murree in these months. Each of these months at Islamabad get 14-15 inches of rainfall which is quite extraordinary in a country like Pakistan where 70 percent of the area of the country gets less than 20 inches of rainfall annually.
The weather of the months of July and August is tolerable to some extent but September is unbearably muggy. With increased humidity and less rain than the preceding months, September at Islamabad is, perhaps, the worst month of the whole year.
In short Islamabad gets heavy rain from the monsoon system being located in close proximity to the outer Himalayas. The months of July and August although humid are less trying than September which is worst of all. At times exceptionally heavy rain, in excess of 40 inches, is recorded in a single day which is harmful both for the humans as well as for the flora and fauna.