Lahore, the second largest city of Pakistan, is the first to receive Monsoon rains from India. The rains commence about the first week of July and continue up to the middle of September. Following is a brief sketch of the weather to be expected at Lahore during the monsoons.
When the cool and refreshing monsoon winds arrive from India, the first noticeable effect is the clear drop in air temperature. The maximum temperature that was hitherto about 100 F drops to 90 F or less. The nights record a similar drop and the nighttime low drops from about 80F to 70 F.
Out of the three months of July, August and September, July is the warmest. Augusts turns pleasant due to the fact that it is height of monsoon in this month. As for September, the first decade of it is comfortable but the second to third decade become hot and muggy; as the monsoon winds begins to withdraw and with the moisture still suspended in the air, the sunshine gets unbearable due to an increase in heat index.
Since Lahore lies at the tail end of the monsoon coverage area, therefore, the rainfall is less reliable and less in amount than that of India. Sometimes it rains for three to four days continuously while on other occasions, it may rain as a heavy downpour for an hour or two. During rain, the weather is cool and pleasant; otherwise hot and sultry.
July gets about 8 inches of rain from the monsoon, August 7 and September about 3 inches. The rain days in July are 8-9, in August 7-8 while in September they are 3-4. So we see that the rainfall as well as the rain-days drop gradually towards September.
At times the western disturbance over north of Pakistan interact with a strong current of monsoon already present at Lahore and exceptionally heavy rainfall results. That happened on August 22-24, 1996 when 17 inches of rain fell in 72 hours. To fully appreciate how great this amount is, we consult the total annual rainfall figure for Lahore. The total annual rainfall at Lahore is 23 inches. It follows, then, that in those 72 hours, three-fourths of total annual rainfall was received.
In short, the weather at Lahore during monsoon is less hot as compared to the pre-monsoon months. Most of the total annual rainfall is received through the monsoon rains. However rainfall amount received is highly variable.