Monsoon winds arrive in Delhi by either the end of June or the first week of July. The arrival of moisture-laden South-Western winds, travelling from the Arabian Sea marks the onset of the humid season in Delhi. This season is marked by high levels of humidity and high heat. Day temperatures drop below 40 °C (104 °F) as humidity suddenly soars. July is marked by high heat and relatively less precipitation (as compared to August). This transition from scorching to sweltering heat between June and July makes the latter feel very uncomfortable. August is Delhi's wettest month. The heat is considerably reduced and it is relatively cooler for most of the month. There is dense cloud formation in the sky and at least a week of distinct, heavy rainfall. By September, the amount and frequency of precipitation drops, though humidity remains high. Towards the end of September, the moisture content in the air begins to fall and monsoon ends by early October. In India, the Rainy season is referred to as "Rituraani", meaning the Queen of Seasons. [Source: Wikipedia]
Don't forget to pack your umbrella ☔