The Great Retreat of the Southwest Monsoon
The primary reason for October's varied weather is the withdrawal of the Southwest Monsoon. This four-month-long season, which brings the bulk of India's annual rainfall, doesn't just switch off. Instead, it undertakes a slow, staggered retreat that can
take over six weeks. The process typically begins in early September from the northwestern parts of the country, like Rajasthan, and gradually moves southeast. By the beginning of October, the monsoon has usually exited the northern plains, leaving behind a completely changed weather pattern. This withdrawal is declared by meteorological authorities based on criteria like five continuous days of no rain, a drop in humidity, and a change in wind patterns.
Pleasant Days and Cool Nights in the North
As the monsoon clouds dissipate from North India, the region experiences clear skies and a significant drop in humidity. This leads to one of the most pleasant times of the year in states like Punjab, Haryana, Rajasthan, and Delhi. Daytime temperatures are warm and sunny, often hovering around 30-34°C, while nights become noticeably cool and comfortable. This is also the period when the first Western Disturbances of the season can begin to affect the Himalayan region. These are extratropical storms originating in the Mediterranean that can bring early snowfall to the high mountains and a further chill to the air in the plains.
The 'October Heat' in Central and West India
While the north cools, a different phenomenon known as 'October Heat' takes hold in parts of western and central India, including Maharashtra and Gujarat. As the monsoon retreats, the skies clear, leading to intense sunshine. However, the ground still holds a lot of moisture from the recent rains. This combination of high temperatures and high humidity creates a sultry, uncomfortable climate that feels like a second summer, often more oppressive than the actual summer months due to the stickiness in the air. This period of heat typically lasts until cooler, drier winter air properly sets in around mid-November.
A Second Rainy Season for the South
In a complete contrast to the rest of the country, southern peninsular India, particularly the southeastern coast, braces for more rain in October. The withdrawal of the Southwest Monsoon paves the way for the onset of the Northeast Monsoon. This system picks up moisture from the Bay of Bengal and brings significant rainfall to Tamil Nadu, Puducherry, parts of Andhra Pradesh, and Kerala. For Tamil Nadu, this is the primary rainy season, accounting for nearly half of its annual rainfall. The normal onset date for this 'retreating monsoon' is around October 20, turning the weather story in the south on its head compared to the drying trend elsewhere.
