The Great Monsoon Retreat
The Southwest Monsoon, which brings about 75% of India's annual rainfall, begins its withdrawal process in September. This isn't like flipping a switch; it's a gradual retreat that starts from the northwestern parts of the country around the beginning
of the month and proceeds southeastward. The India Meteorological Department (IMD) officially declares the withdrawal based on specific criteria, including a cessation of rainfall for five consecutive days and a change in wind patterns. As the moisture-laden monsoon winds weaken and retreat, the atmospheric conditions that dominated for months begin to change, setting the stage for a period of climatic flux.
What Causes the 'September Heat'?
The term 'September Heat' (sometimes called 'October Heat') refers to a noticeable rise in temperatures that occurs as the monsoon recedes. With the rain-bearing clouds disappearing, the skies become clearer. This allows for more direct and intense solar radiation to reach the ground, causing daytime temperatures to soar. In 2025, for example, Delhi experienced one of its hottest Septembers as the monsoon made an early retreat, leading to punishing daytime highs above 38°C. This heat is not a dry heat like in May or June. The retreating monsoon leaves behind significant moisture in the atmosphere and soil, which plays a crucial role in how the heat feels.
The Humidity Factor: A Sticky Situation
The combination of high heat from clear skies and high moisture leftover from the monsoon creates a deeply uncomfortable humid heat. Humidity acts like a blanket, trapping heat near the ground and preventing temperatures from cooling down effectively, especially at night. This is why even when temperatures aren't at their peak summer levels, the 'feels-like' temperature can be dangerously high. This humid heat limits the body's ability to cool itself through sweating, posing a significant health risk. The result is a punishing cycle where spells of intense, sticky heat are broken by sudden, often heavy, rainfall from lingering weather systems.
Forecast for September 2026
For September 2026, forecasts from the IMD and other agencies suggest a complex picture. The presence of strong El Niño conditions is expected to lead to below-normal rainfall for the country as a whole. El Niño is a climate pattern associated with the warming of the ocean surface in the central and eastern tropical Pacific Ocean, which often leads to weaker monsoon rainfall in India. Consequently, temperatures are predicted to be above normal for most parts of the country. However, the monsoon is not disappearing quietly. In the first week of September, a low-pressure system moving in from the Bay of Bengal is expected to bring heavy rainfall to eastern, central, and parts of northern India before conditions potentially turn drier.
A Sign of Changing Times?
While the September heat-rain balance is a natural part of the monsoon's withdrawal, climate change is making these transitions more extreme. Scientists warn that human-driven climate change is making unusual weather patterns, such as intense heatwaves and extreme rainfall events, more frequent. The duel threat of extreme humid heat during monsoon 'breaks' followed by periods of intense rain is becoming a new, challenging norm. This not only disrupts daily life but also puts immense pressure on agriculture, water resources, and public health infrastructure, forcing a rethink of how we adapt to our changing seasonal rhythms.
