Monsoons are not just about rain, they are a dramatic seasonal shift in wind patterns that bring about distinct wet and dry periods.
The word monsoon originates from the Arabic word mausim, meaning “season”, a fitting name for a phenomenon that defines the rhythm of life across vast regions of the globe, particularly in tropical and subtropical regions.
A monsoon climate is marked by a significant reversal in prevailing wind direction, which leads to a stark contrast between dry and rainy seasons. This shift is most pronounced in South and Southeast Asia, where the summer monsoon delivers torrential rains that are vital for agriculture, ecosystems, and water supply. In some areas, such as parts of India, up to 90% of the annual rainfall occurs during the summer monsoon.
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What defines a monsoon?
The science behind monsoons is rooted in basic thermodynamics. Land heats up faster than water. As the sun intensifies over the Indian subcontinent between March and May, land temperatures soar, often reaching the high 40s or even low 50s Celsius. Meanwhile, the surrounding seas remain relatively cool.
This temperature contrast causes warm air over land to rise, creating a low-pressure zone that draws in cooler, moisture-laden air from the Indian Ocean. This incoming air, known as the southwest monsoon, brings the much-anticipated rains.
The monsoon typically begins in southwestern India around late May or early June and progresses north-westward, reaching the northernmost regions by mid-July. However, the Himalayan mountains act as a natural barrier, halting its advance and causing intense rainfall in the northeastern foothills. Mawsynram, located in this region, is one of the wettest places on Earth, receiving nearly 12 metres of rain annually, compared to around one metre in the UK.
Interestingly, in contrast to much of South Asia, it is the northeast monsoon, not the southwest, that brings the primary wet season to southeastern India (including Tamil Nadu) and northeastern Sri Lanka.
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Do monsoons occur anywhere else?
While the Asian monsoon is the most well-known, similar patterns occur in northern Australia, parts of Africa, and the Americas. These systems, though less intense, follow the same principle of seasonal wind reversal.
As the sun weakens in September and the land cools, the winds shift again, marking the end of the wet season and the return of dry conditions. Though essential for life, monsoons can also bring challenges, including flooding and disruption. Understanding them is key to adapting to their powerful, seasonal rhythm.
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