2.2. Seasonal Pattern

Nearly 70% of annual rainfall occurs during the Southwest Monsoon (June–September). The remaining occurs during the post-monsoon, winter (western disturbances), and pre-monsoon (convective thunderstorms) seasons.

In India, a significant portion of rainfall, approximately 70%, occurs during the Southwest Monsoon season, which spans from June to September. This means that during these four months, the majority of rain that falls in a year is received. The rest of the rainfall is distributed throughout the other seasons, including a smaller amount in the post-monsoon season, during the winter months, when western disturbances bring rain, and in pre-monsoon months, primarily due to convective thunderstorms.

Think of the Southwest Monsoon as the big event of the year when most people gather for a festival. Just like how preparation and activities peak during the festival, most of India's rain peaks during these monsoon months. Other times of the year are like quiet days leading up to or recovering from the event, with only a few brief moments of excitement.

The southwest monsoon enters Kerala in early June and sweeps across India by July. Withdrawal starts in September.

The monsoon season in India begins when the southwest monsoon makes its entry into the southern state of Kerala around early June. Over the next few weeks, this monsoon system gradually moves northward, covering the entire country by July. The retreat of the monsoon happens in September, marking the end of this heavy rainfall period. This dynamic showcases not only the arrival of rain but also its movement across different regions over time.

Imagine the monsoon as a wave of water slowly washing up a beach. When it first hits the shore in Kerala, it represents the arrival of the monsoon, and as it continues to roll further up the beach, it symbolizes the way rain spreads across the country, reaching different areas in a series of waves before finally receding back.

The west coast (Western Ghats), northeast (Assam, Meghalaya), and Himalayan foothills receive the highest rainfall, sometimes exceeding 10,000mm in spots like Cherrapunji. Northwestern and interior peninsular regions receive much less, sometimes <500mm annually.

Rainfall in India varies significantly depending on geographical location. The Western Ghats, northeastern states like Assam and Meghalaya, and the foothills of the Himalayas are areas that receive exceptionally high amounts of rainfall, sometimes exceeding 10,000mm annually. In contrast, regions like the northwestern parts of India and some interior peninsular areas receive much lower rainfall, often less than 500mm per year. This stark difference in rainfall distribution is due to topographical factors and weather patterns that influence local climates.

Consider how different areas of a mountainous region might receive rain differently. Just as rainwater runs down a sloped landscape, pooling more in some valleys while missing others, the intense rainfall in the Western Ghats and northeastern regions is much like those valleys that receive the most water, while the higher plateaus or flatter regions receive significantly less rain.

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