Every summer, the same scene unfolds over India: clouds driven from the ocean by the wind release their rain, and how that rain falls on the fields determines the harvest for more than a billion people. Meteorologists have long tried to reduce this mosaic to a single number, the all-India rainfall index, which averages millimeters across the country. Critics have said that such averaging erases all differences, but a new study based on 120 years of observations shows that the index actually captures something important: not the intensity of the downpours, but the area they cover.

The authors took daily data from 1901 to 2020 on a 0.25-degree grid, gathered from about 2600 rain gauges each year. For each summer, they calculated not only the total rainfall but also the number of rainy days (threshold 1 mm per day) and the average rain intensity. Then they built six alternative metrics: the fraction of grid points with excess moisture, the average number of rainy days, the average intensity, and others. It turned out that the all-India index correlates more strongly with the fraction of wet points and the number of rainy days than with intensity: correlation coefficients above 0.9 versus at most 0.77. In other words, the index indeed reflects the spatial extent of anomalies, not their strength.

The second most important pattern of rainfall variability is a tripole: excess moisture over the Western Ghats and the Central Monsoon Zone, with a deficit in the southeast. This pattern is linked to the Indian Ocean Dipole, that is, the difference in sea surface temperatures between the western and eastern Indian Ocean. The dipole has little direct effect on the overall index, but after removing the El Niño signal, its link to the index becomes significant. So an old tool, properly understood, reveals two hands that control Indian rainfall.

The study resolves three long-standing debates. First, the all-India index does reflect the area of wet anomalies, which explains its high correlation with crop yields. Second, the use of coarse grids in the past distorted the picture: with detailed data, it is clear that the Western Ghats and the Central zone vary in concert, not independently. Third, the Indian Ocean Dipole does not ignore the overall index; it acts through the tripole, and its contribution emerges after accounting for El Niño. Now that the meaning of the index is clear, it can serve as a reliable indicator of the spatial extent of drought or excess moisture for agriculture and insurance. The mosaic of rains, assembled into a single pattern, has finally begun to speak in an understandable language.