Every summer, rains in India are unevenly distributed: in some places more than a meter falls in a season, in others three times less. To boil this patchwork down to a single number, meteorologists average all precipitation across the country, yielding the all-India index. For a long time, it was unclear what exactly it measures. A new study based on a 120-year observational record shows that the index rises when rains occur over a larger area and on more days, not when it pours harder in one spot.

The researchers compared the index with six other measures, such as the fraction of stations with above-normal rainfall and the average number of rainy days. It turned out that the index is tightly linked to these two: correlation coefficients above 0.9. Its link to rainfall intensity is weaker. The second most important rainfall pattern is a tripole: excess moisture over the Western Ghats and the Central Monsoon Zone, with a deficit in the southeast. This pattern is governed by the Indian Ocean Dipole, that is, the temperature difference between the western and eastern Indian Ocean. El Niño, a warm current in the Pacific, controls the first pattern by suppressing the number of rainy days. Now that the index's meaning is clear, it can serve as a reliable indicator for agriculture and insurance.