How does one compute the average rainfall of India? Obviously, by averaging the measurements of all raingauges in the country. The process is, however, not so simple and straightforward as it may appear. One reason is that India’s raingauge network is not equally spaced. Another reason is that the rainfall is not evenly distributed across the country. Particularly, rainfall is much higher over the mountains than on the plains. Hence, individual rainfall measurements are representative of smaller or larger areas depending upon where the raingauge is located.

The countrywide averaging of rainfall over India is, therefore, done by multiplying each rainfall value by a weight that represents the area, and an area-weighted average is obtained. Otherwise, if just a simple average be calculated, localized heavy rainfall values, like say over Cherrapunji, would make it unrealistically high.

Way back in 1886, Henry F. Blanford, the first Head of the India Meteorological Department, derived the annual average rainfall of India using a most ingenious method. He used a copy of a large rainfall chart of India on the scale of 1 inch: 64 miles. He then cut it into 25 sections, for the most part following the boundary lines of the rainfall increments. Each fragment of paper was then actually weighed on a sensitive chemical balance. Its area was computed from the weight by comparison with that of a standard area on the chart. The average rainfall in each fragment was computed from the rainfall of all stations within it. The weighted average of the annual Indian rainfall was then obtained by multiplying the weights of the pieces of paper and the rainfall, and then dividing the sum of the products by the sum of the weights of all the pieces. Blanford’s average annual rainfall value turned out to be 42 inches or 106.7 cm for the Indian region including Assam but excluding the Himalayas and the Burmese peninsula. (Reference: The Rainfall of India, Indian Meteorological Memoirs, vol. III, 1886, 658 pp)