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The India Meteorological Department has revised Delhi's August rainfall figures after identifying an entry error in the data recorded for August 6.

The rainfall recorded for that day was initially listed as 56 mm but was later corrected to 18.7 mm. Following the revision, the cumulative rainfall at Safdarjung, Delhi's base weather station, stands at 197.2 mm for August so far.

Earlier, the IMD website had shown cumulative rainfall of 230.5 mm, which suggested that Delhi had already exceeded its August rainfall normal within the first eight days of the month.

According to the IMD's 1991-2020 long-period average, Safdarjung normally receives 226.8 mm of rainfall in August. With the revised figure of 197.2 mm, Delhi is currently 29.6 mm below its monthly normal, having received around 87% of the expected rainfall.

IMD Director Dr Mrutyunjay Mohapatra said the discrepancy appeared to have resulted from a human error in data logging, and the exact reason was being examined.

D R Pattanaik, Scientist G at IMD, explained that 56 mm had been entered instead of the correct 18.7 mm figure for August 6. He said the error affected the cumulative figure displayed on the website, while the daily Delhi weather bulletin had continued to carry the correct rainfall figure.

IMD senior scientist Krishna Mishra said Safdarjung is a manual observatory where meteorological observations are recorded and logged by observers. An incorrect value entered into the system was subsequently included in the cumulative calculation.

How Does IMD Record Weather Data?

Safdarjung is a manual meteorological observatory, although its instruments automatically transmit observations to a receiving centre.

Several weather parameters, including rainfall, temperature, humidity, sunshine duration, wind and cloud cover, are recorded manually by observers. The observations are then sent to the designated Meteorological Centre or Regional Meteorological Centre, where they are scrutinised and entered into standard digital formats.

The information is subsequently fed into the IMD database. Manual and automatic observations follow separate routes before eventually reaching the central database, where quality control, consistency checks and data processing are carried out.

Errors can occur at different stages, including observation, measurement, transmission and data entry. Readings considered suspicious are flagged for further verification instead of being automatically removed.