IMD changes language in weather forecasting

The India Meteorological Department (IMD) has changed the languages and also re-defined terms that are not scientifically precise in weather forecasting.
In this regard, IMD has issued official notification to do away with outdated language to avoid communication gap with the civil administration so they can be more realistic in case of upcoming weather events.
IMD has reviewed the terminology and factors determining each of these terms based on supporting observational data.
Following are the changes
Classification all-India rainfall into five categories:

  • Normal: ± 10 per cent of the long period average (lpa).
  • Below normal: Rainfall lower than 10 per cent below average of the lpa.
  • Above normal: Rainfall greater than 10 per cent above average of the lpa.
  • Deficient year: Rainfall deficit between 10 and 20 per cent up to 40 per cent of India’s spatial area.
  • Large deficient year: Rainfall deficit of over 10 per cent across more than 40 per cent of India’s area.

Criteria to classify seasonal rainfall

  • The rules have been changed to classify seasonal rainfall. IMD has introduced 6 new categories replacing old four rainfall categories (excess, normal, deficient and scanty). New categories are
  • Large excess: 60 per cent and above.
  • Excess: between 20 per cent and 59 per cent.
  • Normal: minus 19 per cent to plus 19 per cent;
  • Deficient: minus 20 per cent to minus 59 per cent.
  • Large deficient: below 60 per cent
  • No Rain: 0 per cent.

IMD also has standardized key terms

  • Heat wave: Temperatures greater than 4.50C above usual temperatures for the region.
  • Severe heat wave: Temperatures greater than or equal to 470C.
  • Cold wave: Temperatures less than 4.50C above usual temperatures for the region.
  • Severe cold wave: Minimum temperature is 20C or lower.



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