IMD changes the terminologies of weather forecasting

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In a major departure, India Meteorological Department (IMD) has changed the terminologies of it's glossary and  re-defined terms that are not scientifically precise in weather forecasting.IMD has issued official notification in this regard to dump the use of outdated term that widens the communication gap with the civil administration. The use of  more realistic and precise term would ensure better understand the terms used in weather forecasts. IMD had formed a committee to look into issues relating to its glossary and it suggested a host of changes. 

Need for change in old terminiologies :

In recent years, Numerical Weather Prediction models have improved IMD’s ability to predict weather and rainfall across various regions and forecasters also have better access to Doppler Weather Radar, satellite and other data to help with decision-making so forecasters need  to take advantage of all these developments and improvements so that more useful information is included in the forecasts with more clarity

Drought is not a simplistic subject. IMD can define a meteorological drought, but agricultural and hydrological droughts are different so the change terminiologies acn help people better understand the terms used in weather forecasts.The key changes are as follows :

The key changes are as follows
New Terminology     Old Terminiology Description
Normal     Normal 

Percentage  departure of realized rainfall is within plus minus 10% of the Long Period Average (lpa).

Below Normal  Below Normal Percentage  departure of realized rainfall is < 10% of the Long Period Average.
Above Normal Above Normal Percentage  departure of realized rainfall is > 10%  of the Long Period Average.
Deficient year All India Drought Year When the rainfall deficency is more than 10% and 20-40 % area of the country is under drough condition.
Large Deficient year  All India Severe Drought Year  When the rainfall deficency is more than 10% and when the spatial coverage of drought is more than 40%.

Classification of  seasonal rainfall :

The recent modification have changed the classification of seasonal rainfall. IMD has introfuced six new categories to replace the four old rainfall categories i.e, excess, normal, deficient and scanty. New categories are as follows :

  1. Large excess: 60 per cent and above.
  2. Excess: between 20 per cent and 59 per cent. 
  3. Normal: minus 19 per cent to plus 19 per cent; 
  4. Deficient: minus 20 per cent to minus 59 per cent.
  5. Large deficient: below 60 per cent No Rain: 0 per cent.

Standardized key terms :

  1. Heat wave: When the temperature is greater than 4.50 C above usual temperatures for the particular region.  
  2. Severe heat wave: When the temperature greater than or equal to 470C. 
  3. Cold wave: When the temperature is less than 4.50C above usual temperatures for the region. 
  4. Severe cold wave: When minimum temperature is 20C or lower.
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