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LATEST NEWS UPDATES | Weather babu, you can't say it 'may' rain -GS Mudur

Weather babu, you can't say it 'may' rain -GS Mudur

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published Published on Jan 8, 2016   modified Modified on Jan 8, 2016
-The Telegraph

New Delhi: The national weather agency has adopted a new rulebook, tweaking figures that define rain conditions, cold and heat waves and abandoning what it has conceded were ambiguous and unhelpful terminology such as "could" and "may".

A forecasting circular issued by the Indian Meteorological Department (IMD) has altered in subtle ways the definitions of cold and heat waves (see chart), introducing uniform cut-offs for locations across the country, and prescribed revised terminology to classify abnormal all-India rainfall.

"We've tried to reduce ambiguity as much as possible and eliminate vague terminology," Madhavan Rajeevan, a senior weather scientist who chaired a scientific panel that tweaked the rulebook, told The Telegraph. "We've also introduced new criteria to classify rain, very hot or very cold weather based on greater scientific analysis of weather patterns over the past several decades," said Rajeevan, who is now secretary in the earth sciences ministry.

The IMD will no longer issue ambiguous forecasts such as rainfall expected in "one or two places" across a state. "To say it will rain over one or two places in, say Bihar or Tamil Nadu, is meaningless - we now have forecast models for district-level forecasts," said Rajeevan. "We want to specify locations of rainfall at least to the district level."

Weather forecasters have been directed to convey "maximum, meaningful, and useful information in local forecasts" to convey the weather situation over the next 24 hours. They can no longer issue forecasts for rain or other weather conditions through words such as "could" and "may".

Such terminology may lead to problems in interpretation and translation into local languages, the circular has said and has directed forecasters to adopt a new set of words - "unlikely", "likely", "very likely" and "most likely" -- that quantitatively express the probability of the weather events.

♦ An event will be classified as "unlikely" when the probability of its occurrence is less than 25 per cent,

♦  "Likely" when the probability is between 25 per cent and 50 per cent,

♦  "Very likely" when the probability ranges from 51 per cent to 75 per cent and

♦  "Most likely" when the probability is 75 per cent or greater.

One key change is to abandon the use of the term "drought" when describing all-India seasonal monsoon rainfall departures from the normal.

"The entire country has never faced a drought," a senior IMD scientist in Pune said. "Even when large parts of the country experience poor monsoon, some parts get normal or even excess rainfall."

The new criteria classify all-India rainfall into five categories: normal (plus or minus 10 per cent of the long period average) below normal (rainfall lower than 10 per cent below average), above normal (rainfall greater than 10 per cent above average), deficient year (rainfall deficit between 10 and 20 per cent up to 40 per cent of India's spatial area) and large deficient year (rainfall deficit of over 10 per cent across more than 40 per cent of India's area).

The temperature cut-offs for heat waves have been standardised across India.

The earlier criteria for heat and cold waves depended on the normal maximum and normal minimum temperatures of the locations - with two sets of criteria for locations where maximum normal is below or above 40°C and where minimum normal is below or above 10°C.

"The new forecasting terminology based on departures from normal brings standardised criteria for heat waves in all locations," said the scientist at IMD Pune. The criterion to define extremely heavy rainfall has been reduced from 24.5cm to 20cm.

The rules also change criteria to classify seasonal rainfall. Instead of the old four rainfall categories (excess, normal, deficient, and scanty), the IMD has introduced six categories - 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; and no rain: (0).

The Telegraph, 8 January, 2016, http://www.telegraphindia.com/1160108/jsp/frontpage/story_62778.jsp#.Vo8nqVI1t_k


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