Weather reports are often full of jargon that does not make sense and probabilities which, to a layman, does not give a clear picture of a forecast.
For instance, a ‘will’ and ‘would’ in a rain forecast may not mean much to the uninitiated but, mathematically, they are quite different when it comes to the percentage chances of rain. This is one of the reasons why weather watchers are so sceptical about the met forecasts.
To understand these scientific terms correctly one has to look into the Indian Meteorological Department’s glossary. Each of the terms denotes the various degrees of probability of the occurrence of weather events.
“Information has no value if we fail to reach the user. Also, the value is reduced if it is not conveyed in the true sense of the term. Therefore, in the IMD’s parlance we have very well defined terminologies indicating the area over which the weather event would occur (spatial distribution), when it is likely to occur (temporal distribution) and the intensity with which it would hit,” said Laxman Singh Rathore, former director general of the Indian Meteorological Department.
Decoding weather report
How much cloudy is a partly cloudy sky? How much rain is intermittent? What are the actual chances of rain when it says rain ‘could occur’? Read on to find the answers
Take for example the regional weather forecasting centre’s daily rain forecast. Sometimes the forecast states that rain ‘could’ occur and sometimes it says that rain ‘would’ occur. So what’s the difference?
“While ‘could’ occur means that there is less than 25% chance of rain, may occur means the chance of rain is 26% to 50%. If the forecaster says rain ‘would’ occur then he means there is 51% to 75% chance of rain.
‘Will’ occur has the highest probability of rainfall with 76% to 100%,” said an official of the local met department.
Same goes with the spatial distribution of the rainfall. If the IMD says ‘isolated’ rain, it means you need to be very lucky to enjoy that rain, because less than one-fourth of Delhi would get it. As per the glossary maximum areas receive rain when it says ‘widespread’.
“These are standard terminologies, which are applicable across India and to all the regional weather forecasting centres across India. They are updated and revised from time to time. The latest glossary was revised in 2015 by a committee set up by the IMD,” said M Mohapatra, who heads the forecasting services at the IMD.
So the next time the forecast says that it ‘could’ rain and you carry an umbrella in your bag before leaving home, then chances are very high that you might not have to take it out.
Source: HindustanTimes