LONG-PERIOD AVERAGE (LPA)

Rainfall in the four-month June-September southwest monsoon season was in the normal range in 2019, 2020, and 2021 as well

2. About Long Period Average (LPA)

• The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA)
• According to the IMD, the “LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30 years, 50 years, etc”
• The IMD’s prediction of a normal monsoon on Thursday was based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average
• The IMD has in the past calculated the LPA at 88 cm for the 1961-2010 period, and at 89 cm for the period 1951-2000
• While this quantitative benchmark refers to the average rainfall recorded from June to September for the entire country, the amount of rain that falls every year varies from region to region and from month to month
• Therefore, along with the countrywide figure, the IMD also maintains LPAs for every meteorological region of the country  this number ranges from around 61 cm for the drier Northwest India to more than 143 cm for the wetter East and Northeast India
• LPA of the southwest monsoon rainfall over Kerala: 556 mm, 659 mm, 427 mm and 252 mm for the months of June, July, August, and September respectively
• Broken down monthwise for the entire country, the LPA figures for the season are 16.36 cm for June, 28.92 cm for July, 26.13 cm for August, and 17.34 cm for September

3. Need of LPA

• The IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations
• Because annual rainfall can vary greatly not just from region to region and from month to month, but also from year to year within a particular region or month, an LPA is needed to smooth out trends so that a reasonably accurate prediction can be made
• A 50-year LPA covers for large variations in either direction caused by freak years of unusually high or low rainfall (as a result of events such as El Nino or La Nina)
• As well as for the periodic drought years and the increasingly common extreme weather events caused by climate change

4.IMD Calculations

Quantitatively, the monsoon seasonal (June to September) rainfall is likely to be 99% of the LPA with a model error of ± 5%. The LPA of the season rainfall over the country as a whole for the period 1971-2020 is 87 cm

The IMD maintains five rainfall distribution categories on an all-India scale. These are:

• Normal or near normal, when the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA
• Below normal, when departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA
• Above normal, when actual rainfall is 104-110% of LPA
• Deficient, when departure of actual rainfall is less than 90% of LPA
• Excess, when the departure of actual rainfall is more than 110% of LPA

• The beginnings of meteorology in India can be traced to ancient times. Early philosophical writings of the 3000 B.C. era, such as the Upanishadas, contain serious discussion about the processes of cloud formation and rain and the seasonal cycles caused by the movement of earth round the sun
• Varahamihira's classical work, the Brihatsamhita, written around 500 A.D., provides a clear evidence that a deep knowledge of atmospheric processes existed even in those times
• It was understood that rains come from the sun (Adityat Jayate Vrishti) and that good rainfall in the rainy season was the key to bountiful agriculture and food for the people
• Kautilya's Arthashastra contains records of scientific measurements of rainfall and its application to the country's revenue and relief work. Kalidasa in his epic, 'Meghdoot', written around the seventh century, even mentions the date of onset of the monsoon over central India and traces the path of the monsoon clouds
• Meteorology, as we perceive it now, may be said to have had its firm scientific foundation in the 17th century after the invention of the thermometer and the barometer and the formulation of laws governing the behaviour of atmospheric gases
• It was in 1636 that Halley, a British scientist, published his treatise on the Indian summer monsoon, which he attributed to a seasonal reversal of winds due to the differential heating of the Asian land mass and the Indian Ocean
• India is fortunate to have some of the oldest meteorological observatories of the world. The British East India Company established several such stations, for example, those at Calcutta in 1785 and Madras (now Chennai) in 1796 for studying the weather and climate of India
• The Asiatic Society of Bengal founded in 1784 at Calcutta, and in 1804 at Bombay (now Mumbai), promoted scientific studies in meteorology in India
• A disastrous tropical cyclone struck Calcutta in 1864 and this was followed by failures of the monsoon rains in 1866 and 1871
• In the year 1875, the Government of India established the India Meteorological Department, bringing all meteorological work in the country under a central authority
• The first Director General of Observatories was Sir John Eliot who was appointed in May 1889 at Calcutta headquarters
• The headquarters of IMD were later shifted to Shimla, then to Poona (now Pune) and finally to New Delhi
• From a modest beginning in 1875, IMD has progressively expanded its infrastructure for meteorological observations, communications, forecasting and weather services and it has achieved a parallel scientific growth
• IMD has always used contemporary technology. In the telegraph age, it made extensive use of weather telegrams for collecting observational data and sending warnings
• Later IMD became the first organisation in India to have a message-switching computer for supporting its global data exchange.
• One of the first few electronic computers introduced in the country was provided to IMD for scientific applications in meteorology
• India was the first developing country in the world to have its own geostationary satellite, INSAT, for continuous weather monitoring of this part of the globe and particularly for cyclone warning
• IMD has continuously ventured into new areas of application and service and steadily built upon its infrastructure in its history of 140 years
• It has simultaneously nurtured the growth of meteorology and atmospheric science in India. Today, meteorology in India is poised at the threshold of an exciting future