Rainfall_Key Terms

Understanding Key Meteorological Terms

Updated Date: 30 Jan 2024

The four meteorological seasons:

The Indian Meteorological Department (IMD) defines four meteorological seasons for any given year:

Winter (Jan – Feb). Winter is also called the dry season.
Pre-monsoon (Mar-May)
Southwest monsoon (Jun-Sep)
Post-monsoon (Oct-Dec). Post-monsoon is also called North-east monsoon.

Long Period Average (LPA)

LPA is the rainfall recorded over a particular region for a given interval (like a month/season/year), averaged over a long period like 30 years, 50 years, etc. Rainfall averaged over 50 years is a 50-year LPA.

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.

Long-Period Average (LPA) Vs Long-Term Mean (LTM)

LPA serves as a hydrological and climatological reference period, providing a benchmark for forecasting. The reference periods are typically 30, 50, or even 100 years. IMD defines a 50-year reference period as a benchmark and uses it for forecasting. In contrast, LTM is a straightforward mathematical mean without a benchmark status or a fixed hydrological reference period.

IMD’s Normal

The Indian Meteorological Department (IMD) defines normal using a standard 50-year LPA. The current normal is the 50-year LPA for the period 1971-2020. 50-year LPA is the benchmark that defines normal rainfall for all spatial scales and is used for forecasting.

Revision of IMD’s Normal

The normal calculation is updated once every decade. In Apr 2022, IMD released the new normal (at the India level) calculated on the basis of rainfall data over a 50-year period from 1971-2020. Any forecasting summaries in terms of departures from the normal for the decade of 2021-2030 will now be based on the 50-year LPA of 1971-2020.

As reported by ‘The Wire’: IMD Director General Mrutyunjay Mohapatra said the definition of the LPA has to be updated every decade, as per international practice. India has not been doing these revisions regularly in the past. Now that the instruments are updated, process of data reception is automated, IMD is able to update the new normal.

WMO’s Normal

The World Meteorological Organization (WMO) establishes normal using a standard 30-year LPA, with the current normal derived from the period 1991-2020.

Revision of WMO’s Normal

WMO updates normal in 30-year epochs, with past reference periods including 1901-1930, 1931-1960, and 1961-1990. For operational services like forecasting, WMO recommends the current normal based on the 30-year LPA for the period 1991-2020. For historical comparison, WMO suggests adopting the 30-year LPA for the period 1961-1990 as a fixed and common reference period to be globally adopted.