Monthly Rainfall Index Calculator (mm)

Monthly Rainfall Index (mm)

The monthly rainfall index is just the total depth of precipitation a calendar month brings, written in millimetres. You get it by adding up the daily totals: P_month = Σ P_day. To say whether a month came in below, within, or above normal, you compare that figure against the climatological normal — INMET currently publishes the 1991–2020 series, which replaced the older 1961–1990 and 1981–2010 baselines.

One metric that comes up a lot is the rainfall anomaly A = (P_obs − P_normal) / P_normal × 100%. Drop below −30% and you are looking at meteorological drought; climb past +30% and the month was wetter than usual. Across Brazil, monthly totals swing wildly, from under 5 mm in the Caatinga dry season to more than 400 mm when the Amazon hits its wet season.

Applications

It shows up in farm planning, where it shapes irrigation schedules and crop calendars, and in sizing reservoirs and dams. Hydrological balance studies rely on it too, as does drought monitoring such as the ANA Monitor de Secas, and the energy sector uses it to forecast hydropower output.

FAQ

What is a climatological normal? It is a 30-year average of meteorological data that, on WMO recommendation, gets recomputed each decade. INMET’s current normal runs over 1991–2020.

How is monthly rainfall measured? A conventional pluviometer like the Ville de Paris gathers rain across a full 24 h, while automatic stations log totals every 10 min. Either way, the monthly accumulation comes out in mm.

Where do I find Brazilian rainfall normals? INMET puts them out for free, station by station, at portal.inmet.gov.br/normais. You will find monthly and annual precipitation totals there for every state.