Recent media weather news has linked the number 18 with rainfall falling over a period of time as shown by the headlines below :

One particular bit of weather trivia is about record rainfall over England in the past 18 months ending March 2024, which has been the wettest since rainfall records began though some reports say the "wettest ever in history".  Behind ever weather record and story there is data.  Looking through the daily rainfall series for Birmingham, since 1920, just over 100 years, reveals that there have been 9 days (being the 24 hours 09h to 09h) when the recorded rainfall has been exactly 18.0mm.

Monthly rainfall records for  England start in 1795, these area composite areal average across England (& Wales) and form the England & Wales Precipitation series (EWP).

Another monthly series which is also an areal average from a network of rain-gauges is the Midland Rainfall Series (MRS), starting in 1838 and extending across the counties from Derbyshire to Herefordshire to Buckinghamshire and Nottinghamshire.  Other regional series also exist.  Another series is from a single station, Birmingham and starts in 1793, but continuous from 1871 and is not included in the either the EWP or the MRS.   The plotted time series of the is shown on the graphs below.

The graphs show respective time series as BRS (top), MRS (middle) and EWP (btm), vertical scales are the same for each but the horizontal varies given the different record lengths.  The data shown is the 18 month rainfall anomaly as a percentage from the long-term average, in each case being 1961-1990.    The accompanying table shows the top 10 wettest for each series.  The year shown is the 18 month ending in the March of the year, so for instance 2024 is the 18 months commencing October 2022, all of 2023 and ending March 2024., along side each total rainfall rainfall is a count of the number of wet months during the 18 month period, a wet month in each series defined as 120% of LTA. 5 to note are

* 2024 is the wettest in both MRS and EWP though 5th in BRS

* top 10 years are common to all series

* there is a cluster of wet 18 month periods since 2000

* EWP 18mth total are 20% higher than both MRS and BRS which are similar

* the number of wet months in each top 10 is twice the LTA of wet months in 18

Graphs and table can be viewed offline by downloading

John