Ireland’s Median Rainfall in January (1850–2014)

##Introduction Rainfall is a defining feature of Ireland’s climate, influencing agriculture, water resources, and everyday life across the country. During the winter months, rainfall patterns are strongly shaped by Atlantic weather systems, bringing frequent and sometimes intense precipitation. January is typically one of the wettest months of the year, making it particularly useful for examining how rainfall varies spatially across Ireland.

This blog explores patterns in January rainfall using long-term observations from 25 weather stations distributed across the country. By mapping median January rainfall values, the analysis highlights regional contrasts and provides insight into how Ireland’s geography and prevailing weather systems influence rainfall distribution.

##Data used This report uses historical monthly rainfall totals (mm) from 25 weather stations across Ireland, covering 1850–2014. Two datasets are used:

rain: monthly rainfall data with variables Year, Month, Rainfall, Station

stations: station metadata including latitude and longitude

##Why January? January was chosen for this analysis because it is typically one of the wettest months in Ireland. Rainfall during this period is strongly influenced by large-scale Atlantic weather systems, which produce widespread and persistent precipitation. As a result, spatial differences in rainfall between western, eastern, and inland regions are particularly clear.

##Load packages and data

library(tidyverse)

── Attaching core tidyverse packages ──────────────────────── tidyverse 2.0.0 ──
✔ dplyr     1.1.4     ✔ readr     2.1.5
✔ forcats   1.0.1     ✔ stringr   1.5.2
✔ ggplot2   4.0.1     ✔ tibble    3.3.0
✔ lubridate 1.9.4     ✔ tidyr     1.3.1
✔ purrr     1.1.0     
── Conflicts ────────────────────────────────────────── tidyverse_conflicts() ──
✖ dplyr::filter() masks stats::filter()
✖ dplyr::lag()    masks stats::lag()
ℹ Use the conflicted package (<http://conflicted.r-lib.org/>) to force all conflicts to become errors

library(leaflet)
library(janitor)

Attaching package: 'janitor'

The following objects are masked from 'package:stats':

    chisq.test, fisher.test

library(lubridate)

Load data saved as an RData file

load("rainfall.RData")

Confirm year range

range(rain$Year, na.rm = TRUE)

[1] 1850 2014

##Method To examine typical winter rainfall patterns across Ireland, this analysis focuses on January rainfall records from 25 weather stations. The dataset was first filtered to include only January observations, ensuring that the analysis reflects consistent seasonal conditions across all years. For each station, the median January rainfall was calculated using data spanning the full period from 1850 to 2014.

The median was chosen rather than the mean because it provides a more robust measure of central tendency, reducing the influence of exceptionally wet or dry years that could otherwise skew the results. The resulting station-level median rainfall values were then joined to geographic information containing the latitude and longitude of each station. These data were visualised using an interactive Leaflet map, with rainfall levels represented by color-coded markers to allow spatial patterns to be explored easily.

##Calculate median January rainfall

jan_median <- rain %>%
filter(Month == "Jan") %>%
group_by(Station) %>%
summarise(
median_jan_rain = median(Rainfall, na.rm = TRUE),
.groups = "drop"
)

nrow(jan_median)

[1] 25

head(jan_median)

# A tibble: 6 × 2
  Station    median_jan_rain
  <chr>                <dbl>
1 Ardara               172. 
2 Armagh                75  
3 Athboy                87.1
4 Belfast              102. 
5 Birr                  77.5
6 Cappoquinn           147. 

##Join rainfall summary to station locations

stations_map <- stations %>%
left_join(jan_median, by = "Station")

nrow(stations_map)

[1] 25

sum(is.na(stations_map$median_jan_rain))

[1] 0

##Interactive map

bins <- c(0, 50, 75, 100, 125, 150, 175, 200, Inf)

pal <- colorBin(
palette = "YlGnBu",
domain  = stations_map$median_jan_rain,
bins    = bins
)

leaflet(stations_map, height = 600) %>%
addProviderTiles(providers$CartoDB.Positron) %>%
addCircleMarkers(
lng = ~Long, lat = ~Lat,
radius = 7,
color = ~pal(median_jan_rain),
stroke = TRUE, weight = 1,
fillOpacity = 0.85,
popup = ~paste0(
"<b>", Station, "</b><br>",
"Median January rainfall: ",
round(median_jan_rain, 1), " mm"
)
) %>%
addLegend(
position = "bottomright",
pal = pal,
values = ~median_jan_rain,
title = "Median January rainfall (mm)",
opacity = 1
)

##Exploring the map

The interactive map below allows users to explore spatial patterns in median January rainfall across Ireland. Each weather station is represented by a circular marker, with colors indicating different rainfall levels. Darker colors correspond to higher median rainfall values, while lighter colors represent drier locations.

Users can zoom in to examine specific regions and click on individual stations to view the station name along with its median January rainfall. The legend provides context for the color scale, making it easier to compare rainfall levels between different parts of the country. This interactive approach allows patterns and regional contrasts to be explored more intuitively than would be possible using static figures alone. ##Discussion of patterns

Map shows clear spatial variation in median January rainfall across Ireland. Stations located in the west and northwest generally experience higher median rainfall values, while many inland and eastern stations record considerably lower amounts. This pronounced west–east gradient reflects Ireland’s exposure to moist Atlantic air masses, which bring frequent rainfall to western coastal areas during the winter months.

As these air masses move eastward across the island, much of their moisture is lost, resulting in drier conditions in the east. Local topography further enhances this pattern, with upland and coastal regions experiencing greater rainfall due to uplift, while low-lying inland areas receive less precipitation. January is typically one of the wettest months of the year in Ireland, and the patterns observed here highlight how both atmospheric circulation and physical geography combine to shape regional rainfall variability across the country.

Limitations

While the dataset spans a long historical period (174 years), the number of weather stations is limited, therefore some regions may be underrepresented. In addition, the use of median rainfall values does not display year to year variability that clearly, meaning that extreme rainfall events are not captured.Future analyses may chose to explore seasonal comparisons or temporal trends to provide a more detailed understanding of rainfall variability across Ireland.