The data that this blog entry is based on comes from the historical monthly rainfall records of 25 stations throughout Ireland (1850–2014). The median rainfalls for January is the focus.
stns_sf <- st_read("weather_stations.geojson", quiet = TRUE)jan_medians <- stns_sf %>%
filter(Month == "Jan") %>%
group_by(Station) %>%
summarise(
Median_Rainfall = median(Rainfall, na.rm = TRUE),
.groups = "drop"
)jan_map_data <- st_transform(jan_medians, 4326)Looking at the Map, a distinct geographical distribution of the median January rainfall over Ireland is observed. A strong west-to-east gradient exists, with the west coast stations always registering higher median rainfall amounts than those in the east and southeast. This situation illustrates the predominant effect of wet westerly airflows related to the North Atlantic, which bring to Ireland during winter (Met Éireann, 2023; Barry and Chorley, 2009) frequent frontal and cyclonic systems.
Western stations, particularly those in counties like Mayo, Galway, Kerry, and Donegal, report the highest monthly rainfall values for January. This phenomenon is closely associated with the prevailing south-west wind interaction and Ireland’s topography. The moist air is forced to rise over the mountains, like the MacGillycuddy’s Reeks, the Twelve Bens, and the Donegal Highlands, where it cools and condenses, leading to an increase in precipitation through the process of orographic uplift (Barry and Chorley, 2010).
On the contrary, eastern stations — near Dublin, Wicklow, and the southeast — have much lower median rainfall than the western ones. The latter regions are located in the rain shadow of the western mountains and receive more often the influence of drier maritime or continental air masses. Thus, precipitation totals are lower compared to the west that faces the Atlantic (Met Éireann, 2023).
Anaylsing the results, the use of median rainfall instead of mean has been particularly important. The distributions of rainfall are usually positively skewed and have rare events of very high rainfall that can lead to an average that is not representative of the situation. The analysis by the median is thus a more robust representation of what is usual in January, and it also to a large extent, prevents such intense winter storm events (Wilks, 2011) from having a large impact on the results.
In this regard, one can say that the map closely depicts the existing knowledge about the climate of Ireland in terms of rainfall. Winter precipitation distribution across Ireland is a result of interplay between large-scale atmospheric circulation, the Atlantic Ocean’s proximity, and local topography (Barry and Chorley, 2009; Met Éireann, 2023). The implications of these results are far-reaching as they will serve as a strong baseline for the further analysis of seasonal variability and also for the assessment of future climate change impacts on Ireland’s winter rainfall patterns (IPCC, 2021).
Barry, R.G. and Chorley, R.J. (2009) Atmosphere, Weather and Climate. 9th edn. London: Routledge.
IPCC (2021) Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.
Met Éireann (2023) Climate of Ireland. Available at: https://www.met.ie/climate/what-we-measure (Accessed: 2026).
Wilks, D.S. (2011) Statistical Methods in the Atmospheric Sciences. 3rd edn. Oxford: Academic Press.