January Median Rainfall Map of Ireland (1850 - 2014)

Author

Lutea Magaia Alberto

Introduction

Understanding rainfall patterns is very important for climate analysis, water resource management, agriculture planning and other topics. This blog aims to analyse rainfall data collected across 25 weather stations in Ireland from 1850 to 2014. The analysis was done by calculating the median rainfall for January at each station and visualizing the results on a map to uncover spatial rainfall distribution. These insights can be helpful to better understand Ireland’s hydrological and climate patterns, which are influenced by its geographical location and diverse microclimates (Met Éireann, 2022).

Data Description

The following data sets were used:

  1. Rainfall Data: Contains monthly rainfall records for each station from 1850 to 2014. Key variables including:
    • Year: The year of observation
    • Month: The month of observation
    • Rainfall: The amount of rainfall recorded (in mm)
    • Station: The station where the observation was made
  2. Station metadata: Provides details about the 25 weather stations, including:
    • Station: Name of the station
    • Easting and Northing: Coordinates in Irish Grid format
    • Elevation: Altitude of the station above sea level
  3. County Boundaries: Geo-spatial data defining the boundaries of Irish counties

Code and Map

The following code was used to process the data and create the map:

# Load necessary libraries
library(dplyr)
library(ggplot2)
library(sf)

# Load the data
load("rainfall.RData")
# Suppress output from st_read
invisible(capture.output({
  counties <- st_read("counties.geojson")
  weather_stations <- st_read("weather_stations.geojson")
}))

# Filter January data and calculate median rainfall
rainfall_summary <- rain %>%
  filter(Month == "Jan") %>%
  group_by(Station) %>%
  summarise(median_rainfall_jan = median(Rainfall, na.rm = TRUE))

# Combine median rainfall with station metadata
stations <- stations %>%
  left_join(rainfall_summary, by = "Station")

# Create the map
ggplot() +
  geom_sf(data = counties, fill = "lightgrey", color = "white") +
  geom_point(data = stations, 
             aes(x = Easting, y = Northing, color = median_rainfall_jan), 
             size = 3) +
  scale_color_gradient(low = "blue", high = "red", name = "Median Rainfall\n(January)") +
  labs(title = "Median Rainfall in January for 25 Weather Stations in Ireland",
       caption = "Source: Assignment Data") +
  theme_minimal()

Discussion

In general, the analysis reveals lack of clear and consistent pattern in the distribution of precipitation in January in Ireland. Regions in the southwest and west, particularly those near the Atlantic Ocean, exhibit significantly higher precipitation levels. For example, County Kerry has two stations with high median precipitation in January: Killarney station with 177.7 mm and Valentia (166.0 mm). Similarly, in Donegal, the Ardara station recorded a median of 171.6 mm. These results are consistent with Ireland’s prevailing weather systems, where Atlantic westerlies often bring moist air masses that deposit substantial rainfall in areas exposed to oceanic influences (Sweeney et al., 2008; Met Éireann, 2022). On the other hand, County Dublin has two stations with the lowest median precipitation levels in the country: Dublin Airport station with 63.0 mm and Phoenix Park with 67.6 mm. Moderate precipitation levels in January are found in stations in the southeast and south, such as Rathdrum in Wicklow (126.6 mm), Cappoquinn (147.4 mm) and Waterford Station (102.6 mm) in County Waterford. Walsh (2012) reports that Ireland’s monthly rainfall average for January is approximately 130 mm, making it expected that some stations record values above this average, while other far bellow it.

Conclusion

This analysis highlights Ireland’s spatial variability in rainfall, particularly for the month of January. These patterns suggest that different factors such as geography, proximity to the ocean and elevation can impact rainfall levels.

References

  1. Curley, M., Coonan, B., Ruth, C.E. and Ryan, C. 2023. Ireland’s Climate Averages 1991-2020. Climatological Note No. 22. Met Éireann, Ireland.

  2. Sweeney, J., Albanito, F., Brereton, A. (2008). Climate Change – Refining the Impacts for Ireland. Environmental Protection Agency.

  3. Walsh, S. (2012). A Summary of Climate Averages for Ireland, 1981–2010. Met Éireann.