Temperature

The value of high quality observations to the meteorological and climatological scientific communities cannot be overstated. Historical observations are of particular value since an observation once made in time cannot be retaken, and if observations are not taken we have no way of directly knowing what the past weather and climate were like. The history of organised weather observations in Ireland goes back to the late 1700s with Observatories in Dunsink (1787) and Armagh (1790). In the first half of the 19th century readings were taken at Birr Castle (1845), Markree Castle (1824), the Phoenix Park (1829) and Trinity College, Dublin (1838). From the mid 19th century the number of stations increased and some standardisation of observations took places in line with international developments. By the mid 20th century there were over 100 locations measuring temperature, by the time the Irish Meteorological Service was formed in 1936 the number of rain gauges was over 200. Since then the network has expanded further, there are now just under 500 rain gauge locations in the country.

Map of the National Synoptic and Climate Stations recording Temperature (2017)

Temperature is a key indicator of changes in the climate, measurements and analysis are essential for assessing the affects of climate change. Meteorological stations with long term records are crucial for the observation, analysis and quantification of climate change. The value of high quality observations to the meteorological and climatological scientific communities cannot be overstated. They are also an ‘irreplaceable cultural and scientific heritage of mankind that serve the needs of current and future generations for long-term high quality climate records’ (World Meteorological Organization).

The island of Ireland has a long history of operational weather stations measuring air temperatures. Maximum and minimum air temperature data from five spatially distributed meteorological stations; Armagh Observatory, Co Armagh, Phoenix Park, Co Dublin 1829; Birr Castle*, Co Offaly, Valentia Observatory and Malin Head, Co Donegal have been used to derive the national long-term mean temperature series for Ireland for the period 1900 to 2023.

The national long-term average temperature series for Ireland has been derived using data from the above five spatially distributed meteorological stations. In May 2017, Valentia Observatory was recognised as a Centennial Observing Station status by the World Meteorological Organization (WMO).

* The meteorological station at Birr closed in October 2009, since then adjusted data from the close by meteorological station at Gurteen, Co Tipperary was used.

YouTube video from the World Meteorological Organization on Centennial Observing Stations Worldwide

Plaque at Valentia Observatory marking its recognition as a long-term observing station by the WMO

The Long Term Average period 1961 to 1990 (LTA 1961-1990) or ‘normal’ period is the WMO’s fixed reference period used for climate change and variability studies.

The figure below shows the air temperature anomaly from 1900 to 2023. The anomaly is the difference between the mean annual temperature and the mean annual LTA 1961-1990 of 9.55°C. As you can see mean temperatures have varied considerably from year to year. Warming periods occurred from the 1930s and 1940s and from the late 1980s to the present time. Warming periods since the end of the 20th century have been longer and more frequent.

You can download the provisional Island of Ireland Long-Term Annual Average Temperatures dataset. The Armagh Observatory data is courtesy of the Met Office in the United Kingdom. Anomalies (difference from), in the chart above, are with respective to averages of the standard reference period 1961-1990.

Air temperature has been measured in a systematic manner in Ireland since the early 19th century. Following the establishment of the Irish Meteorological Service in 1936, the climatological network was reorganised and expanded throughout the 1940s and early 1950s.

Figure 3: Stevenson screen. This shields the meteorological instruments inside against precipitation and direct heat radiation.

Contents of a Stevenson Screen

Traditionally, surface air temperature is measured by thermometers which are housed in a white painted wooden louvered shelter known as a Stevenson Screen in degrees Celsius (°C) and is the standard at official Irish weather stations. Images above show the full Stevenson Screen as it appears in the open, also shown is the internal instrumentation (locations: Dublin Airport and Markree).

Did you know? Air temperature measured in direct sunshine, or in a porch, could be up to 10°C or 15°C higher in warm and sunny weather. This is why temperature measurements are made in a standard enclosure out of direct sunshine. Surface air temperature is not the only temperature measured by Met Éireann, others include upper air, earth and soil temperatures.

Temperature(s) are measured at climate stations and traditionally were read once a day at 9 UTC. At synoptic stations, a new generation of stations provide minute by minute readings of the main weather parameters, including temperature. At these automatic stations and at the airports temperatures are measured by means of platinum resistance thermometer, at other locations the temperature is read by means of a mercury-filled thermometer. A project is currently underway to replace mercury thermometers with the platinum resistance thermometers.

inhomogeneities arise in data time series due to changes in instrumentation, observer, location and times of observation, they can also occur due to changes in station exposure due to new buildings, tree growth, et cetera, in the vicinity of the station. Met Éireann is currently working on homogenisation of data series, data used in this section have not been homogenised.

The observations collected in the manner above, allow for analysis to describe the Temperature Climate of Ireland. Seasonal variations of extremes vary as you might expect, winter being the season with coldest extremes and summer the hottest.

The following graph shows the seasonal extremes in Surface Air Temperatures between 1941 and 2016 inclusive at synoptic stations.

Learn more about Meteorological Seasons. For more information on extremes in Irish temperature, visit the Weather Extremes page.

The 1961-1990 LTA or ‘normal’ period is a fixed reference period used for climate change and variability studies.

Long-Term Stations Annual Average compared with LTA 1961-1990

Temperature is a key indicator for changes in the climate. A long-term average national temperature series for Ireland has been derived using data from the five centennial stations at Valentia, Malin Head, Armagh, Birr* and Dublin. The diagram below shows the mean annual temperature anomaly from 1900 to 2022 (this is the difference between the mean annual temperature and the 1961 to 1990 average value). Mean temperatures have varied considerably from year to year. Warming periods occurred from the 1930s and 1940s and from the late 1980s to the present time. Warming periods since the end of the 20th century have been longer and more frequent. The LTA 1961-1990 series is the period used for climate attribution studies worldwide.

Air Temperature (°C) at centennial stations compared to Long Term Average 1961-1990 (LTA) since 1900 (anomalies)

* The station at Birr Castle, Co Offaly closed in October 2009, since then data from the nearby TUSCON station at Gurteen, Co Tipperary was used.

Seasonal (all Stations) compared with LTA 1961-1990

Data from synoptic stations and climatological stations are available in digital format from 1941 and 1961 respectively. From this data, gridded maximum and minimum temperatures have been produced for each month since 1961. Gridded data is averaged over the area of Ireland to produce national maximum and minimum values for each month and season.

All seasons show a rise in temperatures, with maximum temperatures rising slightly more than minimum temperatures in winter and spring, and minimum temperatures rising more than maximum temperatures in summer.

Mean Minimum and Maximum Seasonal Air Temperature Anomalies (°C) difference from LTA 1961-1990

Climate Change indicators LTA 1961-1990

An expert team of the World Meteorological Organisation has described a range of rainfall and temperature climate change indicators, to enable comparison of these variables between nations. In the case of temperatures, these indices include number of frost and ice days, days with temperatures above or below a certain threshold, and length of growing season. Indicators for frost days (the number of days each year when the air temperature falls below zero), and warm days (the number of days each year when the air temperature falls rises above 20 °C), for the Phoenix Park are shown below.

Phoenix Park Climate Change Indicators for Temperature

From these time series it is possible to derive trends over a number of years, or more usually decades. Such trends have been produced for climate stations with a near complete record for the period 1961 to 2016. In deriving such trends, the best straight line fit to the data is used, standard significant tests are used to derive the confidence intervals, a confidence interval of 95% is usually taken as indicating a significant trend, it may not always be appropriate to fit a straight line to the data.

From individual station records, trend maps have been calculated for the indicators. The maps below show the trends for the number of frost days, and days with temperature in excess of 20 °C from 1961 to 2016. In line with the general temperature trend, there is an increase in the number of days with temperatures over 20 °C, and a decrease in the number of days with temperatures below zero degrees Celsius.

Climate Change Indicators for SU20: Warm Days (Tmax>20°C) and FD: Frost Days (Tmin<0°C)

Long Term Averages or ‘normals’ are used for comparison purposes and to put values in context. The LTA 1981-2010 series is the period currently used for the climate periodicals (Statements, both final and provisional).

Excluding upland areas, the mean annual temperatures generally range between 9 and 12°C, from northeast to southwest.

Inland stations show more variation than coastal stations, being colder in Winter and warmer in Summer. There is on average only about one day or less per year when the air temperature stays below freezing point. Minimum air temperature falls below zero on about 40 days per year at the inland stations, but on less than 10 days per year in most coastal areas. Air temperatures inland normally reach 18 to 20 °C during summer days, and about 8°C during wintertime.

Need to read more? S. Walsh (2012). A Summary of Climate Averages for Ireland 1981-2010. Met Éireann, Climatology Note No 14