The moderating influence of the Atlantic is felt throughout Ireland. The country therefore does not experience the same range of temperatures throughout the year as more continental countries do. Although our inland stations show more variation, there is 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.
Temperature is a key indicator of changes in the climate. 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. The temperature climate of Ireland is strongly influenced by our proximity to the Atlantic Ocean. Mean annual temperatures generally range between 9 and 11°C, from northeast to southwest. Traditionally temperature was measured by thermometers which were housed in a white painted wooden louvered shelter known as a Stevenson Screen.
Temperature readings at synoptic stations are made every hour on the hour, at climate stations temperatures are read once a day at 0900 UTC. Met Éireann is in the process of automating its synoptic network of stations, the new generation of stations, known as TUCSON (The Unified Climate and Synoptic Observation Network), 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. Inhomogenieties 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 etc. in the vicinity of the station. At present, Met Éireann is assessing different methods for homogenising data series, data used in this section have not been homogenised.
Time Series and Trends: A number of locations have times series longer than 100 years. Stations with long term records are crucial for the observation, analysis and quantification of climate change. The time series of annual mean air temperature for the Valentia Observatory and the Phoenix Park from 1881 to 2009 is shown below.
A long-term average national temperature series for Ireland has been derived using data from 5 long-term stations at Valentia, Malin Head, Armagh, Birr and Dublin. The diagram below shows the mean annual temperature anomaly from 1900 to 2010 (This is the difference between the mean annual temperature and the 1961 to 1990 normal value). (The station at Birr closed in October 2009, for the final few months of 2009 weighted data from the nearby TUSCON station at Gurteen was used).
The mean temperature varies considerably from year to year. Warming periods occurred from the 1920s to 1940 and from the late 1980s to the present time. The warming at the end of the 20th century was larger and more rapid.
Data from synoptic stations and climatological stations are available in digital format from 1961. 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.
Seasonal temperature differences from 1961 to 1990.
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.
Climate Change indicators
An expert team of the World Meteorological Organisation has described a range of rainfall and temperature climate change indicators, to enable uniform comparison of these variables. 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° Celsius), for the Phoenix Park are shown above.
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 1961-2010 (Note: 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 2010. 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.