The mean annual temperature at low altitudes in Northern Ireland varies from about 8.5 °C to 9.5 °C, with the higher values occurring around or near to the coasts. The mean annual temperature decreases by approximately 0.5 °C for each 100 metres increase in height so that, for example, Parkmore Forest in County Antrim (at 235 metres) has an annual mean temperature of 7.4 °C. On this basis, Slieve Donard (at 852 metres) would have an annual mean temperature of about 4.5 °C.
In winter, temperatures in the UK are influenced to a very large extent by those of the surface of the surrounding sea, which reach their lowest values in late February or early March. Around the coasts February is therefore normally the coldest month, but inland there is little to choose between January and February. The January mean daily minimum temperatures vary from about 0.5 °C in the upland areas to about 2°C on the coast, with the highest values on the coast of County Down.
The coldest nights are those on which there is a covering of snow on the ground; the lowest temperatures occur away from the moderating influence of the sea, on the floors of inland valleys into which cold air can drain. It was under such conditions that the temperature fell to -17.5 °C, the lowest officially accepted value recorded in Northern Ireland, at Magherally near Banbridge (County Down) on 1 January 1979. Most coastal areas do not experience such cold nights; as an example, the lowest temperature recorded at Helens Bay in County Down in the thirty years 1961 to 1990 was -5.4 °C.
July is normally the warmest month in Northern Ireland, with mean daily maximum temperatures varying from about 17 °C in the upland areas and along the north coast to almost 20 °C in low lying areas south of Lough Neagh and in Fermanagh. In the UK, the highest July mean daily maxima occur in the London area (22.5 °C) whilst the lowest occur in the Shetlands (15 °C). The highest temperature ever recorded in Northern Ireland is 30.8 °C at Knockarevan in County Fermanagh on 30 June 1976, and also at Shaw's Bridge in Belfast on 12 July 1983.
The variation of mean daily maximum and minimum temperatures month by month, together with the highest and lowest temperatures recorded, is shown for Aldergrove and Lough Navar Forest.
The average number of days with frost in Northern Ireland varies widely depending on the location. The main controls are distance from the sea and altitude, but the ability for cold air to drain into inland valleys is another important factor. An 'air frost' occurs when the temperature at 1.25 metres above the ground falls below 0 °C, whereas incidence of a 'ground frost' refers to a temperature below 0 °C measured on a grass surface. Sites along the coast typically have fewer than 25 days of air frost each year and inland the number increases with altitude to over 55 days in the highest upland areas. Ground frost occurs on average on less than 80 days each year on the coast and over 115 days in the highest upland areas and the most frost-prone lowland locations, with a similar distribution to air frost.
The graphs show the average frequency of air and ground frost at Aldergrove and Lough Navar Forest.
