Details available for: sunshine - rainfall - wind - temperature - snow - visibility

See also Scotland - Wales - England climates.

On the whole, Northern Ireland is cloudier than England, because of the hilly nature of the terrain and the proximity to the Atlantic. Even so, the coastal strip of County Down manages an annual average total of over 1,400 hours of sunshine. This compares favourably with many coastal areas of England and Wales, though not perhaps with the figures of around 1,750 hours achieved by many places along the south coast of England. The dullest parts of Northern Ireland are the more mountainous areas, with annual average totals of less than 1,100 hours.

Mean daily sunshine figures reach a maximum in May or June, and are at their lowest in December. The key factor is, of course, the variation in the length of the day through the year, but wind and cloud play their part as well.

Facts and figures (bright sunshine)

Maximum duration in a month: 298.0 hours at Mount Stewart (Co. Down) in June 1940.

Minimum duration in a month: 8.3 hours at Silent Valley (Co. Down) in January 1996.

Sunshine graph

Rainfall in Northern Ireland varies widely, with the highest average annual totals being recorded in the Sperrin, Antrim and Mourne Mountains, where the yearly fall of around 1,600 mm is about half that of the English Lake District or the western Highlands of Scotland. In the east, close to the coast, and near to the southern and eastern shores of Lough Neagh, the annual totals of just under 800 mm are similar to those near the Firth of Forth in Scotland, but rather more than those over much of the English Midlands.

The seasonal variation of rainfall in Northern Ireland is not large, but throughout the Province the wettest months are between August and January, unlike places in south-east Scotland or in the English Midlands, where July and August are often the wettest months of the year. This is partly a reflection of the relatively low frequency of thunderstorms in Northern Ireland, compared with that in England. For example, at Armagh, thunder occurs on an average of less than four days a year, compared with 15 to 20 at many places in England. Only in a few locations, mainly away from the coasts, does the frequency of thunder exceed five days a year.

Facts and figures

Maximum in a day (09-09 UTC): 158.9 mm at Tollymore Forest (Co. Down) on 31 October 1968.

Rainfall graph

There is a close relationship between surface isobars (lines joining points of equal air pressure) and the wind speed and direction over open, level terrain. However, in mountainous areas local topography also has a significant effect, with winds tending to blow along well-defined valleys.

Over land, the roughness of the ground causes a decrease in the mean wind speed compared with that which occurs over the sea, with the size of the decrease depending on the nature of the terrain. In major towns and cities the overall mean speed is considerably reduced by the buildings, but local funnelling may occur and the wind may gust to about the same speed as in open country. It is the gusts which cause much of the damage to buildings and trees during major storms. In general, wind speed increases with height, with the strongest winds being observed over the summits of hills and mountains.

A day of gale is defined as a day on which the mean wind speed at the standard measuring height of 10 m above ground attains a value of 34 knots (39 miles per hour, 17.2 metres per second) or more over any period of 10 minutes during the 24 hours. The strongest winds in the United Kingdom are associated with the passage of deep depressions across or close to the British Isles; these are most frequent during the winter, so that is when gales occur most frequently on the exposed western and northern coasts of both Britain and Ireland. For example, the Hebrides experience, on average, about 35 days of gale a year, and the extreme south-west of England about 30. In Northern Ireland, the coastal areas are not as exposed as this, being protected both by the rest of Ireland and the adjacent parts of Scotland. Thus the coastal fringes of Counties Antrim and Down have about 15 gales a year, while the number of days decreases inland to five days or fewer.

In general, wind speed increases with height, with the strongest winds being observed over the summits of hills and mountains. There are no wind-recording stations at high altitudes in Northern Ireland so no data can be given; as an indication, however, Snaefell on the nearby Isle of Man (at 615 m) averages over 200 days of gale a year.

Facts and figures

Highest gust recorded at a low-level site: 108 knots (125 mph) at Kilkeel (Co. Down) on 12 January 1974.

Wind graph

Over Northern Ireland the mean annual temperature at low altitudes 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 m increase in height so that, for example, Parkmore Forest in County Antrim (at 235 m) has an annual mean temperature of 7.4 °C. On this basis, Slieve Donard, the Province's highest mountain (at 852 m) would have an annual mean temperature of about 4.5 °C.

In winter, temperatures in the British Isles 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 thus normally the coldest month, but inland there is little to choose between January and February as the coldest month.

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 the cold air drains. It was under such conditions that the temperature fell to -17.5 °C, the lowest ever recorded in Northern Ireland, at Magherally in County Down on 1 January 1979. 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, and the highest temperatures of all have occurred inland, furthest away from the cooling influence of the Atlantic. 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.

Facts and figures

Air temperature (measured under standard conditions at 1.25 m above the ground).

Highest recorded 30.8°C at Knockarevan (near Belleek, Co. Fermanagh) on 30 June 1976, and at Shaw's Bridge, Belfast (Co. Antrim) on 12 July 1983.

Lowest recorded -17.5°C at Magherally (near Banbridge, Co. Down) on 1 January 1979.

Snow is comparatively rare near sea level in Northern Ireland, but much more frequent over the hills. The average number of days each year when sleet or snow falls in Northern Ireland varies from around 10 near the east coast to over 30 in the mountains of Sperrin, Antrim and Mourne. Snow rarely lies on the ground at sea level before December or after March, and the average annual number of days with snow lying in Northern Ireland varies from less than five around the coasts to over 30 in the mountains.

The number of days of snowfall and snow cover varies enormously from year to year. At many places in the last 50 years it has ranged from none at all in several winters to in excess of 30 days during the winters of 1962/63 and 1981/82. Even places near the coast experienced prolonged snow cover during these two winters. In heavy snowfalls there can be quite extensive drifting of the snow in strong winds, especially over the higher ground, resulting in severe dislocation of transport. Fortunately such occasions are comparatively rare.

Facts and figures

Snow graph

Given the distance of many parts of Northern Ireland from the industrial and populous areas of Britain and mainland Europe, much of the Province enjoys excellent visibility. The industrial areas are all close to the coast, and are thus relatively breezy and free of serious reductions of visibility by smoke.

Any early morning mist or fog will usually clear rapidly, though it can be much more persistent during the winter months.

Over the higher ground in Northern Ireland fog statistics are scarce, but given the hilly nature of the country and its relative proximity to the sea, hill fog can be both extensive and frequent and is a potential hazard to be borne in mind by walkers.