What a lot of weather we're having today.

The above is a saying in Northern Ireland which evolved because the location of Northern Ireland, and the resulting climate, leads to the weather being fairly changeable.

The climate of Northern Ireland is hard to classify, and would sometimes depend on not only which month it was but also to what day and time of day you were referring. Meteorologists officially classify Northern Ireland's climate as Temperate Northern European. A major influence on the climate is the relatively warm North Atlantic Drift Current, or Gulf Stream. This makes the climate warmer and more temperate than is usual for this latitude, without the wide fluctuations suffered at other points so far north. Ireland is also the windbreak for England and Wales, sheltering them from the prevailing Atlantic winds that often reach gale force.

A Wild Wind Blows

Friends of the Earth¹ say that Northern Ireland has one of the best wind resources in Europe. The frequent occurrence of high winds is due to the constant passing of deep depressions. These persistently come off the Atlantic over the whole of Northern Britain and are then slowly dispersed as they hit land. Also, the glacial valleys and high mountains help to funnel the wind, once it hits the land, into certain routes of least resistance, causing a wind tunnel effect in certain locations. This funnelling effect can lead to gusts of wind that greatly exceed the mean wind speed. Wind speeds are already high due to the maritime effects of a vast ocean where the prevailing pressure systems head.

A gale day is a day when the mean wind speed, at the standard measuring height (ten metres above ground level), exceeds 34 knots (39 miles per hour or 17.2 metres per second) over any period of 24 hours. The systems that most frequently cause these conditions develop during the winter months. In the UK, gales most often occur in the Hebrides, off the Scottish coast, which experience 33 gale days per year. Northern Ireland is not so exposed, being protected by the rest of Ireland, especially Donegal from the west, and the rest of the UK from most other directions. On the coasts of Antrim and Down they average 15 gale days per year, and less inland.

The strongest winds in Northern Ireland are experienced on the many summits and mountains. However, there is no recording activity up in the strongest, highest and most gale-prone parts of Northern Ireland, so these cannot be monitored, unless they eventually develop a range of wind farms there.

The fastest recorded gust of wind in Northern Ireland was observed at the County Down fishing village of Kilkeel on 12 January, 1974 — a gust of 108 knots (125mph).

Where Is the Sun?

Apparently the Romans never invaded Hibernia, as they called Ireland, because it was always covered in cloud. Things have not changed, and Ireland still has less sunshine than England. However, Friends of the Earth say there is sufficient sunshine to be used as a back-up to the wind farms, which would enable Northern Ireland to move to renewable energy use.

One of the main reasons for all this cloud cover is of course the hilly nature of Northern Ireland's geography. However, on the extreme east of Northern Ireland the coastal strip of County Down benefits from being in a rain shadow. Here there is an average of over 1400 hours of sunshine², allowing the area to have a very prosperous market-farming sector on the rich, volcanic, alluvial soils. However, up in the hills there are increased levels of precipitation, and sunshine drops to an average of 1100 hours per annum. Obviously the best months to see the Sun in Northern Ireland are May and June when the Northern Hemisphere is pointing in its general direction.

The best sunshine record is 298 hours, in June, 1940, at Mount Stewart on that County Down coastal strip. The lowest for a month was a mere 8.3 recorded at Silent Valley, County Down, in the Mournes in January, 1996. This is not necessarily a bad thing, unless you are a hill walker, as this is the where the main reservoir for Belfast is located.

Rain, Rain Everywhere...

... or so it seems.

Maybe the Romans feared that the rain that fell from the Hibernian skies would rust their breast plates and helmets. This was, of course, before the days of acid rains and nuclear power plants being located just across the Irish Sea. Rain does fall on Ireland: that is why it is the Emerald Isle, after all. All this precipitation does have some advantages, a thriving farming community for one. Most of the rain obviously falls on the high ground of the Mourne and Sperrin Mountains, where up to 1600mm falls yearly. This is only about half of what falls in England's Lake District or Scotland's Western Highlands. In the coastal areas of Down and Antrim the yearly average only reaches 800mm.

There are only small seasonal variations in rainfall, but the wettest months are from August to January. The fact that this is not extended into July, as in other parts of the UK, is accounted for by the relatively small amount of thunderstorms. There are only four thunderstorm days a year in Northern Ireland, compared to 15 or 20 in many parts of the UK. The American Benjamin Franklin must have been relieved not to have been born in Northern Ireland, or he might never have got his kite and electricity experiment to work.

The wettest day recorded in Northern Ireland was at Tollymore Forest, in the shadows of the Mournes, just outside Newcastle, County Down, when on 31 October, 1968, 158.9mm of rain fell.

Don't Forget your Coat and Scarf

When the climate is described as temperate we must remember that this use of the word is relative. The mean annual temperature at low altitudes is about 8.5°C to 9.5°C. This rises slightly near the coast, where the effect of the North Atlantic Drift can be felt. However, for each 100m of increased altitude the average drops off by 0.5°C. That means that the Highest Peak, Slieve Donard at 852m, would have a mean temperature of approximately 4.5°C.

The sea greatly affects the land temperature in Northern Ireland. Nowhere is it more than 80km to the coast. The result of this is that when the sea temperature around Britain is at its lowest, in late February and early March, this is reflected in Northern Ireland's temperature. The coldest nights occur when snow has actually fallen, and areas that are sheltered from the influences of the sea tend to have the lowest temperature. These include those parts that are inland, especially the deep valley floors where cold air drifts downwards.

Compared with locations at similar latitudes, the low temperatures are not that low. The worst was -17.5°C recorded at Magherally, County Down on 1 January, 1979. The coastal areas are a great deal warmer, the lowest recorded at a coastal station being -5.4°C at Helen's Bay just outside Belfast.

It does get warmer in the summer, and summer sportspeople enjoy days of genuine short-sleeve weather, although they usually have a supply of warm and protective clothing ready just in case. The maximum recorded temperature came during the 1976 heatwave and was 30.8°C at Knockarevan, County Fermanagh. The same temperature was recorded again at Shaw's Bridge in South West Belfast on 12 July, 1983.

Will there be Snow this Christmas?

Most likely not.

Snow is rare in Northern Ireland. As less rain falls there than in the Highlands of Scotland or the Lake District, and the influence of the sea helps to keep the temperature up, snow is very rare at sea level, but slightly more frequent on the hills. The average number of days with snow is about ten on the east coast, rising to about 30 on the Sperrin, Antrim, and Mourne Mountains. The temperature, the sea breeze and salt air mean that snow very rarely falls at sea level before December or after March, though it has been known outside these times (as late as June one year). If snow does fall heavily and the wind is also high, drifting can very quickly affect the transport system; however, the occurrence of both at the same time is rare.

The worst winters were 1962-3 and 1981-2 where even in coastal areas snow lay on the ground for more than 30 days and traffic there came to a grinding halt at times.

Monthly Almanac