Weather and Climate in Northern Ireland

The climate of Northern Ireland is influenced mainly by the relatively warm surface waters of the North Atlantic Drift or Gulf Stream, which keeps the atmosphere at a fairly constant level all year round. Our relatively mild, moist climate means that a wide variety of plants can be grown and has led to the development of such great collections of plants in the gardens of the British Isles.

Extremes of temperature are rare, the coldest usually occuring in January and February, with average daily maximums ranging from 7 C at the coast to 5 C over the uplands and the average minimums from 2.5 C to 0 C, respectively. The most extreme low can be -17.5 C on inland valley floors. The warmest month is July with maximum daily temperatures betweeen 18 - 19 C at low altitudes and less than 17 C over uplands. The highest temperature can be around 30 C.

The rainfall is highest in the upland regions which receive about 1600 mm per year, whereas in the Lough Neagh and Upper Bann lowlands the annual totals are less than 750 mm. There is also a west to east decrease in rainfall with the Ards Peninsula receiving less than 800 mm. The wettest months are between August and January. The main influence over the amount of rainfall reaching our shores is the Jet Stream. This is a flow of wind which moves the rain clouds, in the summer the flow is towards the North East and is normally concentrated over the Atlantic, well to the west of Ireland. If it drifts to the East as it did this summer (2002), then the rainfall becomes record-breaking.

Snowfall varies greatly from year to year with an average of less than 10 days annually near to sea level and more than 30 days in upland areas. Inland areas are more at risk from frost, eg. there is a 10% chance of an air frost in coastal areas of County Down after 28th April, but in the Lough Neagh basin there is a 50% chance until after the 1st of May. In the late Autumn the probability is less than 5% before the 1st of November along the Down coast compared to 50% before this date, inland.

As the growing season is determined mainly by temperature, requiring at least 5.6 C, it's length varies accross the Province and decreases by 20 to 30 days with every 100 m of altitude. The longest season of more than 280 days occours around Belfast Lough, East Down and the Ards Peninsula. The central lowlands have about 265 days, with less than 205 days on the highest areas. It is usually taken that the last day of frost is around the first of June so tender bedding plants are best kept with some protection at night until after this date.

Another climatic feature of interest to the gardener is the wind and Northern Ireland receives its fair share, although it is protected slightly by the rest of Ireland and Scotland. The main directions are between South South West and West North West (200° - 280°). The average speed varies from 6.7 metres per second on the North Antrim coast to less than 4.1 metres per second in sheltered inland areas. The highest winds occur between November and March with an average of 15 days of gales (ie. 10 minutes at or above 17.2 metres/ second) along the Antrim and Down coasts, and about 5 days per year inland.

The strength of the wind is determined by the movement of air between areas of high and low pressure. Pressure is measured in Bars and on weather maps lines called Isobars mark areas of equal pressure with high and low spots as roughly concentric circles. The closer these lines are together mean that the air will be moving fast. The strength of the wind is usually given in miles per hour by the weatherman, but another measure of interest to seafarers is the Beaufort Scale, devised by Admiral Beaufort in the early 19th century before proper measuring instruments were available, to indicate the amount of sail to deploy. It rises from strength 0 - "calm"; through strength (or force) 5 - "fresh breeze" - at about 20 miles per hour; to to strength 12 - "gale" and "hurricane".
Another seafarer, Admiral Fitzroy had a rule to to predict the wind strength - "Barometer falls for warm, wet, or more wind; rises for cold, dry, or less wind."

The subject of interest lately is climate change or global warming and this will have some bearing on what goes on in the garden as well as in the wider world. For example many diseases and pests are killed during cold spells so milder winters will lead to an increase in their occurance. Slugs and their eggs are killed by the frost and the fewer of these around the better.

The winter cull also kills rats which carry diseases such as plague. Recently Anopheles mosquitoes have colonised parts of Lundy Island, most likely by "jumping ship" from a cargo heading up the Bristol Channel. So far they are free from the malaria bug which they carry in other parts of the world, but the potential is there for them to be infected and warmer conditions in the rest of the British Isles could eventually lead to it's spread. Other pests and diseases can be brought due to the globalisation of commerse and a warmer climate might mean that they become endemic. Many plants, fruit and vegetables are grown all over the world to satisfy the modern taste, so there is great potential for future problems.
The change in climate is likely to lead to an increase in the severity of storms and changes in wind direction leading to a different pattern of coastal erosion. For example the South Down coastline is made up of softer rocks so would erode quite fast. There is less likelihood of coastal flooding as the land mass of Northern Ireland is still rising so cancelling out the effect of melting ice caps.

The milder winters have also meant that we can no longer pack the lawnmower away in October as there is still some growth which requires trimming throughout the winter to keep the lawn looking well. This should only be done in drier conditions and when there is no frost, otherwise the grass could be damaged.

There will be an increase in winter rainfall which will lead to more waterlogging of soil and many of the less tolerant plants will no longer survive. The summers will be drier so lawns may become brown patches. These extremes will reduce the range of plants which can be grown and could change the whole character of the traditional garden.
Weed seeds also take advantage of milder periods and continue to germinate and grow throughout winter albeit at a reduced rate. If the climate is to get warmer then the amount of maintenance will increase.

To check the current weather condition of Northern Ireland please click here

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