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"Climate is what we expect, weather is what we get" - Mark Twain
The climate of Northern Ireland is influenced 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. This is a current of warmer water which arises off the coast of the mid-Americas and flows north easterly towards Northern Europe, enveloping the British Isles which are at the same Latitude as otherwise much colder countries. 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 throughout the region.
The largest influence which determines the rainfall and wind speeds is the Jet Stream. This is a band of fast-moving air which flows in a generally eastward direction at about six miles up in the atmosphere and generally follows the boundary between the cold polar air and the warmer air in the rest of the northern hemisphere. The existence of the Jet Stream has only been known since the 1940s when high altitude flying began - aeroplanes experienced unexpected headwinds which affected navigational calculations.
There are a number of Streams which encircle the globe. The Northern Jet Stream arises above North West Africa and flows eastward, but moving slightly northward in a spiral over Southern Asia, across the Pacific Ocean, North America, the Atlantic Ocean and ends over the British Isles. The route can vary giving a wavy motion so it fluctuates to the North and South. This meandering causes the changes to our weather. The temperature at the northern edge can also be different to that on the southern edge, this temperature gradient causes spiralling currents of air. These give rise to hurricanes and tornadoes. If the Jet Stream moves slightly the weather can change dramatically, as it did in the summer of 2007, which resulted in the flash floods and the highest rainfall figures ever recorded in a British summer. It can also be 'blocked' by air currents from mainland Europe as in 1963 when there were months of freezing conditions.
The record low temperatures in December 2010 were due to the Jet Stream moving to the south of the British Isles which allowed cold easterly and northerly air to flow south-westerly and sit over the area. The record low temperatures around Christmas 2010 were again due to the blocking effect of air currents from continental Europe.
Extremes of temperature are quite rare in Northern Ireland, 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 - though this was exceeded in the winter 2010-11 cold spell with a new record low of minus 18 C at Castlederg in Co Tyrone.
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.
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, the faster the air moves along the gradient giving rise to stronger winds. The strength of the wind is usually given in miles per hour by the weatherman.
Nature of Wind Miles per Hour Force in pounds
per square foot
Perceptable 1 to 3 0.005 to 0.044 Gentle Breeze 5 to 5 0.079 to 0.123 Breeze 10 to 15 0.490 to 0.107 Brisk Wind 20 to 25 1.96 to 3.07 High Wind 30 to 35 4.42 to 6.02 Very High Wind 40 to 45 7.87 to 9.96 Strom 50 12.3 Hurricane 80 to 100 31.48 to 49.2
The Beaufort Scale used by seafarers to indicate the amount of sail to deploy, was devised by Admiral Beaufort in the early 19th century before proper measuring instruments were available:-
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."
force 0 "calm" force 5 "fresh breeze" about 20 miles per hour force 8 "gale" force 12 "hurricane"
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. Studying the periodic changes of plants and animals that are influenced by seasonal changes is known as Phenology. Observations are recorded annually when particular events occur, such as when leaf buds break in the spring, when plants flower or when migrating birds arrive and leave. The first records were kept by interested individuals, but in 1875 the Royal Meteorological Society established a national network of record keeping with the data published annually. This ended in 1948 but the increased awareness of climate change prompted the Woodland Trust to join with the Centre for Ecology & Hydrology in Cambridge to promote a revived scheme in 2000 called the Nature's Calendar survey which now has about 50,000 people involved.
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