Down Garden Services

CLIMATE

"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.

Extremes of temperature are 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. 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:-

force 0	"calm"
force 5	"fresh breeze"	about 20 miles per hour
force 8	"gale"
force 12	"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."

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Climate Change

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 pests and diseases are killed during cold spells so milder winters will lead to an increase in their occurrence. 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 globalisation, 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.
Some alien garden pests have moved here from continental Europe in recent years and are moving northward within the British Isles. The Red Lily Beetle was first reported in the south of England in the 1940s and remained there for many years due to the colder northern conditions, but recently it has progressed until it is now here in Northern Ireland. Other insect pests following a similar pattern are the Rosemary Beetle and the Berberis Sawfly.

The types and severity of plant diseases are going to be influenced by the drier conditions expected in summer. Plants will be under stress which makes them more susceptible. Drier roots encourages Powdery Mildew and the waterlogging expected in winter will make rots more prevelant. On the positive side, Potato Blight will be discouraged by the drier summers.

The lengthening of the growing period can have serious effects on the breeding in animals. Hedgehogs have been producing late litters of young which are unable to feed for long enough to store reserves to carry them through hibernation. Also birds which feed their young on caterpillars start their mating process in response to lengthening daylight which is remaining constant - Photoperiodism. If the spring comes earlier due to a rise in temperature, the plant growth and therefore the time when insects reproduce, will be earlier as well, so the food for the nestlings will be gone.

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. The spring flowers come earlier so this is exacerbating the problem of pollen allergies and when it is available for insects.
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. If plants can be kept relatively dry during the winter they can tolerate fairly low temperatures, but if the soil remains wet for prolonged periods they will rot.
The summers would be expected to be relatively drier so lawns may become brown patches, but there is evidence that the Jet Stream is drifting slightly northward so it is likely that there may be wetter periods, with flash-flooding. This change in the position is thought to be due to the rise in temperatures.
These extremes will reduce the range of plants which can be grown and could change the whole character of the traditional garden. Fruit production has already been affected by the milder winters of the past few years. Blackcurrants have been ripening over longer periods causing problems for commercial growers especially, who require them to ripen together for mass harvesting - also in the garden for jam-making. It is thought that the fruiting buds need a period of cold in the dormant season to keep them all synchronised.

Weed seeds also take advantage of milder periods, continuing to germinate and grow throughout winter albeit at a reduced rate. If the climate is to get warmer then the amount of garden maintenance will increase.

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 as an after-effect of the last ice age when it sank under the weight of the ice, so possibly cancelling out the effect of melting ice caps.

Climate Change  Weather Folklore   Local Weather Forecast      /\ top

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