Northern Ireland is one of the windier parts of the UK, with the windiest areas being over the highest ground and along the coasts of Counties Antrim and Down.
The strongest winds are associated with the passage of deep areas of low pressure close to or across the UK. The frequency and strength of these depressions is greatest in the winter half of the year, especially from November to January, and this is when mean speeds and gusts (short duration peak values) are strongest.
The variation in monthly mean speeds (average of a continuous record) and highest gusts ('instantaneous' speed averaged over about 3 seconds) at Aldergrove is shown below.
Another measure of wind exposure is the number of days when gale force is reached. If the wind reaches a mean speed of 34 knots or more over any 10 consecutive minutes, then that day is classed as having a gale. At low altitudes in Northern Ireland, gales occur most frequently on the coasts of Antrim and Down with about 15 days of gale each year on average. The number of days decreases inland to 5 days or fewer around and west of Lough Neagh. Wind speed is sensitive to local topographic effects and land use - places sheltered by hills or in urban areas will have lower wind speeds and fewer days of gale.
There have been several noteworthy gales affecting Northern Ireland, accompanied by property damage and disruption to travel and power supplies. The highest gust recorded at a low-level site is 108 knots (125 m.p.h.) at Kilkeel (Co. Down) on 12 January 1974. This storm damaged many buildings, blew down trees and interrupted the power supply to 150,000 consumers. More recently, gales on 7-8 January 2005 with gusts up to about 75 knots also caused some tree damage and power cuts to 4000 homes.
Wind direction is defined as the direction from which the wind is blowing. As Atlantic depressions pass the UK the wind typically starts to blow from the south or south west, but later comes from the west or north-west as the depression moves away. The range of directions between south and north-west accounts for the majority of occasions and the strongest winds nearly always blow from these directions.
The annual wind rose for Aldergrove is typical of low lying, inland locations in Northern Ireland, with a prevailing south-westerly wind direction through the year. However, there is a high frequency of north, north-east and easterly winds in Spring.
