Select Committee on Northern Ireland Affairs Minutes of Evidence


Memorandum submitted by the Meteorological Office

  Thank you for your letter of 18 March about the Boxing Day storm in Northern Ireland.

Mean Wind Speeds

  The Met. Office classifies wind events by the highest record 10-minute mean wind speed. The highest 10-minute wind speeds recorded at anemometer stations in Northern Ireland and at neighbouring Met. Eireann Stations on 26 December 1998 are shown in the following table.
StationMaximum 10 minute Mean Speed Knots Beaufort Classification
Northern Ireland
Hillsborough37Gale Force 8
Ballykelly155 Storm Force 10
Gelnanne141 Severe Gale Force 9
Castlederg48Storm Force 10
Ballypatrick1, 230Near Gale Force 7
Aldergrove49Storm Force 10
St. Angelo1, 242Severe Gale Force 9
Killough56Violent Storm Force 11
Malinhead, County Donegal68 Hurricane Force 12
Belmullet, County Mayo61 Violent Storm Force 11
1 Where recorded 10-minute mean wind speeds are not readily available, the 10-minute wind speed has been statistically calculated from the recorded hourly mean wind speed.
2 A full record is not available from these stations as power was lost from mid-afternoon.

  It is worth noting that Met. Office anemometer stations in Northern Ireland are mostly sited on low, open ground. Orographic effects can greatly enhance winds and it is therefore reasonable to infer that winds greater than Force 10 occurred in some inland parts of Northern Ireland.

Gust Speeds

  The Boxing Day storm was characterised by the strength of gust speeds, that is wind speed measured over a 3-second period, and the highest gusts recorded on 26 December are shown in the following table. The maximum gust at Aldergrove set a new record on the highest gust recorded there since records began in 1928.
StationMaximum Gust Knots Maximum Gust MPH
Northern Ireland
St Angelo6271
Malinhead, County Donegal96 109
Belmullet, County Mayo94 107


  The duration of a wind event is based on mean hourly wind speeds. Our records show that the duration of the storm at Aldergrove was 10 consecutive hours based on Gale Force 8 criteria and five consecutive hours based on Severe Gale Force 9 criteria. These durations are also the longest on record.

Return Periods

  To estimate the return of a storm, we use a statistical technique known as Extreme Value Analysis. In short, this involves picking out the largest gust speed in each year of record and then putting a statistical distribution to these values. Aldergrove has the longest continuous wind records for any location in Northern Ireland dating back to 1928. Unfortunately, the other operational stations listed above do not have sufficient duration of record on which to calculate Return Periods. Based on the Aldergrove records, Extreme Value Analysis suggests that the return period of the storm lies in the 200-400 years range. It should be noted however that, given the amount of extrapolation involved, Extreme Value Analysis is an inexact science, with changes in instrumentation and exposure over time adding further uncertainty. We have therefore, carried out the analysis over a shorter time period, when these factors were constant. This has resulted in a fairly wide range to the possible return period. Nevertheless, it is evident that the Return Period of this storm can be considered to be over 100 years at Aldergrove. Put another way, there is less than one in 100 chance of this type of storm occurring in any year, (based on the assumption that such storms are independent). Of course, this means there could be a similar event next year.

12 April 1999

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