APPENDIX 23
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.
| Station | Maximum 10 minute Mean Speed Knots
| Beaufort Classification |
| | |
| | |
| Northern Ireland | |
|
| Hillsborough | 37 | Gale Force 8
|
| Ballykelly | 155
| Storm Force 10 |
| Gelnanne | 141 |
Severe Gale Force 9 |
| Castlederg | 48 | Storm Force 10
|
| Ballypatrick | 1, 230 | Near Gale Force 7
|
| Aldergrove | 49 | Storm Force 10
|
| St. Angelo | 1, 242 | Severe Gale Force 9
|
| Killough | 56 | Violent Storm Force 11
|
| Eire | |
|
| Malinhead, County Donegal | 68
| Hurricane Force 12 |
| Belmullet, County Mayo | 61
| 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.
| Station | Maximum Gust Knots
| Maximum Gust MPH |
| | |
| | |
| Northern Ireland | |
|
| Hillsborough | 66 | 76
|
| Ballykelly | 81 | 93
|
| Glenanne | 62 | 71
|
| Castlederg | 84 | 97
|
| Ballypatrick | 59 | 68
|
| Aldergrove | 76 | 87
|
| St Angelo | 62 | 71
|
| Killough | 80 | 92
|
| Eire | |
|
| Malinhead, County Donegal | 96
| 109 |
| Belmullet, County Mayo | 94
| 107 |
Duration
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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