Climate Statement for December 2020
Cool, wet and windy at times
Blocking high-pressure systems to the east and at times to the west and north, led to a pattern with deep Atlantic troughs of low-pressure repeatedly stalling close to or over Ireland during December 2020. This kept Ireland on the cool, northern side of a highly amplified, meandering Jetstream. After a few dry days at the beginning of December, a trough of low-pressure moved in from the northwest and stalled over Ireland and the UK for most of the first week in a cyclonic or northerly airflow. This brought strong winds, with bands of rain or showers at first, but also some dry, cold days with frost and fog at night towards the end of the week as the low filled. This setup continued during the second and third weeks as further deep troughs of low pressure stalled just to the west of Ireland bringing further frontal depressions across the country in a cyclonic or westerly airflow. Active weather fronts brought widespread heavy rain on the 9th and 13th. A transient area of high pressure brought a few cold, dry days over the Christmas period, before more active weather fronts, associated with storm Bella, brought a spell of very wet and windy weather across the country from the northwest on the 26th and 27th. Storm Bella, which intensified to the south of Greenland, moved south-eastwards and stalled just to the east of Ireland, establishing a cold northerly airflow for the remainder of the month, with further bands of rain or showers, some wintry.
Rainfall: Above average in most places, highest in the South and West
The majority of monthly rainfall totals were above their Long-Term Average (LTA). Percentage of monthly rainfall values ranged from 94% (monthly rainfall total of 89.3 mm) at Mullingar, Co Westmeath to 157% (monthly rainfall total of 195.6 mm) at Claremorris, Co Mayo. Monthly rainfall totals ranged from 77.9 mm (103% of its LTA) at Casement Aerodrome, Co Dublin to 244.2 mm (148% of its LTA) at Valentia Observatory, Co Kerry. The highest daily rainfall total was 27.5 mm at Newport, Co Mayo on Sunday 13th. The number of rain days ranged from 20 days at Dublin Airport to 31 days at a few stations. The number of wet days1 ranged from 14 days at Dublin Airport to 30 days at Valentia Observatory, Co Kerry. The number of very wet days3 ranged from 1 day at Mullingar, Co Westmeath to 8 days at Valentia Observatory, Co Kerry.
Temperature: Below average in most places
Nearly all mean air temperatures were below their Long-Term Average (LTA) for the month. Deviations from mean air temperature ranged from -0.9 °C (4.9 °C mean temperature) at Markree, Co Sligo to 0.3 °C (5.8 °C mean temperature) at Finner, Co Donegal. Mean temperatures for the month ranged from 4.3 °C (at its LTA) at Knock Airport, Co Mayo (its coldest December since 2010) to 7.8 °C (0.2 °C below its LTA) at Sherkin Island, Co Cork (its coldest December since 2010). The month’s highest temperature was reported at Dublin Airport on Friday 18th with a temperature of 14.2 °C. The month’s lowest air minimum was recorded on Monday 7th at Dunsany, Co Meath with -4.8 °C while the lowest grass minimum was -9.4 °C reported at Athenry, Co Galway on Thursday 31st. All stations reported ground frost during the month. The number of days with ground frost ranged from 6 days at Malin Head, Co Donegal to 21 days at Markree, Co Sligo. More than half of stations reported air frost. The number of days with air frost ranged from zero days at a few stations to 11 days at Dunsany, Co Meath. 22 stations had their coldest December since 2010.
Sunshine: Highest in the South and East
All available sunshine totals were above their Long-Term Average (LTA). Percentage of monthly sunshine values ranged from 113% (monthly sunshine total of 54.0 hours) at Casement Aerodrome, Co Dublin to 145% (monthly sunshine total of 76.3 hours) at Cork Airport, Co Cork (its sunniest December since 2010). Monthly sunshine totals ranged from 31.2 hours (No LTA comparison*) at Belmullet, Co Mayo to 73.3 hours (No LTA comparison*) at Johnstown Castle, Co Wexford. The highest number of daily sunshine hours recorded this month was 7.0 hours at both Johnstown Castle , Co Wexford on Sunday 6th and Casement Aerodrome, Co Dublin on Saturday 12th. The number of dull days2 ranged from 8 days at Johnstown Castle, Co Wexford to 17 days at Malin Head, Co Donegal.
Wind: Storm force winds reported
Monthly mean wind speeds ranged from 6.5 knots (12.0 km/h) at Ballyhaise, Co Cavan to 18.0 knots (33.3 km/h) at Malin Head, Co Donegal. Gales were reported on 14 days during the month with strong gales reported on the 4th, 16th, 26th, 27th and 28th. Mean wind speeds reached storm force on Friday 4th at Malin Head, Co Donegal. The number of days with gales ranged from zero days at Dublin Airport, Co Dublin to 8 days at both Roche’s Point, Co Cork and Malin Head, Co Donegal. The number of days with up to strong gales ranged from zero days at most stations to 3 days at Mace Head, Co Galway, Sherkin Island, Co Cork and Malin Head, Co Donegal. The number of days with storm force winds was 1 day at Malin Head, Co Donegal on Friday the 4th. The month’s highest gust was reported at Mace Head, Co Galway on Saturday 26th during storm Bella with 66 knots (122 km/h). The month’s highest 10-minute mean wind speed was 51 knots (95 km/h) at Malin Head, Co Donegal on Friday 4th.
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