Warm, dry for most and dull
The very dry weather from the end of August continued for most of the first week of September as high-pressure to the north of Ireland slowly pulled away to the east. Atlantic weather fronts brought rain to the West and North on the 5th, but the South and East stayed mostly dry. Cloudy conditions during the first half of the week broke up to give some warm sunshine towards the end of the week. Mist, fog and drizzle were common features at night. Low-pressure moved up from the south at the beginning of the second week giving some widespread heavy and sometimes thundery rain or showers. The rest of the sec-ond week continued mostly cloudy with scattered showers and slack winds, but stayed relatively warm, as a weak area of high pressure passed from west to east over the country. The third and most of the fourth weeks saw low-pressure to the north and high-pressure to the south steer a mild and cloudy tropical maritime air mass over Ireland in a westerly airflow. This brought several Atlantic weather fronts across the country with some spells of rain or showers, but amounts stayed generally low, especially in the East. An active cold front, which brought widespread heavy rain on the 26th and 27th, introduced a much cooler polar maritime air mass. This marked a change to more autumnal conditions that continued to the end of the month with rain or showers on each day.
Rainfall: Below average in most places, driest in the East
The majority of monthly rainfall totals across the country were below their Long-Term Average (LTA). Percentage of monthly rainfall values ranged from 53% (monthly rainfall total of 37.7 mm) at Dunsany, Co Meath to 133% (the month’s highest monthly rainfall total of 147.8 mm) at Knock Airport, Co Mayo. Monthly rainfall totals were lowest at Phoenix Park, Co Dublin with 33.4 mm (56% of its LTA). The highest daily rainfall total was 31.6 mm at Mace Head, Co Galway on Wednesday 8th. The number of rain days ranged from 8 days at Phoenix Park, Co Dublin to 23 days at both Markree, Co Sligo and Valentia Obser-vatory, Co Kerry. The number of wet days1 ranged from 7 days at Phoenix Park, Co Dublin to 18 days at a few stations in the West. The number of very wet days3 ranged from zero days at a few stations to 7 days at Newport, Co Mayo. Seven stations had absolute droughts4 between Sunday 22nd August and Wednes-day 8th September. These were Phoenix Park, Co Dublin, Oak Park, Co Carlow (lasting 17 days), Shannon Airport, Co Clare, Johnstown Castle, Co Wexford, Athenry, Co Galway, Valentia Observatory, Co Kerry (lasting 16 days) and Casement Aerodrome, Co Dublin (lasting 15 days). Fourteen stations had dry spells6 between Saturday 21st August and Wednesday 8th September (lasting between 15 and 18 days).
Temperature: Above average everywhere, record breaking in places
All mean air temperatures across the country were above their Long-Term Average (LTA) for the month. Deviations from mean air temperatures ranged from 1.1 °C (14.3 °C mean temperature) at Malin Head, Co Donegal to 2.3 °C (15.5 °C mean temperature) at Phoenix Park, Co Dublin. Mean temperatures for the month ranged from 13.6 °C (1.6 °C above its LTA) at Knock Airport, Co Mayo to 15.7 °C at both Sherkin Island, Co Cork (1.4 °C above its LTA) and Shan-non Airport, Co Clare (1.5 °C above its LTA) (its warmest September on record (length 71 years)). The month’s lowest temperatures were recorded on Wednesday 29th with the lowest air minimum reported at Athenry, Co Galway with a temperature of 3.1 °C and the lowest grass minimum reported at Mullingar, Co Westmeath with -0.5 °C. The highest maximum temperature of 27.9 °C was reported at both Shannon Airport, Co Clare (its highest max temperature for September on record (length 75 years)) and Valentia Observatory, Co Kerry (its highest max temperature for September since 1991), both on Tue 7th. There was no air frost reported this month. Two stations reported ground frost. The number of days with ground frost ranged from zero days at Phoenix Park, Co Dublin to 1 day at both Mullingar, Co Westmeath and Claremorris, Co Mayo. Along with Shannon Airport, nine other stations had their highest maximum temperature for September on record, all on Tuesday 7th (record lengths ranging from 11 to 75 years). Along with Shannon Airport, 11 other Stations had their warmest September on record (record lengths ranging from 11 to 75 years).
Sunshine: Dull, lowest in the Northwest
Percentage of monthly sunshine values ranged from 75% (monthly sunshine total of 89.6 hours) at Shannon Airport, Co Clare to 79% (monthly sunshine total of 99.0 hours) at Casement Aerodrome, Co Dublin. Monthly sunshine totals ranged from 75.8 hours (No LTA comparison*) at Belmullet, Co Mayo to 102.6 hours (No LTA comparison*) at Dublin Airport, Co Dublin. The highest number of daily sunshine hours recorded this month was 13.0 hours at Malin Head, Co Donegal on Wednesday 1st. The number of dull days ranged from 7 days at Dublin Airport, Co Dublin to 12 days at Cork Airport, Co Cork.
Wind: Gales reported
Monthly mean wind speeds ranged from 4.3 knots (8.0 km/h) at Moore Park, Co Cork to 12.1 knots (22.4 km/h) at Mace Head, Co Galway. Gales were report-ed on Sunday 26th and Wednesday 29th. The number of days with gale force winds ranged from zero days at most stations to 2 days at Belmullet, Co Mayo. There were no strong gales or storms reported this month. Both the month’s highest gust and 10-minute mean wind speed was reported at Belmullet, Co Mayo. The highest gust was on Sunday 26th and Wednesday 29th, respectively with 47 knots (87 km/h) while the month’s highest 10-minute mean wind speed was 36 knots (67 km/h) – Gale Force 8 – on Wednesday 29th.
Extreme values at synoptic stations – September 2021
Rainfall % of 1981-2010 Monthly Average – September 2021 (Provisional)
