Very Dry, Cool and Sunny
April was a very dry month with high pressure dominating. The high-pressure systems often set up to the west or northwest of Ireland, which brought cool polar air masses over the country on several occasions through the month, leading to below average temperatures everywhere, but also above average sunshine. The first week saw high pressure to the northwest build towards Greenland, allowing a very cold polar air mass sweep south over the country in a northerly airflow. This brought rain or showers, some wintry, towards the end of the first week but amounts stayed low. The cold northerly airflow re-established during the second week as high pressure intensified once again to the northwest of Ireland with low pressure moving over Scandinavia. There were occasional showers, again some wintry and with hail, but the week finished dry as high-pressure developed directly over Ireland. The high pressure moved towards Scandinavia during the third week and some weak weather fronts crossed the country, but rainfall amounts again stayed low, especially in the South and East. At the beginning of the fourth week, high pressure again built over Ireland and gradually moved to the north, bringing a dry easterly airflow over the country for most of the week. This brought a few warm days, especially in the Midlands and West, before low pressure moved south over the UK towards the end of the month, establishing a cool and showery north-easterly airflow over the Ireland.
Rainfall: Very Dry Everywhere
All monthly rainfall totals across the country were below their Long-Term Average (LTA). Percentage of monthly rainfall values ranged from 17% (monthly rainfall total of 12.3 mm) at Johnstown Castle, Co Wexford to 59% (monthly rainfall total of 42.4 mm) at Belmullet, Co Mayo. Monthly rainfall totals ranged from 10.8 mm (20% of its LTA) at Dublin Airport, Co Dublin to 45.6 mm (47% of its LTA) at Newport, Co Mayo. The highest daily rainfall total was 9.3 mm at Roche’s Point, Co Cork on Thursday 29th. The number of rain days ranged from 8 days at Johnstown Castle, Co Wexford to 17 days at both Belmullet, Co Mayo and Knock Airport, Co Mayo. The number of wet days1 ranged from 4 days at Johnstown Castle, Co Wexford to 16 days at Belmullet, Co Mayo. There were no very wet days3 reported this month. Four stations had their driest April on record. These were Oak Park, Co Carlow with 14.4 mm (26% of its LTA) (record length 15 years), Sherkin Island, Co Cork with 13.5 mm (18% of its LTA) (record length 48 years), Gurteen, Co Tipperary with 13.4 mm ( 22% of its LTA) (record length 13 years) and Johnstown Castle, Co Wexford with 12.3 mm (17% of its LTA) (record length 80 years). Three stations had dry spells6 between the 12th and 28th April. These were Dublin Airport, Co Dublin, Casement Aerodrome, Co Dublin and Johnstown Castle, Co Wexford. Johnstown Castle, Co Wexford also had a partial drought5 between 28th March and 27th April.
Temperature: Cool Nearly Everywhere
Nearly all mean air temperatures were below their Long-Term Average (LTA) for the month. Deviations from mean air temperature ranged from -2.5 °C (5.6 °C, the month’s lowest mean temperature) at Dublin Airport, Co Dublin (its coldest April on record (length 78 years)) to 0.0 °C (9.1 °C, the month’s highest mean temperature) at Newport, Co Mayo. The month’s highest mean temperature of 9.1 °C was also recorded at Valentia Observatory, Co Kerry (0.2 °C below its LTA). The month’s highest temperature was reported at Valentia Observatory, Co Kerry on Thursday 1st with a temperature of 21.2 °C (its highest April temperature since 2011). The month’s lowest air minimum of -4.7 °C was reported on Saturday 10th and Sunday 11th at both Dublin Airport, Co Dublin and Casement Aerodrome, Co Dublin respectively, while the lowest grass minimum was -9.6 °C reported at both Ballyhaise, Co Cavan and Mount Dillon, Co Roscommon on Saturday 10th. All stations reported ground frost during the month. The number of days with ground frost ranged from 7 days at both Sherkin Island, Co Cork and Belmullet, Co Mayo to 26 days at Dublin Airport, Co Dublin. More than half of stations reported air frost. The number of days with air frost ranged from zero days at a few stations to 19 days at Dublin Airport, Co Dublin.
Sunshine: Above Average Everywhere
All available sunshine totals were above their Long-Term Average (LTA). Percentage of monthly sunshine values ranged from 131% (monthly sunshine total of 209.8 hours) at Cork Airport to 141% (monthly sunshine total of 218.5 hours) at Shannon Airport, Co Clare. Monthly sunshine totals ranged from 202.4 hours (133% of its LTA) at Casement Aerodrome, Co Dublin to 236.4 hours (No LTA comparison*) at Valentia Observatory, Co Kerry. The highest number of daily sunshine hours recorded this month was 14.0 hours at Gurteen, Co Tipperary on Sunday 25th. The number of dull days2 ranged from 2 days at a few stations to 5 days at Belmullet, Co Mayo.
Wind: Gales Reported
Monthly mean wind speeds ranged from 4.9 knots (9.1 km/h) at Ballyhaise, Co Cavan to 12.4 knots (23.0 km/h) at Malin Head, Co Donegal. Gales were reported on Monday 5th. The number of days with gale force winds ranged from zero days at most stations to 1 day at Malin Head, Co Donegal. 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 Malin Head, Co Donegal on the Monday 5th. The highest gust was 46 knots (85 km/h) while the month’s highest 10-minute mean wind speed was 35 knots (65 km/h).
The full report is available here.
Extreme Values at synoptic stations for April 2021
Rainfall % of 1981-2010 Monthly Average – April 2021 (Provisional)
Total Monthly Rainfall (mm) for March 2021 (Provisional)