PARTS OF IRELAND are currently in the middle of a heatwave, but what exactly counts as a heatwave?

There is no single global definition of a heatwave, but Met Éireann notes that they are typically understood to be periods of unusually high temperatures that last a few days.

Just how hot it needs to be, and for how long, to declare a heatwave varies.

In Ireland, the definition of a heatwave is when five or more consecutive Summer Days of 25 degrees are observed at a Met Éireann station.

Today, for the fifth consecutive day, three weather stations recorded temperatures of 25 degrees of more and therefore have reached the heatwave criteria.

These three stations are Oak Park in Co Carlow, Moore Park in Co Cork, and Gurteen in Co Tipperary.

A Met Éireann spokesperson told The Journal that the World Meteorological Organisation recommends that heatwave temperatures should occur with the 95th – 99th percentile range and therefore “rare to the region”.

“As different countries experience different heat, different Met Services have different heatwave definitions,” added the spokesperson.

Met Éireann added that when analysing Ireland’s temperatures, the optimum definition for a heatwave is the one chosen, five or more days with temperatures greater than 25 degrees.

In the UK, the temperature threshold for most regions in also 25 degrees but the duration is shorter, at only three consecutive days.

In 2022, rising temperatures meant a change to the heatwave temperature thresholds for eight counties in England.

Six counties in England moved from a 27°C threshold to 28°C, one from 26 to 27°C, and one from 25 to 26°C.

Moving back to Ireland, Met Éireann said that when a heatwave occurs, the average duration is six-days long.

The forecaster noted that “it would be rare to have a heatwave longer than nine-days”.

The longest heatwave on record for Ireland was 14-days from Saturday 14 to Friday 27 August 1976, at both Birr Castle, Co Offaly (highest 28.2 °C) and Ballybrittas, Co Laois (highest 29.5 °C).

The last time a heatwave was observed in Ireland began Monday 4 to Saturday 9 September 2023, a period of six-days.

This was recorded at four stations: Gurteen, Co Tipperary; Oak Park, Co Carlow; Shannon Airport, Co Clare; and Mount Dillon, Co Roscommon.

Heatwaves were also recorded during this period over the course of five days at three climate stations, in Greenshill, Co Kilkenny, Athy, Co. Kildare and Listowel, Co Kerry.

Meanwhile, Met Éireann said that heatwaves are most common during summer when high pressure develops over the country.

High pressure systems, also known as anticyclones, are slow moving and can persist for a prolonged period of time, resulting in dry and settled weather.

In anticyclones, cool air descends from aloft and diverges outwards at lower levels – as the air descends, it is warmed by compression.

The current spell of hot weather is being driven by a plume of very warm air moving north from Europe, where several countries have experienced extreme heat in recent days.

Elsewhere, three stations provisionally recorded tropical nights last night.

Tropical nights are rare in Ireland and occurs when temperatures do not drop below 20 degrees during a 24-hour period.

A tropical night has occurred, provisionally, at Oak Park in Co Carlow, Casement Aerodrome in Co Dublin, and at Cork Airport.

Climate change

Heatwaves have become more frequent and more intense globally since the 1950s and Met Éireann said human-caused climate change is the main driver of this increase in extreme heat.

The forecaster said some recent global heatwaves would have been “virtually impossible” without human influence on the climate system.

Climate change has also been linked to Ireland’s warmest summer on record, which was recorded last year.

Average minimum temperatures experienced last year were estimated to be around 40 times more likely than in a pre-industrial climate.

Met Éireann said that climate projections for Ireland indicate that there will be increases in summer days (maximum temperature above 25 °C), warm nights (minimum temperature above 15 °C) and heatwaves.

The forecaster added that it is “virtually certain that increases in the frequency and magnitude of warm daily temperature extremes and decreases in cold extremes will occur in the 21st century at the global scale”.