Last updated Thursday 17 July 2025

When is meteorological summer?

Meteorological seasons are used by all National Meteorological Services. Summer then runs from 1st June to 31st August inclusive, in the Northern Hemisphere. Using meteorological seasons makes data collection and seasonal comparison much simpler and based upon the annual temperature cycle and grouping the year into four fixed three-month periods where the warmest season is Summer, coldest Winter, and transition seasons are Spring and Autumn. Therefore each season has the same start and end dates every year.

You can read more on the differences between Irish, meteorological and astronomical seasons here.

What is the climate of Ireland like in summer?

Summer in Ireland brings longer daylight hours, milder temperatures, and increased sunshine, offering many vital opportunities for outdoor activity. After the darker, wetter seasons, summer is important for many who live in a temperate, often overcast climate. Based upon the most recently climatological reference period (1991-2020), summers in Ireland are:

Source: Met Éireann. (2023). Ireland's climate averages 1991–2020 [PDF]. Retrieved from www.met.ie/climate/30-year-averages.

While summer warmth is often welcome, rising temperatures, both daytime highs and overnight minimums, do pose risks to human health, livestock, crops, and public infrastructure. Prolonged heat can strain those with chronic illness, children and outdoor workers, particularly during still, humid spells. Livestock face heat stress and water demand rises sharply. Soil moisture can drop quickly during dry periods, reducing grass growth and increasing wildfire risk in uplands. Urban areas may suffer from heat retention overnight, affecting sleep and recovery. Transport networks, energy systems, and healthcare services may come under pressure during hot weather.

Marine area

It takes longer for sea temperatures to rise and fall than air temperatures; because of this, summer sea temperature only reach their peak, around 14 to 15 °C, in August and sea temperatures remain elevated into mid-Autumn. Summer sea temperatures are slightly higher on off western coasts due to the impact of Atlantic currents.

As summer is the calmest season for wind in Ireland, it is unsurprising that it is also when Ireland's seas are at their calmest. During summer, average significant wave heights are typically 1.0 to 1.5 m in the Irish Sea and 1.5 to 2.5 m off the West coast.

Both high sea temperatures and calmer seas support water-based activities. However, temperatures are usually still low enough to carry the risk of cold water shock, particularly in the early summer. See watersafety.ie/open-water-swimming for guidance on swimming safely in Ireland's waters. These higher temperatures and calmer seas may also promote the development of algal blooms in inland waters.

Summer Extremes

Summers that were once unusual are now common, and cooler summers have become increasingly rare. Hottest and coldest in the table below mean highest or lowest average temperature, relative to its climate record.

Source: Met Éireann. (2025). Datasets: Island of Ireland Long-Term Temperature Series 1900-2024 and Ireland Gridded Rainfall Series 1941-2024

What is a heatwave?

The definition of a heatwave in Ireland is:

A heatwave occurs when five or more consecutive Summer Days (> 25.0 °C) are observed at a Met Éireann station.

There is no single global definition of a heatwave, but they are generally understood to be periods of unusually high temperatures that last a few days. Internationally, the concept of a Summer Day is often used in climatology, defined as a day when the maximum shaded air temperature exceeds 25.0 °C. Once a heatwave occurs, the average duration is 6 days long. It would be rare to have a heatwave longer than 9 days.

• The longest heatwave on record for Ireland was 14 days from Saturday 14 to Friday 27 August 1976 inclusive 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 (6 days) at four synoptics stations (Gurteen, Co Tipperary, Oak Park, Co Carlow, Shannon Airport, Co Clare and Mount Dillon, Co Roscommon) and three climate stations (Greenshill, Co Kilkenny, Athy, Co. Kildare and Listowel, Co Kerry (5 days)). This was the only heatwave observed during 2023.

Why do heatwaves happen?

Heatwaves are most common in 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.

Climate change

Heatwaves have become more frequent and more intense globally since the 1950s. Human-caused climate change is the main driver of this increase in extreme heat. Some recent heatwaves would have been extremely unlikely to occur without human influence on the climate system. According to climate projections, it is virtually certain that the duration, frequency and intensity of heatwaves will increase over most land areas. Because of global warming, the frequency, duration, and severity of heat waves are predicted to increase in most parts of the world including Ireland. The IPCC Special Report on Extremes shows that heatwaves will be more frequent, longer and more intense in the 21st century.

What you can do during a short sharp warm spell or heat wave?

• Never leave children or pets in parked cars.
• Close curtains to keep rooms cool.
• Stay in the shade or indoors during hottest hours (12–4pm)
• Drink water regularly, even if not thirsty.
• Before going outside, use high-factor sunscreen and reapply often.
• Learn signs of heat exhaustion and how to respond.

The SunSmart 5 S's poster from the HSE outlines essential steps to reduce your risk of sun damage.

Follow these practices daily, especially during high UV months in Ireland.

Click on the image across to read more on how to Be Summer-Ready and preparing you, your home and your business for summer.

Further reading

• Met Éireann: Highest Temperature Recorded in Ireland since 1887
• Met Éireann: Heatwave in parts of Europe June 2022
• Copernicus: Europe continued to swelter in July [2022]
• Met Éireann: Temperature extremes and heatwaves during July 2019
• Met Éireann: An analysis of heatwaves and droughts affected Ireland and Europe in the summer of 2018

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