The 2021 Britain and Ireland heat wave was a period of unusually hot weather in July 2021 that led to record-breaking temperatures in the UK and Ireland.

On 19 July, the Met Office issued its first ever extreme heat warning for parts of the UK.^[1][2] Temperatures soared across the United Kingdom over a weekend which saw all four nations record the hottest day of the year.^[3][4] On 17 July, temperatures reached 31.2 °C (88.2 °F) in County Down, Northern Ireland.^[5] On 18 July, temperatures reached 31.6 °C (88.9 °F) at Heathrow Airport, London and 30.2 °C (86.4 °F) in Cardiff, Wales.^[6][7]

In the Republic of Ireland, Met Éireann issued its first ever Status Orange high temperature warning for six counties on 20 July,^[8] after temperatures reached 29.5 °C (85.1 °F) in Athenry, County Galway on 17 July.^[9]

On 21 July, temperatures reached 32.2 °C (90.0 °F) at Heathrow, London.^[10] In Ireland, temperatures reached 30.8 °C (87.4 °F) at Mount Dillon, County Roscommon. In Northern Ireland a new record for maximum temperature was set, with 31.3 °C (88.3 °F) was reached at Castlederg, County Tyrone.^[11]

Despite the extreme weather warning, the 2021 July heatwave was relatively mild in comparison to heatwaves in the UK and Ireland of previous and subsequent years, with heatwaves in previous years surpassing the maximum temperature of this heatwave, particularly in the south east.^[12][13][14]