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A tropical night is a term used in many European countries to describe days when the temperature does not fall under 20 °C (68.0 °F) during the night time. This definition is in use in countries including the United Kingdom, Republic of Ireland, Spain, Sweden, Norway, the Netherlands, Germany, Finland, Lithuania, Latvia, Hungary, Serbia and Croatia.
Tropical nights are common during heat waves and occur mostly over seas, coasts and lakes. Heat gets stored in the water during periods of sunny and warm weather during the day, which is then emitted during the night and keeps the night temperature up.
The Met Office began tracking 'tropical nights' in 2018. This criterion is infrequently met, with the 30 years between 1961 and 1990 seeing 44 tropical nights, most of them associated with the hot summers of 1976 and 1983. From 1991 to 11 August 2020, 84 such nights were recorded, with 21 of them occurring since 2008. Five nights that stayed above 20 °C were recorded in 2018, and four in 2019. By 11 August 2020, four tropical nights had been recorded for that year, one in June and three in August.
In Croatia, this occurrence is usually termed 'warm night' (Croatian: topla noć),: 32  but also tropska noć ('tropical night'). A 'very warm night' (vrlo topla noć) occurs when the temperature stays above 25 °C (77.0 °F) overnight. Tropical nights happen regularly at the seaside in summer, and less frequently inland. In the 1961–1990 period, there was an average of 10–20 tropical nights a month during the summer at the seaside,: 32 but less than one per year in most of continental Croatia.: 41 However, they have become more frequent in Zagreb since 2000. During 1990–2014, Zagreb recorded an increasing trend of 19.5 additional tropical nights per decade. In August 2018, the Zagreb–Grič Observatory registered 24 tropical nights, beating the previous record from 2003.
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