Date of first tropical day in a year

Scatterplot above shows the date of the first tropical day observed in that particular year, i.e. the first day of that calendar year, when the temperature reached at least 30 °C. The date is based on an assessment of 133 stations in the Czech Republic with sufficiently long data series (to prevent potential bias). Years 1962 and 1968 were years when a tropical day was observed earliest (Apr 24). In contrast, latest tropical day was seen in the year 1974 (Jul 28). Linear trend clearly shows that the date of a first tropical day in a calendar year gradually shifts towards the beginning of the year, which is related to the long-term warming in the Czech Republic. On average, the first tropical day occurs 2.6 days earlier each decade.