Beginning of seasons [series]

The maps show the beginning of the different seasons indicated by the average date of the normal period 1961-1990. The analysis is based on averages from a large number of observation sites. The map is published in the Swedish National Atlas Climate, Lakes and Watercourses. SMHI defines winter as the period when the average temperature of the day is permanently 0 degrees or lower. If the average daily temperature is 0.0 °C or lower for five consecutive days, we say that winter arrived the first of these days. Even if there is a return to higher temperatures thereafter, it still counts as winter. The meteorological definition of spring is that the daily average temperature should be rising and be between 0 and 10 degrees plus degrees. If the average daily temperature is in the range from 0.1 °C to 9.9 °C, we call this a day with spring temperature. If this happens seven days consecutively, we say that spring arrived the first of these days. Although there will be a return to lower temperatures thereafter, it still counts as spring. SMHI’s definition of ours contains an additional condition. The start of the spring may not take place before 15 February. This is normally the time for high winter in southern Sweden. The meteorological definition of summer is that the daily average temperature shall be at least 10 degrees per day. If the average daily temperature is 10.0 °C or higher five consecutive days, we say that summer arrived the first of these days. Although there will be a return to lower temperatures thereafter, it still counts as summer. The meteorological definition of autumn is that the mean daily temperature should be lower and be between 0 and 10 degrees plus degrees. If the average daily temperature is 9.9 °C or lower for five consecutive days, we say that autumn arrived the first of these days. Even if there is a return to higher temperatures thereafter, it still counts as autumn. As a further condition, autumn cannot start before 1 August. It’s a week after the average date for the hottest day of the year. For more info see: https://www.smhi.se/kunskapsbanken/meteorologi/arstider-1.1082