The dates of the seasons vary according to the reference used. Calendar, astronomical and meteorological seasons do not begin and end at the same dates. What is the difference between these different seasons?
The seasons depend on the quantity of energy (heat) that the Earth receives from the Sun. This quantity is determined by the inclination of the Earth’s axis of rotation and its position in relation to the Sun. During the boreal summer, the Earth’s orientation allows the northern hemisphere to receive most of the heat with the Sun at its zenith at the level of the Tropic of Cancer (23,5°N) on the summer solstice. On the winter solstice the Sun is at its zenith at the level of the Tropic of Capricorn (23,5°S) and the northern hemisphere receives a lot less heat. Between the two solstices, the spring and autumn equinoxes correspond to the times when the Sun is at its zenith at the level of the Equator and the two hemispheres receive a similar amount of sunshine.
The dates of the equinoxes and solstices determine the calendar seasons. In the northern hemisphere spring begins on the spring equinox (around March 20), summer begins on the summer solstice (around June 21), autumn begins on the autumn equinox (around September 22) and winter on the winter solstice (around December 21).
The astronomical seasons are determined by the duration of sunshine. In Switzerland the period of maximum sunshine occurs around the northern summer solstice, meaning between May 6 and August 7 approximately. Winter corresponds to the period of minimum sunshine around the winter solstice between November 7 and February 4 approximately. The period from February 5 to May 5 corresponds to spring and the period from August 8 to November 6 to autumn. However, it should be noted that the term astronomical seasons is often used when talking about calendar seasons.
Meteorological seasons are staggered in relation to calendar and astronomical seasons. This can be explained by the atmosphere’s inertia. In summer, for example, although Switzerland receives maximum sunshine around June 21, the maximum temperatures are recorded some three weeks later around the middle of July. Meteorological summer therefore begins on June 1 and ends on August 31. The lowest temperatures are recorded around the middle of January: meteorological winter extends from December 1 to February 28. For climatological statistics it is also easier to separate the seasons into periods of 3 full months. Thus the months of March, April and May correspond to spring; the months of June, July and August to summer; the months of September, October and November to autumn and the months of December, January and February to winter.