The season is each of the time periods into which the calendar year is divided.
There are different ways of defining a season: the most traditional is the astronomical subdivision, for which a season is the time interval between an equinox and a solstice. In this case, four seasons are distinguished: spring, summer, autumn, winter; each of them has a constant duration of three months and well defined throughout the year, regardless of latitude and geographical location.
Then there is the meteorological subdivision, which instead takes into account the climatic and environmental changes that occur in a given place during the year, and therefore does not necessarily coincide with the astronomical subdivision of the seasons.
Seasons according to latitudes
In particular, the meteorological subdivision of the seasons varies approximately on the basis of the following astronomical zones, due to the inclination of the earth's axis and latitude:
in temperate zones there are generally four meteorological seasons more or less similar to astronomical ones, but their duration varies according to the latitude and the local microclimate induced by the surrounding geography;
in the polar regions there are generally only two seasons (often called the midnight sun and polar night, or simply summer and winter) determined by the presence or absence of the sun above the horizon.
finally, also in tropical areas, it is preferred to divide the year into only two seasons, defining them as the rainy season and the dry season (although there are often also a hot and a cold season), determined by the main annual climatic changes.
The influence of the Earth's inclination on the seasons
As already mentioned, the change of seasons is determined by the inclination of the Earth's rotation axis, which changes the angle of incidence of the sun's rays on its surface.
When a hemisphere is in winter, this is due to the fact that the sun's rays hit the surface with a lesser inclination than the horizon; as a consequence there is a lower degree of irradiation, the atmosphere and the surface absorb less heat and the whole hemisphere is colder. Conversely, when in a hemisphere it is summer, the rays tend to perpendicular to the horizon and both the atmosphere and the surface absorb more heat, with a consequent increase in temperature.
The effect of the seasons is increasingly evident as we move from the equator towards the poles because, due to the different inclination of the earth's surface with respect to the sun's rays, the difference in heat absorbed between the condition of maximum irradiation and that of minimum irradiation becomes bigger and bigger with increasing latitude. The cycle of seasons of one hemisphere is the opposite of that of the other. When it is summer in the northern hemisphere it is winter in the southern hemisphere and when it is spring in the northern hemisphere it is autumn in the southern hemisphere.
The inclination is about 23 ° 27 'with respect to the perpendicular to the plane of the ecliptic.
If the axis of rotation were perfectly perpendicular to the orbital plane, astronomical seasons would not exist, as exposure to heat and light in a given portion of the planet would be constant throughout the year.
The equator, with the Sun perpetually at the zenith, would have the maximum insolation, while the poles would always be cold, with the Sun constantly on the horizon line; we would not speak of tropics (the latitudes closest to the equator where the Sun can reach its zenith) and polar circles (the latitudes closest to the poles, in which there is at least one day without light); the climate is generally determined only by latitude and not by the time of year; the duration of the night would be equal to that of the day anywhere on the Earth (i