A season is one of the major divisions of the year, generally based on yearly periodic changes in weather.

Seasons happen due to the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution. In Temperate and Polar regions, the seasons are marked by changes in the amount of sunlight which may cause animals to go into hibernation or to migrate, and plants to be dormant.

The Northern hemisphere is closer to the sun at times of the year than the southern, it is exposed to more light causing summer to come into action at different times as the earth revolves.

In temperate and polar regions generally four seasons are recognized: spring, summer, autumn, winter.

In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, because the amount of precipitation may vary more dramatically than the average temperature. In other tropical areas a three-way division into hot, rainy and cool season is used.

In some parts of the world, special "seasons" are loosely defined based upon important events such as a hurricane season, tornado season or a wildfire season.

Chinese seasons are traditionally based on 24 periods known as solar terms, and begin at the midpoint of solstices and equinoxes.1

Causes and effects

The seasons result from the Earth's axis being tilted to its orbital plane; it deviates by an angle of approximately 23.5 degrees. Thus, at any given time during summer or winter, one part of the planet is more directly exposed to the rays of the Sun (see Fig. 1). This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the northern and southern hemispheres experience opposite seasons.

It is easy to observe the effect of the axis tilt from the change in day length, and altitude of the Sun at noon (the culmination of the Sun), during a year.

Seasonal weather differences between hemispheres are further caused by the elliptical orbit of Earth. Earth reaches perihelion (the point in its orbit closest to the Sun) in January, and it reaches aphelion (farthest point from the Sun) in July. Even though the effect this has on Earth's seasons is minor, it does noticeably soften the northern hemisphere's winters and summers. In the southern hemisphere, the opposite effect is observed.

Seasonal weather fluctuations (changes) also depend on factors such as proximity to oceans or other large bodies of water, currents in those oceans, El Niño/ENSO and other oceanic cycles, and prevailing winds.