Concepts & Causes of Seasons

A season is a subdivision of the year, marked by changes in weather, ecology, and hours of daylight. Seasons result from 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 intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant.
During May, June and July, the northern hemisphere is exposed to more direct sunlight because the hemisphere faces the sun. The same is true of the southern hemisphere in November, December and January. It is the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux. However, due to seasonal lag, June, July and August are the hottest months in the northern hemisphere and December, January and February are the hottest months in the southern hemisphere.
In temperate and subpolar regions, generally four calendar-based seasons (with their adjectives) are recognized: spring (vernal), summer (estival), autumn (autumnal) and winter (hibernal). However, ecologists mostly use a six season model for temperate climate regions that includes pre-spring (prevernal) and late summer (serotinal) as distinct seasons along with the traditional four.
Hot regions have two or three seasons; the rainy (or wet, or monsoon) season and the dry season, and in some tropical areas, a cool or mild season.
In some parts of the world, special "seasons" are loosely defined based on important events such as a hurricane season, tornado season or a wildfire season.


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. Therefore, at any given time, regardless of season, the northern and southern hemispheres experience opposite seasons.
The effect of axis tilt is observable from the change in day length, and altitude of the Sun at noon (the culmination of the Sun), during a year.

Elliptical Earth orbit

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.

Distance from sun

Compared to axial tilt, other factors contribute little to seasonal temperature changes. The seasons are not the result of the variation in Earth’s distance to the sun because of its elliptical orbit.Orbital eccentricity can influence temperatures, but on Earth, this effect is small and is more than counteracted by other factors; research shows that the Earth as a whole is actually slightly warmer when farther from the sun. This is because the northern hemisphere has more land than the southern, and land warms more readily than sea. Mars however experiences wide temperature variations and violent dust storms every year at perihelion.

Maritime and hemispheric

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.
In the temperate and polar regions, seasons are marked by changes in the amount of sunlight, which in turn often causes cycles of dormancy in plants and hibernation in animals. These effects vary with latitude and with proximity to bodies of water. For example, the South Pole is in the middle of the continent of Antarctica and therefore a considerable distance from the moderating influence of the southern oceans. The North Pole is in the Arctic Ocean, and thus its temperature extremes are buffered by the water. The result is that the South Pole is consistently colder during the southern winter than the North Pole during the northern winter.
The cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.

Tropics

In 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. For example, in Nicaragua the dry season (November to April) is called 'summer' and the rainy season (May to October) is called 'winter', even though it is located in the northern hemisphere. In other tropical areas a three-way division into hot, rainy, and cool season is used. There is no noticeable change in the amount of sunlight at different time of the year. However, many regions (such as the northern Indian ocean) are subject to monsoon rain and wind cycles.
A study of temperature records over the past 300 years^. shows that the climatic seasons, and thus the seasonal year, are governed by the anomalistic year rather than the tropical year.

Mid latitude thermal lag

In meteorological terms, the summer solstice and winter solstice (or the maximum and minimum insolation, respectively) do not fall in the middles of summer and winter. The heights of these seasons occur up to seven weeks later because of seasonal lag. Seasons, though, are not always defined in meteorological terms.
In astronomical reckoning, the solstices and equinoxes ought to be the middle of the respective seasons, but, because of thermal lag, regions with a continental climate which predominate in the Northern hemisphere often consider these four dates to be the start of the seasons as in the diagram, with the cross-quarter days considered seasonal midpoints. The length of these seasons is not uniform because of the elliptical orbit of the earth and its different speeds along that orbit.