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 June, July, and August; the northern hemisphere is exposed to more direct sunlight because the northern hemisphere faces the sun. The same is true of the southern hemisphere in December, January, and February. Contrary to what some people believe, the seasons do not result from the varying distance between the Earth and the Sun. Instead the tilt of the Earth causes the Sun to be higher in the sky during the summer months which increases the solar flux.
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.
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.
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.
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.
In the tropics, there is no noticeable change in the amount of sunlight. 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.
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 a month later because of seasonal lag. Seasons, though, are not always defined in meteorological terms.
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. Mars however experiences wide temperature variations and violent dust storms every year at perihelion. 23
In the weeks surrounding 21 June, the sun is at its highest, and it appears to circle the sky without going below the horizon. Eventually, it does go below the horizon, for progressively longer periods each day until, around the middle of October, it disappears for the last time. For a few more weeks, "day" is marked by decreasing periods of twilight. Eventually, for the weeks surrounding 21 December, it is continuously dark. In later winter, the first faint wash of light briefly touches the horizon (for just minutes per day), and then increases in duration and pre-dawn brightness each day until sunrise in February.
Meteorological seasons are reckoned by temperature, with summer being the hottest quarter of the year and winter the coldest quarter of the year.
Using this reckoning, the Roman calendar began the year and the spring season on the first of March, with each season occupying three months. This reckoning is also used in Denmark, the former USSR, and Australia. In the modern United Kingdom there are no hard and fast rules about seasons, and informally many people use this reckoning.
So, in meteorology for the Northern hemisphere: spring begins on March 1, summer on June 1, autumn on September 1, and winter on December 1. Conversely, for the Southern hemisphere: spring begins on September 1, summer begins on December 1, autumn on March 1, and winter on June 1.
In astronomical reckoning, the seasons begin at the solstices and equinoxes. The cross-quarter days are 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 .
In the conventional United States calendar: Winter (89 days) begins on 21 December, the winter solstice; spring (92 days) on 20 March, the vernal equinox; summer (93 days) on 21 June, the summer solstice; and autumn (90 days) on 22 September, the autumnal equinox.
Because of the differences in the Northern and Southern Hemispheres, it is no longer considered appropriate to use the northern-seasonal designations for the astronomical quarter days. The modern convention for them is: March Equinox; June Solstice; September Equinox; and December Solstice
There is an error in the above diagram: the blue text in the top right should read, "Northern Winter / Southern Summer"
In traditional reckoning, the seasons begin at the cross-quarter days. The solstices and equinoxes are the midpoints of these seasons. For example, the days of greatest and least insolation are considered the "midsummer" and "midwinter" respectively.
This reckoning is used by various traditional cultures in the Northern Hemisphere, including East Asian and Irish cultures. In Iran, Afghanistan and some other parts of middle east the beginning of the astronomical spring is the beginning of the new year which is called Nowruz.
So, according to traditional reckoning, winter begins between 5 November and 10 November, Samhain, 立冬 (lìdōng or rittou); spring between 2 February and 7 February, Imbolc, 立春 (lìchūn or rissyun); summer between 4 May and 10 May, Beltane, 立夏 (lìxià or rikka); and autumn between 3 August and 10 August, Lughnasadh, 立秋 (lìqiū or rissyuu). The middle of each season is considered Mid-winter, between 20 December and 23 December, 冬至 (dōngzhì or touji); Mid-spring, between 19 March and 22 March, 春分 (chūnfēn or syunbun); Mid-summer, between 19 June and 23 June, 夏至 (xiàzhì or geshi); and Mid-autumn, between 21 September and 24 September, 秋分 (qiūfēn or syuubun). .
In Australia, the traditional aboriginal people defined the seasons by what was happening to the plants, animals and weather around them. This led to each separate tribal group having different seasons, some with up to 8 seasons each year. However, most modern Aboriginal Australians follow the meteorological seasons as do non-Aboriginal Australians.