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.
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
[2] 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 seven weeks 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.
[3] 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.
[4] Mars however experiences wide temperature variations and
violent dust storms every year at perihelion.
[5]
