Causes and effects
Illumination of Earth at each change of astronomical season
The seasons result from the Earth's
axis of rotation being
tilted with respect to its
orbital plane by an angle of approximately 23.5
 (This tilt is also known as "obliquity of the
Regardless of the time of year, the
southern hemispheres always experience opposite seasons. This is because during
winter, one part of the planet is more directly exposed to the rays of the Sun (see Fig. 1) than the other, and this exposure alternates as the Earth revolves in its orbit. For approximately half of the year (from around March 20 to around September 22), the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June 21. For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December 21. The two instants when the Sun is directly overhead at the
Equator are the
equinoxes. Also at that moment, both the
North Pole and the
South Pole of the Earth are just on the
terminator, and hence day and night are equally divided between the two hemispheres. Around the
March equinox, the Northern Hemisphere will be experiencing
spring as the hours of
daylight increase, and the Southern Hemisphere is experiencing
autumn as daylight hours shorten.
The effect of axial tilt is observable as the change in
day length and
altitude of the Sun at solar
noon (the Sun's
culmination) during the
year. The low angle of Sun during the winter months means that incoming rays of solar radiation are
spread over a larger area of the Earth's surface, so the light received is more indirect and of lower intensity. Between this effect and the shorter daylight hours, the axial tilt of the Earth accounts for most of the seasonal variation in climate in both hemispheres.
A deciduous tree in winter
Illumination of Earth by Sun at the northern solstice.
Illumination of Earth by Sun at the southern solstice.
Diagram of the Earth's seasons as seen from the north. Far right: southern solstice
Diagram of the Earth's seasons as seen from the south. Far left: northern solstice
Animation of Earth as seen daily from the Sun looking at
UTC+02:00, showing the solstice and changing seasons.
Two images showing the amount of reflected sunlight at southern and northern summer solstices respectively (watts / m²).
Elliptical Earth orbit
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
 In fact, Earth reaches
perihelion (the point in its orbit closest to the Sun) in January, and it reaches
aphelion (the point farthest from the Sun) in July, so the slight contribution of
orbital eccentricity opposes the temperature trends of the seasons in the Northern Hemisphere.
 In general, the effect of orbital eccentricity on Earth's seasons is a 7% variation in sunlight received.
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.
 Any noticeable intensification of southern winters and summers due to Earth's elliptical orbit is mitigated by the abundance of water in the Southern Hemisphere.
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
A deciduous tree in autumn (fall)
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 seasonal cycle in the polar and temperate zones of one hemisphere is opposite to that of the other. When it is summer in the Northern Hemisphere, it is winter in the Southern, and vice versa.
subtropical regions see little annual fluctuation of sunlight. However, seasonal shifts occur along a rainy, low-pressure belt called the
Intertropical Convergence Zone (ICZ). As a result, the amount of
precipitation tends to vary more dramatically than the average temperature. When the Zone is north of the Equator, the northern tropics experience their wet season while the southern tropics have their dry season. This pattern reverses when the Zone migrates to a position south of the Equator.
Mid-latitude thermal lag
In meteorological terms, the
solstices (the maximum and minimum
insolation) do not fall in the middles of summer and winter. The heights of these seasons occur up to 7 weeks later because of
seasonal lag. Seasons, though, are not always defined in meteorological terms.
astronomical reckoning by hours of
daylight alone, the solstices and
equinoxes are in the middle of the respective seasons. Because of seasonal lag due to
thermal absorption and release by the oceans, 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 Earth's
elliptical orbit and its
different speeds along that orbit.