Causes and effects
Illumination of the earth at each change of astronomical season
This diagram shows how the tilt of the Earth's axis aligns with incoming sunlight around the
of the northern hemisphere. Regardless of the time of day (i.e. the Earth's rotation on its axis), the
will be dark, and the
will be illuminated; see also
. In addition to the density of
of light in the
is greater when it falls at a shallow angle.
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
northern and southern hemispheres always experience opposite seasons. This is because during summer or 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 northern and southern 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
culmination of the Sun) during a
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 elliptical orbit.
 In fact, 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, 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 the southern hemisphere's 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)
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.
subtropical regions there is little annual fluctuation of sunlight. However, there are seasonal shifts of a rainy global-scale low pressure belt called the
Intertropical convergence zone. As a result, the amount of
precipitation tends to vary more dramatically than the average temperature. When the convergence zone is north of the equator, the tropical areas of the northern hemisphere experience their wet season while the tropics south of the equator have their dry season. This pattern reverses when the convergence zone migrates to a position south of the equator.
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.
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 the elliptical orbit of the earth and its
different speeds along that orbit.