Seasons divide the year into different periods, which are delimited by astronomical data – including calendar data – or by characteristic climatic properties. In everyday language, this means mainly meteorological, clearly distinguishable periods of the year. In the temperate latitudes, they are spring, summer, fall (autumn), and winter; in the tropics, there are rainy seasons, transitional seasons, and dry seasons.
The description of the seasons in this article refers to the Northern Hemisphere of the earth, in the Southern Hemisphere they are offset by half a year. Summer and winter can each also be understood as halves of a year, for example as the northern summer half-year or the southern winter half-year.
Different cultures distinguish different times of the year. For example, the Sami people in Scandinavia know eight seasons, and the Australian Aborigines in Arnhem Land have six seasons. In Russia, Rasputitsa is known as the mud season, during spring snowmelt and autumn rains.
Formation of seasons
On Earth, the intensity, duration, and angle of incidence of sunlight in a geographic location vary over the course of a year. These changes are small near the equator and more pronounced towards the poles. They repeat themselves as seasonal fluctuations after the earth orbits the sun. It is not the distance from the sun that determines the seasons experienced in different regions of the earth, but rather the position of the earth’s axis of rotation relative to the plane of its orbit.
As with a spinning top, the earth’s axis retains its orientation in space and is (almost) fixed in space at a certain angle to the ecliptic plane because of the conservation of angular momentum. This inclination of the earth’s axis is not perpendicular, but is (currently) about 66.6°, so the equatorial plane is inclined by about 23.4° (23° 26′) with respect to the orbital plane. This is why the angle of incidence of sunlight (height of the sun at noon) changes over the course of the year as the earth orbits the sun. In addition, due to the rotation of the earth around itself, the duration of daylight (bright day) changes as day length, in the regions far from the poles outside the polar circles from one earth revolution to the next. The longer and steeper sunlight hit the surface, the more this region can be heated.
Change in the angle of incidence and duration of exposure
Seasonal course of the mean air temperature in the Northern Hemisphere (NH), Southern Hemisphere (SH), and whole earth (GLO) according to measurements in 1961-1990.
The first decisive factor for the formation of the seasons is how much the respective proportion of the sun’s radiant power that a certain geographical region receives fluctuates over the course of the year. The irradiance related to the relief of the surface depends on the angle of incidence and the duration of the irradiance. The angle of incidence reaches It’s daily maximum at midday and this sun elevation at noon fluctuates by ± 23.4° for all locations outside the polar regions over the course of the year, with the average elevation angle becoming flattered towards the poles. On the other hand, the possible daily exposure time, the daylight, is the same on average, but the annual range of fluctuation in day length increases with increasing geographical latitude. Since both factors, the angle, and duration of solar radiation, are related across the diurnal arc – the highest position of the sun and the longest day coincide – and their fluctuations add up, the formation of the seasons depends primarily on the geographical latitude of a region.
Solar climate zones related to the angle of incidence of the sunlight can therefore be differentiated solely according to the circle of latitude. Thus, the tropics near the equator are defined as tropical zones between the tropics (23.4° latitude) compared to the ectopic zones – which then include the subtropics or mid-latitudes and (from about 66.6° latitude) the polar zones – with increasingly stronger ones further away from the equator pronounced seasonal differences. This gives rise to two basic types of climate: the daytime climate of the tropics and the seasonal climate of the ectropes; determined by the time interval in which the highest and lowest average temperatures occur.
During the period between the March and September equinoxes, the Northern Hemisphere is tilted more toward the Sun, causing the Sun to traverse a high arc for an observer located there. When the sun is high in the sky, the solar radiation hits the earth’s surface steeply and thus delivers a relatively high energy input per area. Furthermore, the greater part of the apparent path of the sun that is traveled around the earth every day lies above the horizon as a daily arc, so that the days are long and there is plenty of time for the energy input. The thus increased energy input causes a warming of the Northern Hemisphere (Northern Hemisphere) during this period.
If the earth is half a year later at the opposite point of its orbit, the Northern Hemisphere is inclined towards the sun – due to the relatively fixed position of the earth’s axis apart from precession and nutation. For an observer in the Northern Hemisphere, this results in a low daily sun path. When the sun is low in the sky, the solar radiation hits the earth’s surface more flatly, so that it is distributed over a larger area and brings in less energy. In addition, only the smaller part of the daily path of the sun lies above the horizon, so the energy input can only take place for a short period of time. The result is a cooling of the Northern Hemisphere.
Warming and cooling are first reflected in the air temperatures (see figure); Because of thermal inertia, ground temperatures follow the sun’s highs and lows with some lag. The differences in the daily arc of the sun’s course increase with higher geographical latitude and have increasingly stronger effects (up to the polar night), towards the equator, the seasonal fluctuations become smaller.
What are the conditions in the Southern Hemisphere?
The seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere: if it is summer in the south, it is winter in the north, and vice versa.
In the tropical and subtropical regions on both sides of the earth’s equator, seasonal changes are less pronounced; instead, rainy and dry seasons occur. In the tropics near the equator, two rainy seasons can be distinguished over the course of the year. With increasing geographical latitude, they merge into one another and thus become two peaks of a single rainy season, which differs depending on the hemisphere.
Change in distance from the sun
Although the Earth’s orbit around the Sun is elliptical rather than circular, so the distance to the Sun varies, the resulting differences are in light intensity alone and are not large because of the Earth’s orbit’s small orbital eccentricity. The Earth passes through the point furthest from the sun in the first week of July, in summer in the Northern Hemisphere. The slightly different distance of the earth from the sun over the course of the year due to the eccentric orbit is therefore not the cause of the seasons. The change between perihelion and aphelion only currently makes the southern winters somewhat severer and the northern winters somewhat milder (shorter and closer to the sun) than they would be for a circular Earth orbit. Under current circumstances, the earth is at its closest point to the sun (perihelion) in northern winter – around January 3 at a distance of around 147.1 million km; in southern winter it is furthest away from the sun (aphelion) – around July 5 at a distance of around 152.1 million km. As discussed above, the reason for the seasons on Earth is the angle and duration of the sun’s rays. For Central Europe, the extremes of the angles in summer are 60° to 65° and the possible sunshine duration in Central Germany is 16-17 hours, in winter it is 7-8 hours or angles of only 13° to 18°.
Shifting of the seasons
Due to the gravitational effect of the moon and sun on the rotating body of the earth, the earth’s axis performs a precession movement, so that the position of the reference points of solstices and equinoxes gradually shifts and in about 26,000 years goes backward (retrograde) around the earth’s orbit (cycle of precession). The tropical year related to the vernal equinox, therefore, lasts about 20 minutes shorter than a complete orbit of the earth around the sun related to the background of the fixed stars, a sidereal year. The so-called civil year of the calendar is based on the length of the tropical year. The calendar year in the Gregorian calendar is approximated to the tropical length of the year by inserting leap days, which results in typical shifts for calendar indications of the beginning of the seasons, for example for the beginning of autumn.
In addition, as a result of orbital disturbances caused by other planets, the apse line (straight through aphelion and perihelion) rotates once in a good 111,000 years in the right direction (prograde). Because of these opposing movements, perihelion runs through all seasons once every 21,000 years. In about ten thousand years, the point closest to the sun will coincide with the northern summer solstice. The winter seasons in the Northern Hemisphere will then be longer and farther from the sun than they are today. In return, the Southern Hemisphere will experience shorter and sunnier winters.
Astronomical Seasons definitions
Astronomically, the seasons are determined according to the apparent geocentric ecliptic longitude of the position of the sun. Taking aberration and nutation into account, the apparent annual path of the sun is viewed from a hypothetical observation point at the center of the earth and divided into four sections. Each of the orbit sections is delimited by an equinoctial point (of the equinox, at 0° or at 180°) and a solstitial point (of the solstice, at 90° or at 270°).
The astronomical seasons are defined as the periods of time that elapse while passing through one of the four sections and do not last the same length of time due to the different angular velocities. Due to the geocentric definition related to the center of the earth, an astronomical season begins or ends at the same time worldwide, regardless of location (which, however, corresponds to different times in different time zones).
Astronomical spring begins when the Sun’s apparent geocentric longitude is 0°. This is the time of the vernal equinox (primary equinox). Except for a few seconds, it coincides with the moment when the sun crosses the celestial equator from south to north.
Astronomical summer begins when the Sun’s apparent geocentric longitude is 90°. This is the time of the summer solstice. Except for a few minutes, it coincides with the time when the sun reaches its greatest northern declination and thus its northernmost position on the celestial sphere.
Astronomical fall begins when the Sun’s apparent geocentric longitude reaches 180°. This is the time of the autumnal equinox (secondary equinox). Except for a few seconds, it coincides with the time at which the sun crosses the celestial equator from north to south.
Astronomical winter begins when the Sun’s apparent geocentric longitude is 270°. This is the time of the winter solstice. Except for a few minutes, it coincides with the time when the sun reaches its greatest southern declination and thus its southernmost position on the celestial sphere.
The beginnings of the seasons are not exactly identical to crossing the celestial equator or reaching the greatest declination, because it is actually the center of gravity of the Earth-Moon system that moves evenly around the Sun in the “Earth’s orbital plane”, while the Earth itself is that center of gravity encircles and is usually slightly above or below this level. Thus, as seen by the geocentric observer, the Sun does not travel exactly on the ecliptic (it has non-zero ecliptic latitude). Therefore, on the one hand, it does not pass exactly through the spring and autumn equinoxes, and on the other hand, its variable elliptical width means that the maximum declination is usually not assumed exactly at the solstices.
The period of time from one astronomical beginning of spring to the next, a tropical year, lasts on average 365 days, 5 hours, and 49 minutes. Therefore, the next beginning of spring falls at a time almost 6 hours later than the previous one. As a comparison of the times in the table shows, there are small deviations from the mean for consecutive years. They are caused by the orbital disturbances caused by other planets and the already mentioned difference between the center of the earth and the earth-moon center of gravity (see also the article on the earth’s orbit).
For a period of four years, this means a time that is almost 24 hours later. The beginning of spring falls four years later at about the same time of day, but in a common calendar year of 365 days on a date one day later. A leap day was therefore introduced with the Julian calendar, which is switched on every four years (in leap years such as 2016, 2020, and 2024) so that the date of the beginning of spring in a calendar year does not keep shifting to a later calendar date. However, this calendar correction, which is set at 24 hours per 4 years, is too large by an average of about 44 minutes (4 · 5 h 49 m = 23 h 16 m), so with this regulation, the spring date given in the Julian calendar gradually shifts to an earlier calendar date. This overcompensation is corrected in the Gregorian calendar by omitting the leap day in the secular years three times in four centuries. This was the case in the past in the 1700s, 1800s, and 1900s; However, 2000 was a leap year. With this regulation, the Gregorian calendar year has a length of 365.2425 days and comes pretty close to the astronomically determined mean length of a tropical year.
The calendar dates of the astronomical dates for the beginning of spring and the other seasons are therefore not fixed. In addition to a daily change of almost six hours compared to the previous year, their fluctuations show a pattern influenced by the switching rules. The beginning of the season in question can take place on different calendar days over the years. In the 21st century, summer can begin on June 20 or 21 in the Central European Time Zone (CET or CEST), autumn can begin on September 22 or 23, and winter can begin on September 21, 22, or 23 December and the beginning of spring on March 19, 20 or 21:
The beginning of summer in the Central European Time zone with daylight saving time (CEST) fell on June 21 at the beginning of the 21st century and on June 20 for the first time in 2020. Towards the end of the century, the 20th becomes more frequent than the 21st. The missing leap day in 2100 pushes the beginning of summer back to June 21st for a while.
The beginning of autumn in the Central European Time zone with daylight saving time fell on September 22 or 23 with about the same frequency as the beginning of the century. The 22nd then becomes increasingly frequent and in 2067 the 23rd becomes the date of the beginning of autumn for the last time in this century (provided that in those years daylight saving time (CEST) is still given, otherwise in 2063). With the change of century rule, the 23rd again appears alongside the 22nd of September as the beginning of autumn.
In the first decade, the beginning of winter fell almost equally often on December 21 and December 22. In the future, the date of the 21st will become more common; in 2047, the 22nd will appear as a date for the last time in this century. In the year 2084 December, 20th will be the beginning of winter for the first time since 1696. After the turn of the century, winter begins again on December 21 or 22.
In the early years of the 21st century, the beginning of spring fell on March 20th or 21st in the Central European Time Zone (CET). In 2011, for the last time in this century, the date was March 21st. Since then, it has been March 20th. In 2048, the beginning of spring will fall on March 19 for the first time. After that, more and more often, towards the end of the century March 19th and 20th will occur with about the same frequency. Because of the missing leap day in 2100, the beginning of spring at the beginning of the 22nd century will again oscillate between March 20th and 21st (see figure below).
The so-called meteorological seasons are simply divided according to the calendar months and always include three complete months. This puts them about three weeks earlier than the astronomical seasons. With the meteorological definition, the warmest months in the Northern Hemisphere, such as in North America, on average June, July, and August fall in the (meteorological) summer, and the coldest months on average December, January, and February fall in the (meteorological) winter (see figure above, seasonal temperature profile (NH)). These quarterly periods, determined according to the beginning and end of calendar months, often allow simpler statistical recording of meteorological data; they are not aligned with current weather changes.
When Do the Seasons Start and End?
- Spring: March 1st – May 31st
- Summer: June 1st – August 31st
- Fall: September 1st – November 30th
- Winter: December 1st – February 28th/29th
What is the name of six seasons?
Ecologists often use a six-season model for temperate climate regions which are not tied to any fixed calendar dates: prevernal, vernal, estival, serotinal, autumnal, and hibernal. Many tropical regions have two seasons: the rainy, wet, or monsoon season and the dry season.
What are the 6 seasons, in India?
- Vasanta (Spring),
- Varsha (Monsoon),
- Hemanta (pre-winter)
- Shishira (winter)
And these are called Ritu’s.