In general, the effect of orbital eccentricity on Earth’s seasons is a 7% variation in sunlight received. 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. The effect of axial tilt is observable as the change in day length and the altitude of the Sun at solar noon (the Sun’s culmination) during the year. Regardless of the time of year, the Northern Hemisphere and Southern Hemisphere always experience opposite seasons. Because of seasonal lag, June, July, and August are the warmest months in the Northern Hemisphere while December, January, and February are the warmest months in the Southern Hemisphere.
Tropics
First light comes in late January because the sky has twilight, being a glow on the horizon, for increasing hours each day, for more than a month before the Sun first appears with its disc above the horizon. By October 13 the Sun is above the horizon for only 1 hour 30 minutes, and on October 14 it does not rise above the horizon at all and remains below the horizon until it rises again on 27 February. On 6 April the Sun is perceived as rising at 0522 UTC and remains above the horizon until it sets below the horizon again on 6 September at 0335 UTC. At progressively higher latitudes, the maximum periods of “midnight sun” and “polar night” are progressively longer.
- As mentioned above, Varro wrote that spring, summer, autumn, and winter start on the 23rd day of the Sun’s passage through Aquarius, Taurus, Leo, and Scorpio, respectively, and that (in the Julian Calendar) these days were February 7, May 9, August 11, and November 10.
- All the times are given in UTC (roughly speaking, the time at Greenwich, ignoring British Summer Time).
- Seasonal weather and climate conditions can become important in the context of military operations.
- Floral and animal activity variation near the equator depends more on wet/dry cycles than seasonal temperature variations, with different species flowering (or emerging from cocoons) at specific times before, during, or after the monsoon season.
- The four seasons have been in use since at least Roman times, as in Rerum rusticarum of Varro Varro says that spring, summer, autumn, and winter start on the 23rd day of the sun’s passage through Aquarius, Taurus, Leo, and Scorpio, respectively.
Other Word Forms
At the present time, the astronomical timing has winter starting at the winter solstice, spring at the spring equinox, and so on. As mentioned above, Varro wrote that spring, summer, autumn, and winter start on the 23rd day of the Sun’s passage through Aquarius, Taurus, Leo, and Scorpio, respectively, and that (in the Julian Calendar) these days were February 7, May 9, August 11, and November 10. Astronomical timing as the basis for designating the temperate seasons dates back at least to the Julian Calendar used by the ancient Romans.
- The most historically important of these are the three seasons—flood, growth, and low water—which were previously defined by the former annual flooding of the Nile in Egypt.
- Six ecological seasons can be distinguished without fixed calendar-based dates like the meteorological and astronomical seasons.
- There is no noticeable change in the amount of sunlight at different times of the year.
- As a result, the amount of precipitation tends to vary more dramatically than the average temperature or daylight hours.
- The effect of axial tilt is observable as the change in day length and the altitude of the Sun at solar noon (the Sun’s culmination) during the year.
- He points out that the lengths are not equal, being 91 (in non-leap years), 94, 91, and 89 days for spring, summer, autumn, and winter, respectively.
Solar terms
The tropical and (to a lesser degree) subtropical regions see little annual fluctuation of sunlight and temperature due to Earth’s moderate 23.4-degree tilt being insufficient to appreciably affect the strength of the Sun’s rays annually. 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. The low angle of the 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. In India, from ancient times to the present day, six seasons or Ritu based on south Asian religious or cultural calendars are recognised and identified for purposes such as agriculture and trade. Seasons often hold special significance for agrarian societies, whose lives revolve around planting and harvest times, and the change of seasons is often attended by ritual.
Related Words
Indigenous people in polar, temperate and tropical climates of northern Eurasia, the Americas, Africa, Oceania, and Australia have traditionally defined the seasons ecologically by observing the activity of the plants, animals and weather around them. The precise timing of the seasons is determined by the exact times of the Sun reaching the tropics of Cancer and Capricorn for the solstices and the times of the Sun’s transit over the equator for the equinoxes, or a traditional date close to these times. The four seasons have been in use since at least Roman times, as in Rerum rusticarum of Varro Varro says that spring, summer, autumn, and winter start on the 23rd day of the sun’s passage through Aquarius, Taurus, Leo, and Scorpio, respectively. Floral and animal activity variation near the equator depends more on wet/dry cycles than seasonal temperature variations, with different species flowering (or emerging from cocoons) at specific times before, during, or after the monsoon season. There is no noticeable change in the amount of sunlight at different times of the year.
Calendar-based reckoning defines the seasons in relative rather than absolute terms, so the coldest quarter-year is considered winter even if floral activity is regularly observed during it, despite the traditional association of flowers with spring and summer. As a result, the amount of precipitation tends to vary more dramatically than the average temperature or daylight hours. The slight differences between the solstices and the equinoxes cause seasonal shifts along a rainy low-pressure belt called the Intertropical Convergence Zone (ICZ). The seasonal cycle in the polar and temperate zones of one hemisphere is opposite to that of the other. 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.
The North American Cree and possibly other Algonquian https://royaldogs-ca.com/ speaking peoples used or still use a 6-season system. In Australia, some tribes have up to eight seasons in a year, as do the Sami people in Scandinavia. Oceanic regions tend to experience the beginning of the hibernal season up to a month later than continental climates. Six ecological seasons can be distinguished without fixed calendar-based dates like the meteorological and astronomical seasons. In this sense, ecological seasons are defined in absolute terms, unlike calendar-based methods in which the seasons are relative.
Each separate tribal group traditionally observes different seasons determined according to local criteria that can vary from the hibernation of polar bears on the arctic tundras to the growing seasons of plants in the tropical rainforests. The actual dates for each season vary by climate region and can shift from one year to the next. The solar seasons change at the cross-quarter days, which are about 3–4 weeks earlier than the meteorological seasons and 6–7 weeks earlier than seasons starting at equinoxes and solstices.
In Finland, “permanently” is defined as when the mean daily averaged temperature remains above or below the defined limit for seven consecutive days. For the southern hemisphere temperate zone, spring begins on 1 September, summer on 1 December, autumn on 1 March, and winter on 1 June. According to this definition, for temperate areas in the northern hemisphere, spring begins on 1 March, summer on 1 June, autumn on 1 September, and winter on 1 December. In 1780 the Societas Meteorologica Palatina (which became defunct in 1795), an early international organization for meteorology, defined seasons as groupings of three whole months as identified by the Gregorian calendar. These observances are often declared “official” within their respective areas by the local or national media, even when the weather or climate is contradictory. Nine years before he wrote, Julius Caesar had reformed the calendar, so Varro was able to assign the dates of February 7, May 9, August 11, and November 10 to the start of spring, summer, autumn, and winter.
Any point north of the Arctic Circle or south of the Antarctic Circle will have one period in the summer called “polar day” when the sun does not set, and one period in the winter called ‘polar night’ when the sun does not rise. The most historically important of these are the three seasons—flood, growth, and low water—which were previously defined by the former annual flooding of the Nile in Egypt. Thus, the tropics are characterized by numerous “mini-seasons” within the larger seasonal blocks of time. Instead, many regions (such as the northern Indian Ocean) have varying monsoon rain and wind cycles.
People living farther to the east (Asia and Australia), whose local times are in advance, see the astronomical seasons apparently start later; for example, in Tonga (UTC+13), an equinox occurred on September 24, 1999, a date on which the equinox will not fall again until 2103. Currently, the most common equinox and solstice dates are March 20, June 21, September 22 or 23, and December 21; the four-year average slowly shifts to earlier times as a century progresses. As noted, a variety of dates and even exact times are used in different countries or regions to mark changes of the calendar seasons. Pre-modern armies, especially in Europe, tended to campaign in the summer months – peasant conscripts tended to melt away at harvest time, nor did it make economic sense in an agricultural society to neglect the sowing season. Seasonal weather and climate conditions can become important in the context of military operations.
The beginning of spring is defined as when the mean daily temperature permanently rises above 0 °C. In Sweden and Finland, meteorologists and news outlets use the concept of thermal seasons, which are defined based on mean daily temperatures. These are mainly a matter of custom and not generally proclaimed by governments north or south of the equator for civil purposes.
This also means that in many years of the twentieth century, the dates March 21, June 22, September 23, and December 22 were much more common, so older books teach (and older people may still remember) these dates. The times of the equinoxes and solstices are not fixed with respect to the modern Gregorian calendar, but fall about six hours later every year, amounting to one full day in four years. Thus the time from the March equinox to the September equinox is 7.56 days longer than from the September equinox to the March equinox.