Causes and effects of seasons
– Seasons are based on changes in weather, ecology, and the number of daylight hours in a given region.
– Seasons result from the axial parallelism of Earth’s tilted orbit around the Sun.
– In temperate and polar regions, seasons are marked by changes in sunlight intensity, causing animals to hibernate or migrate and plants to be dormant.
– Different cultures define the number and nature of seasons based on regional variations.
– The Northern Hemisphere experiences the most direct sunlight during May, June, and July, while the Southern Hemisphere experiences it in November, December, and January.
– The tilt of the Earth’s axis causes variations in day length and altitude of the Sun, leading to seasonal climate variation.
– The low angle of the Sun during winter months spreads solar radiation over a larger area, resulting in indirect and lower-intensity light.
– Proximity to bodies of water affects the seasonal variations in temperature.
– Changes in sunlight in temperate and polar regions cause cycles of dormancy in plants and hibernation in animals.
– The South Pole is consistently colder during the southern winter than the North Pole during the northern winter due to its distance from the moderating influence of oceans.
– The seasonal cycle in one hemisphere is opposite to that of the other.
– The abundance of water in the Southern Hemisphere mitigates any noticeable intensification of southern winters and summers due to Earth’s elliptical orbit.
– Seasonal weather fluctuations depend on factors such as proximity to oceans, oceanic cycles, prevailing winds, and currents.
Axial parallelism and tilt
– Axial parallelism is a characteristic of the Earth’s orbit where the direction of the axis remains parallel to itself throughout its orbit.
– Earth’s orbit exhibits approximate axial parallelism, causing the Earth’s seasons.
– Minor variation in the direction of the axis, known as axial precession, takes place over 26,000 years and is not noticeable in modern human civilization.
– Seasons result from the Earth’s axis of rotation being tilted with respect to its orbital plane by approximately 23.4 degrees.
– Regardless of the time of year, the northern and southern hemispheres always experience opposite seasons.
Elliptical Earth orbit
– The seasons are not the result of the variation in Earth’s distance to the Sun due to its elliptical orbit.
– Earth reaches perihelion (closest to the Sun) in January and aphelion (farthest from the Sun) in July.
– Orbital eccentricity has a minimal effect on Earth’s seasons, contributing only a 7% variation in sunlight received.
– The slight contribution of orbital eccentricity opposes the temperature trends of the seasons in the Northern Hemisphere.
Different methods of seasonal reckoning
– Most calendar-based partitions use a four-season model.
– Warmest and coldest seasons are separated by two intermediate seasons.
– Calendar-based reckoning defines seasons in relative terms.
– The four seasons have been in use since Roman times.
– Different countries or regions use various dates and times to mark changes in seasons.
– These observances are often declared official by local or national media.
– However, they are mainly a matter of custom and not proclaimed by governments for civil purposes.
– Weather or climate may contradict the official dates.
– Official dates vary across different countries and regions.
– Meteorological seasons are defined by temperature.
– Summer is the hottest quarter of the year, while winter is the coldest.
– The Societas Meteorologica Palatina defined seasons as groupings of three whole months.
– Professional meteorologists worldwide use this definition.
– Spring begins on March 1 in the northern hemisphere and September 1 in the southern hemisphere.
– Astronomical timing has been used to designate temperate seasons since ancient times.
– Varro and Pliny the Elder provided dates based on the sun’s passage through zodiac signs.
– Astronomical timing now aligns with the winter solstice, spring equinox, etc.
– The lengths of seasons are not equal due to Earth’s elliptical orbit.
– Astronomical timing is used worldwide, but some cultures may have variations.
– Solar timing is based on insolation, or Sun energy.
– The solstices and equinoxes are seen as the midpoints of the seasons.
– This method was used by the Roman scholar Varro and medieval Europe.
– It is still ceremonially observed in Ireland and some East Asian countries.
– Summer is defined as the quarter of the year with the greatest insolation, while winter is defined as the quarter with the least.
– The solar seasons change at the cross-quarter days.
– These days are about 3-4 weeks earlier than the meteorological seasons.
– They are also 6-7 weeks earlier than seasons starting at equinoxes and solstices.
– The day of greatest insolation is designated midsummer.
– The Celtic calendar and traditional Chinese calendar both have cross-quarter days.
– The four seasons in China are: spring (春), summer (夏), autumn (秋), and winter (冬).
– Each season centers on the respective solstice or equinox.
– Some calendars in south Asia use a six-season partition.
– The number of seasons between summer and winter can range from one to three.
– The dates for these seasons are fixed at even intervals of months.
– The Odia Calendar is a similar system but with different start and end times.
– Ecological seasons are defined by specific floral and animal events.
– They are absolute terms, unlike calendar-based methods.
– If specific conditions associated with a particular ecological season do not occur in a region, that area does not experience that season regularly.
– Observing changes in daily floral and animal events can indicate a change in seasons.
– In Great Britain, the onset of spring used to be defined by when the maximum daily temperature reached a certain threshold.
– Six ecological seasons can be distinguished without fixed calendar-based dates.
– Oceanic regions experience the beginning of the hibernal season up to a month later than continental climates.
