The Seasons
The seasons are divisions of the year which relate to the annual changes in the weather. The classical division into the four seasons – spring, summer, autumn and winter – derives from the weather pattern in middle latitudes where the onset of growth in plants and the reawakening of nature from a period of stasis can be separated from the hot season, when crops ripen and trees lose their leaves, and the cold season. At other latitudes such a division is not appropriate.
The cause of the seasons is that the plane of the Earth's orbit around the Sun is inclined to the plane of the Equator. This means that the direction of the Sun's rays relative to the ground and the number of daylight hours change during the year and hence the amount of solar energy received at different latitudes changes during the year.
Astronomically the arrangement of the planes of the orbit of the Earth and its equator are such that the planes intersect at two times, the Equinoxes, when the length of the day and night are equal. Mid-way between these are the Solstices, when the Sun is at its highest and lowest in the sky at mid-day. These times can be determined very accurately and, as they occur near the times when the seasons are changing, have been used to indicate the start of each season. Thus spring is deemed to start at the Vernal Equinox (near March 21), summer at the Summer Solstice (near June 21), autumn at the Autumn Equinox (near September 21) and winter at the Winter Solstice (near December 21). In the southern hemisphere the cycle is displaced by half a year.
The use of these astronomically defined dates for the start of the seasons is due, in the main, to the need, seen by diary manufacturers and quiz masters and the like, for definitive dates. In reality there are no hard and fast criteria to determine the start of each season; the onset of spring, for instance, could be the date on which the first daffodil is seen or the first birds make their nests or some such criterion. The dates of nearly all of these are not only extremely difficult to determine but also vary quite dramatically through the United Kingdom, let alone the rest of the World.
As a compromise, because they are well defined dates, the equinoxes and solstices are probably as good as any other criterion. Some prefer the simple use of three calendar months for each season with spring being March, April and May, etc.
An additional complication exists in England where one of the Quarter Days, when rents were due, is called Midsummer Day and occurs on June 24.
Factors affecting the timing of climatic change
(This does not refer to change of climate due to for example, the green-house effect but to the normal seasonal changes during the year.)
The major change in the energy received from the Sun at any one place on the Earth is due to the height of the Sun in the sky at mid-day and the associated length of the hours of sunlight. These are highest at the Summer Solstice and lowest at the Winter Solstice. These times do not, however, coincide with the hottest and coldest weather. This is because there is a considerable inertia in the Earth's response to the change in energy received and this delays the warming and cooling of the atmosphere and the oceans by several weeks.
In temperate latitudes the cycle of four seasons per year is a good representation of the weather pattern but in other latitudes this is not so. In the polar regions there are effectively only two seasons, winter when the Sun never rises and summer when it never sets. In the tropics, where the weather is dominated by the location of the equatorial convergence zone of rainfall, there can be two wet and two dry seasons, both hot, each year. In the far-east the weather is dominated by the monsoons and there are three seasons, cool-dry, hot-dry and hot-wet.
In addition to the effect of the variation in the height of the Sun in the sky, which is dominant, there is a small effect due to the elliptical path of the Earth around the Sun. This, in effect, means that the Earth is closer to the Sun in the southern summer than it is in the northern summer. Hence northern winters are warmer than southern ones and southern summers are warmer than northern ones. The actual dates of perihelion (the Earth closest to the Sun) and aphelion (furthest from the Sun) are around 3 January and 3 July respectively.
