Seasons.

Seasons. In the article EARTH the motions of the earth on which the changes of the seasons ultimately depend are explained. The chief cause of the greater heat of summer and cold of winter is that the rays of the sun fall more obliquely on the earth's surface in the latter season than in the former (see CLIMATE). Another concurrent cause is the greater length of the day in summer, and of the night in winter. Within the tropics the sun's rays have at no time so much obliquity as to make one part of the year very sensibly colder than another. But the zone of equatorial calms in which rainfall is practically continuous is shifted northward when the sun moves northward in the northern summer, and is similarly shifted southward in the southern summer. As the wet-zone swings to and fro, following the sun, the regions it traverses experience alternate wet and dry seasons. Those regions lying near the mean position of the wet-zone have thus two wet and two dry seasons in the year, the regions near its extreme positions having one wet and one dry season. Wet and dry seasons are also produced by the Monsoons (q.v.), themselves due to the relative seasonal change of temperature between land and sea (see also RAIN). In the temperate regions of the globe the year is naturally divided into four seasons—Spring, Summer, Autumn, and Winter. In the arctic and antarctic regions spring and autumn are very brief, and the natural division of the year is simply into summer and winter, the winter being long, and the summer short; and this is very much the case also in regions of the temperate zones lying near the arctic and antarctic circles. In subtropical regions the distinction of four seasons is, in like manner, very imperfectly marked. Conventionally (as in almanacs) it is assumed that each season commences at the equinox or solstice—e.g. that in the northern hemisphere spring commences at the vernal equinox about March 20, and summer at the summer solstice on June 21, although this is popularly spoken of as 'Midsummer Day;' and the 'summer months' in common English parlance include May, June, and July, winter being November, December, and January, and spring and autumn accordingly. Practically our division of the seasons depends more on seedtime and harvest than on the extremes of annual heat and cold. The greatest heat of summer is reached a considerable time after the summer solstice, the period when the sun's rays are most nearly vertical, and the day is longest. The greatest cold of winter in like manner occurs after the winter solstice—the period when the day is shortest, and the sun's rays are most oblique. The reason in the former case is that as summer advances the earth itself becomes more heated by the continued action of the sun's rays; in the latter, that it retains a portion of the heat which it has imbibed during summer, just as the warmest part of the day is somewhat after mid-day, and the coldest part of the night is towards morning. The four seasons of temperate regions are distinguished by the phenomena of plant-life, such as the budding, blossoming, fruit-bearing, and leafless repose of deciduous trees. Associated with these annual changes there are modifications of structure and function adapted to the seasonal variation of climate in different localities. Similar habits of Hibernation (q.v.) or of change in the thickness and colour of fur or feathers are found in the animals of regions where the seasons are sharply contrasted in climate. The intellectual superiority of the races inhabiting temperate regions is in part traceable to the constant necessity for forethought in providing for the regularly recurring season of winter when natural resources cease to be available.