The two instants when the Sun is directly overhead at the Equator are the equinoxes.
For the other half of the year, the same happens, but in the Southern Hemisphere instead of the Northern, with the maximum around December 21. For approximately half of the year (from around March 20 to around September 22), the Northern Hemisphere tips toward the Sun, with the maximum amount occurring on about June 21. This is because during summer or winter, one part of the planet is more directly exposed to the rays of the Sun than the other, and this exposure alternates as the Earth revolves in its orbit. Regardless of the time of year, the northern and southern hemispheres always experience opposite seasons. (This tilt is also known as "obliquity of the ecliptic".) The seasons result from the Earth's axis of rotation being tilted with respect to its orbital plane by an angle of approximately 23.4 degrees. In addition to the density of incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle. the Earth's rotation on its axis), the North Pole will be dark and the South Pole will be illuminated see also arctic winter. This diagram shows how the tilt of Earth's axis aligns with incoming sunlight around the winter solstice of the Northern Hemisphere. Some examples of historical importance are the ancient Egyptian seasons- flood, growth, and low water-which were previously defined by the former annual flooding of the Nile in Egypt.
"Seasons" can also be dictated by the timing of important ecological events such as hurricane season, tornado season, and wildfire season. Some have a third cool, mild, or harmattan season. Many tropical regions have two seasons: the rainy, wet, or monsoon season and the dry season. 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. In temperate and sub-polar regions, four seasons based on the Gregorian calendar are generally recognized: spring, summer, autumn (or fall), and winter.
However, due to 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. It is Earth's axial tilt that causes the Sun to be higher in the sky during the summer months, which increases the solar flux. The same is true of the Southern Hemisphere in November, December, and January. The Northern Hemisphere experiences most direct sunlight during May, June, and July, as the hemisphere faces the Sun. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical cultures whose number of seasons varies. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region.
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