Variations in the average weather at different times of the year are known as seasons, controlled by the average amount of solar radiation received at the surface in a specific place for a certain time. several factors determine the amount of radiation received at a particular point on the surface, including the angle at which the sun's rays hit the surface, the length of time the rays warm the surface, and the distance to the sun. As the Earth orbits the sun approximately once every 365 days, it follows an elliptical orbit that brings it closest to the sun in January (91 million miles, or 147 million kilometers) and farthest from the sun in July (94.5 million miles, or 152 million kilometers). Therefore the sun's rays are slightly more intense in January than in July but, as any Northern Hemisphere resident can testify, this must not be the main controlling factor determining seasonal warmth since winters in the Northern Hemisphere are colder than summers. Where the sun's rays hit a surface directly, at right angles to the surface, they are most effective at warming the surface since they are not being spread out over a larger area on an inclined surface. Also, where the sun's rays enter the atmosphere directly, they travel through the least amount of atmosphere, so are weakened much less than rays that must travel obliquely through the atmosphere, which absorbs some of their energy. The Earth's rotational axis is presently inclined at 23.5° from perpendicular to the plane on which it rotates around the sun (the ecliptic plane), causing different hemispheres of the planet to be tilted toward or away from the sun during different seasons. During the Northern Hemisphere summer the Northern Hemisphere is tilted toward the sun, so it receives more direct sunlight rays than the southern Hemisphere, causing more heating in the north than in the south. Also, since the Northern Hemisphere is tilted toward the sun in summer, it receives direct sunlight for longer periods of time than the southern hemisphere, enhancing this effect. on the summer solstice on June 21, the sun's rays are directly hitting 23.5°N latitude (called the tropic of Cancer) at noon. Because of the tilt of the planet, the sun does not set below the horizon for all points north of the Arctic Circle (66.5°N). Points farther south have progressively shorter days, and points farther north have progressively longer days. At the North Pole the sun rises above the horizon on March 20 and does not set again until six months later, on september 22. since the sun's rays are so oblique in these northern latitudes, however, they receive less solar radiation than areas farther south where the rays hit more directly but for shorter times. As the Earth rotates around the sun, it finds the southern hemisphere tilted at its maximum amount toward the sun on December 21 (summer solstice in the southern hemisphere), and the situation is reversed from the Northern hemisphere summer, so that the same effects occur in the southern latitudes.
seasonal variations in temperature and rainfall at specific places are complicated by global atmospheric circulation cells, proximity to large bodies of water and warm or cold ocean currents, and monsoon-type effects in some parts of the world. seasons in some places are hot and wet, others are hot and dry, cold and wet, or cold and dry.
Supercontinents affect the supply of nutrients to the oceans and thus seasonality. Large supercontinents that contain most of the planet's landmass cause increased seasonality, and thus lead to an increase in the nutrient supply through overturning of the ocean waters. During breakup and dispersal, smaller continents have less seasonality, yielding decreased vertical mixing, leaving fewer nutrients in shelf waters. Seafloor spreading also increases the nutrient supply to the ocean; the more active the sea-floor spreading system, the more interaction there is between ocean waters and crustal minerals that dissolve to form nutrients in the seawater.
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