A season is a period of the year that is distinguished by special climate conditions. The four seasons—spring, summer, fall, and winter—follow one another regularly. Each has its own light, temperature, and weather patterns that repeat yearly. In the Northern Hemisphere, winter generally begins on December 21 or 22. This is the winter solstice, the day of the year with the shortest period of daylight. Summer begins on June 20 or 21, the summer solstice, which has the most daylight of any day in the year. Spring and fall, or autumn, begin on equinoxes, days that have equal amounts of daylight and darkness. The vernal, or spring, equinox falls on March 20 or 21, and the autumnal equinox is on September 22 or 23. The seasons in the Northern Hemisphere are the opposite of those in the Southern Hemisphere. This means that in Argentina and Australia, winter begins in June. The winter solstice in the Southern Hemisphere is June 20 or 21, while the summer solstice, the longest day of the year, is December 21 or 22. Seasons occur because Earth is tilted on its axis relative to the orbital plane, the invisible, flat disc where most objects in the solar system orbit the sun. Earth’s axis is an invisible line that runs through its center, from pole to pole. Earth rotates around its axis. In June, when the Northern Hemisphere is tilted toward the sun, the sun’s rays hit it for a greater part of the day than in winter. This means it gets more hours of daylight. In December, when the Northern Hemisphere is tilted away from the sun, with fewer hours of daylight. Seasons have an enormous influence on vegetation and plant growth. Winter typically has cold weather, little daylight, and limited plant growth. In spring, plants sprout, tree leaves unfurl, and flowers blossom. Summer is the warmest time of the year and has the most daylight, so plants grow quickly. In autumn, temperatures drop, and many trees lose their leaves. The four-season year is typical only in the mid-latitudes. The mid-latitudes are places that are neither near the poles nor near the Equator. The farther north you go, the bigger the differences in the seasons. Helsinki, Finland, sees 18.5 hours of daylight in the middle of June. In mid-December, however, it is light for less than 6 hours. Athens, Greece, in southern Europe, has a smaller variation. It has 14.5 hours of daylight in June and 9.5 hours in December. Places near the Equator experience little seasonal variation. They have about the same amount of daylight and darkness throughout the year. These places remain warm year-round. Near the Equator, regions typically have alternating rainy and dry seasons. Polar regions experience seasonal variation, although they are generally colder than other places on Earth. Near the poles, the amount of daylight changes dramatically between summer and winter. In Barrow, Alaska, the northernmost city in the U.S., it stays light all day long between mid-May and early August. The city is in total darkness between mid-November and January.
The image below (apparently from "physicalgeography.net" is somewhat instructive: Notice that even in the middle of summer, India (at about 30 °N) still gets slightly more sun than the Britain (at 60 °N). That's because although the days are longer in Britain, the sun is lower in the sky and so its heat is spread out over a wider area. But the North Pole gets even more sun than either Britain or India in June, and the equator gets substantially less. So there is something to explain. The reason is thermal inertia.  This graph shows insolation averaged over 24 hours at four different Latitudes. This shows the theoretical insolation. The model used to calculate these curves includes the effects of Earth's elliptical orbit, but do not take account of any atmospheric effects, such as the energy absorbed by the atmosphere, nor any weather effects such as clouds. It shows that a perfectly black disc with an area of 1 m² placed just above the atmosphere and oriented horizontal to the Earth surface would receive an average of about 515 Joules per second at the North Pole on June 21st and an average of about 380 Joules per second at the Equator. The main reason for the Equator value being so low is that it is night time for about 12 hours at the equator. Imagine pushing a heavy ball. It can take some time for it to begin to move quickly because it has inertia. Similarly, it takes time for the ground and water to heat up when the sun shines upon them. India gets much more sun in winter, and so the ground and water remain warm. When summer comes, it heats up further and ends up much hotter than Britain. In Britain there is very little sun in winter, and the ground and water cool down (and freeze). Then, even when the sun comes in summer, it takes time to warm up. By the time the ground and water have warmed up, the sun is already starting to weaken. And so the temperature never rises as much as it can in India. There are lots of details and local effects (the gulf stream in Britain, and the monsoon in India are significant) but the basic reason that India is hotter in summer is that it gets more sun in winter! Pidwirny, M. (2006). "Earth-Sun Relationships and Insolation". Fundamentals of Physical Geography, 2nd Edition.
« Back to Meteorology, Climatology page Because the earth’s axis is tilted. It is all about the tilt of the Earth’s axis. Many people believe that the temperature changes because the Earth is closer to the sun in summer and farther from the sun in winter. In fact, the Earth is farthest from the sun in July and is closest to the sun in January! During the summer, the sun’s rays hit the Earth at a steep angle. The light does not spread out as much, thus increasing the amount of energy hitting any given spot. Also, the long daylight hours allow the Earth plenty of time to reach warm temperatures. During the winter, the sun’s rays hit the Earth at a shallow angle. These rays are more spread out, which minimizes the amount of energy that hits any given spot. Also, the long nights and short days prevent the Earth from warming up. Thus, we have winter! Winter in Yellowstone National Park. National Park Service Web site. Rural Alabama in the spring. Carol M. Highsmith Collection, Library of Congress. Soybean and corn fields ready for harvesting in late summer in Carroll County, Indiana. Carol M. Highsmith Collection, Library of Congress. Colorado’s Delores River Valley in autumn. Carol M. Highsmith Collection, Library of Congress. Published: 11/19/2019. Author: Science Reference Section, Library of Congress
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Why does the Northern Hemisphere have hotter summers and colder winters than the Southern Hemisphere?
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