Seasons

Seasons are the two (fair and rainy) or four (spring, summer, autumn, and winter) periods of the year that are typically distinguished by specific atmospheric conditions. Many plant and animal life cycles and periods of activity are based on the seasons, which, in turn, are defined by the relative positions of the earth and the sun.

Energy from the Sun

The seasons are the natural, weather-related divisions of the year. In the earth's north and south temperate zones, the four seasons of spring, summer, autumn, and winter are the normal progression of annual climatic change. These represent distinct phases in the weather patterns of those regions, each lasting approximately three months. As latitudes change, however, seasonal variations of weather and the length of the seasons change to reflect local conditions. At latitudes increasingly distant from the equator, colder weather becomes more dominant, and warmer seasons become shorter and cooler. However, warmer conditions dominate at latitudes approaching the equator, and colder seasons shrink to nonexistence. In tropical areas, the seasons may be classified merely as “rainy” and “dry” according to the amounts of rainfall and sunshine received.

The seasons and their weather patterns all result from the variable distribution of energy from the sun according to the relative positions and motions of the sun and Earth. The seasons change because the solar energy received increases and decreases in an annual cycle. This cycle results from Earth’s axis being tilted at an angle of 23.5 degrees relative to the plane formed by Earth’s orbit around the sun.

Earth makes a complete orbit around the sun every 365 days, 5 hours, and 49 minutes. As it does so, the tilt of Earth’s axis does not appreciably change. Thus, the north end of the axis tilts toward the sun in June; six months later, when Earth has traveled to the opposite point in its orbital path, the north end of the axis tilts away from the sun.

Light and solar energy are most intense when they strike Earth directly, perpendicular to the planet’s surface because its energy is then the most concentrated. Light falling at an oblique angle is spread out as it strikes the surface, thus more diffuse and less effective in delivering heat to the surface. When the North Pole tilts toward the Sun, more sunlight falls on the Northern Hemisphere than the Southern Hemisphere. This sunlight is more direct; it hits the ground surface vertically at the Tropic of Cancer on the summer solstice. This results in two factors that together cause the warmer summer weather: The sun is higher in the sky than in autumn and winter, and the number of daylight hours is increased. The higher the sun is in the sky, the more concentrated its heat is, and thus, the warmer that part of the surface becomes.

A common misconception is that summer occurs because the earth is closer to the sun. Earth is closest to the sun (at perihelion) about January 3 of each year, which is winter in the Northern Hemisphere. Earth is farthest from the sun on about July 3. Earth’s orbit is an ellipse (an elongated circle). Still, the shape of this ellipse is so nearly circular that its effect (that is, the variation in distance to the sun) is not as significant a factor in Earth’s weather as is the angle at which the sunlight strikes Earth. This angle is greatest in the Northern Hemisphere in June, when the sun shines directly over the Tropic of Cancer. It is greatest in the Southern Hemisphere in December, when the sun shines directly over the Tropic of Capricorn. It takes an enormous amount of solar energy to warm the earth’s atmosphere, lakes, oceans, and land, and thus, the warmest part of summer does not occur on the summer solstice itself but instead about one month later.

Effects of Latitude

Latitude affects seasonal change in two ways. One of these is the number of hours of daylight. As one goes from the equator (0 degrees latitude) toward either of the poles, the latitude increases until one is at the poles, where the latitude is 90 degrees. On the day of the summer solstice, daylight is at a maximum for areas north of the equator and at a minimum for areas south of the equator. The farther north one goes on that date, the longer the daylight. At 20 degrees north of the equator (approximately the latitude of Mexico City), daylight on that date is 13 hours and 12 minutes long; at 40 degrees north (approximately the latitude of New York, Rome, or Beijing), it is 14 hours, 52 minutes; at 60 degrees north (approximately the latitude of Anchorage or Oslo), daylight is 18 hours and 27 minutes.

The opposite is true at the winter solstice. At 20 degrees north of the equator, daylight on that date is 10 hours and 48 minutes long; at 40 degrees north, it is 9 hours and 8 minutes; and at 60 degrees north, it lasts only 5 hours and 33 minutes. Therefore, the closer one is to the poles, the more extreme the variations of daylight and darkness.

The other way latitude affects seasonal change is the angle of incoming sunlight. As one goes north from the Tropic of Cancer (or south from the Tropic of Capricorn), the angle of incoming solar light decreases. Even though far northern areas have extremely long periods of daylight from May to July, the angle of sunlight is so low that the solar energy is spread very thinly across the surface. Therefore, the earth does not receive as much heat in those locations, and thus, summer temperatures in the region never get very high.

Two other common misconceptions are that the sun is straight overhead at noon every day and rises due east and sets due west every day. The sun is never straight overhead at any part of any day in any location north of the Tropic of Cancer or south of the Tropic of Capricorn. The location of sunrise and sunset changes with the seasons. In June, the sun rises in the northeast and sets in the northwest; in December, it rises in the southeast and sets in the southwest. The exact location is dependent upon latitude. The only days the sun rises due east and sets due west are the vernal and autumnal equinoxes.

Other Factors Affecting Seasons

Seasons and seasonal weather are affected by other factors such as altitude, nearby mountain ranges, ocean currents, and proximity to water. Mountains lose heat in a particular manner, and the air at the top of a mountain is much cooler than at the bottom. A typical example is Mount Kilimanjaro in Tanzania, the bottom of which is located in a tropical rainforest, but the top of which is covered by snow and ice.

The Gulf Stream in the Atlantic Ocean absorbs a great deal of solar energy as it passes through tropical areas near Florida. It then flows northeastward, carrying its relatively warm water to Western Europe. Therefore, Western Europe generally experiences much warmer weather than areas of eastern North America at the same latitude. The same process occurs in the Pacific Ocean, where the Japan Current carries warm water to the western North American shore near Oregon and Washington.

Mountain ranges often deplete air currents of their moisture and deprive downwind areas of rainfall. The Cascade Mountains, for example, force moist Pacific air upward as it passes over them to the east. This causes the air to cool, its contained water vapor to condense, and rain to fall on the windward side of the mountains, a region known as a “rain shadow.” When the air gets to the other side of the mountain range, much of its moisture is gone, and the potential for rainfall is much lower. These factors cause western Oregon and western Washington to get much more rainfall than most places in North America but leave Idaho and Montana with very little.

Lakes and oceans heat up more slowly than land areas do. As a result, land areas near large bodies of water do not experience the extremes of heat and cold that areas farther from the water do. For example, downtown Chicago, at the Lake Michigan shoreline, does not get as cold on a winter night as Batavia, Illinois, which is 57 kilometers west of the Chicago lakefront. For the same reason, downtown Chicago does not get as hot on a summer afternoon as Batavia.

Many areas near the tropics do not experience the four seasons already described but rather have an annual pattern of dry and wet seasons. In southern Asia, for example, the sun's lower angle in the winter causes temperatures to cool below the temperature of the nearby warm Indian Ocean. This causes winds to blow off the land toward the ocean, resulting in several months of fair or dry weather. By summer, the higher sun has caused the continent to warm slightly above the ocean's temperature, and the wind pattern is reversed. Moist, warm air flows from the ocean to the land, causing heavy rainfall; this cyclical wind system is called a monsoon. The name is also given to the rains caused by this system.

Study of the Seasons

The ability to predict the seasons accurately was vitally important to early civilizations. Because of their dependence upon farming, ancient peoples had to know when a river would flood and when to plant crops. Early peoples noted that the appearance of certain constellations and the locations of sunrise and sunset varied when warmer or cooler weather could be expected.

Several early civilizations, including Egypt, Babylon, India, and China, have left indirect evidence of astronomical writings from as early as 2500 Before the Common Era (BCE). Stonehenge, built in southwest England in about 1800 BCE, has a large stone that marks the direction of sunrise at the summer solstice. Medicine wheels made by Indigenous Americans two thousand years ago also show the direction of the summer solstice sunrise.

A critical barrier in defining the seasons was the difficulty of determining the exact length of one year. For many centuries, astronomers and civil rulers tried to define the year based on a number of lunar months. Interestingly, the Mayan civilization developed a calendar that agrees with present-day calculations of the length of a year to a high degree. The Egyptians, who needed a means to forecast the flooding of the Nile, may have been one of the earliest civilizations to rely on a solar calendar. By 46 BCE, however, errors in the accepted calendar were so great that Julius Caesar redefined the year, entirely independently of the Moon, as 365 days, with one extra day added every four years, to create what is known as the Julian calendar. The Gregorian calendar, adopted in 1582 and still used today, corrected a small error in the Julian calendar by eliminating the extra day from would-be leap years divisible by one hundred but not by four hundred.

An understanding of the causes of seasonal change begins with an understanding of the relative positions and motions of the sun and Earth. In the sixth century BCE., Pythagoras theorized that Earth was a sphere resting in the center of the universe. Even though several of his students believed that the apparent movements of the Sun and the stars were caused by the movement of Earth, that explanation was rejected by many people, including Aristotle. The Greek astronomer Ptolemy confirmed Aristotle’s geocentric views. His Almagest (circa 150 Common Era or CE) was the standard work on astronomy for 1,400 years.

In 1543, Nicolaus Copernicus published his explanation of the sun-centered solar system and a new explanation for changing seasons. His text shows the position of the tilted Earth in relation to the sun at the two equinoxes and solstices. Although Copernicus’s heliocentric solar system and explanation for the seasons are commonly taught in schools today, his ideas did not win general acceptance for nearly two hundred years. Galileo Galilei’s use of the telescope confirmed Copernicus’s theories, as did Sir Isaac Newton’s 1687 theory of gravitation. The last minor obstacle to universal acceptance of the Copernican system was that the distant stars did not appear to move as Earth revolved around the sun. That opposition disappeared after Friedrich Bessel’s 1838 discovery of stellar parallax, which confirmed that those stars move, albeit by very small amounts.

Significance

Understanding the seasons is just as crucial to modern civilization as ancient ones. In the twenty-first century, the need to understand seasons is more complex but just as vital. Knowing the seasons and the length of a growing season still affects decisions about which crops can be planted in a given area. Certain crops require longer growing seasons—typically described in terms of “heat units”—than others and must be grown in southern areas; some perennial flowers, such as tulips, require a cold dormant period and cannot be grown in areas without a cold winter season. Homeowners who purchase trees and other living plants from landscaping catalogs need to know about their seasonal climates to buy plants suited to local conditions. Knowing how high the sun will be in the summer can help a homeowner or a landscape architect decide the best location for shade-producing trees, so they do not interfere with a sun-loving vegetable garden.

Seasonal changes of the sun can help the environmentally conscious architect decide where to place windows and whether to design roof overhangs. For example, when most new houses and office buildings will likely use air conditioning in summer, midday sunlight streaming in windows can drive up interior temperatures and the electricity required to maintain a cool interior temperature. To minimize this, homes and other buildings can be designed with roof overhangs that shade the windows from direct sunlight. In winter, when the buildings could use this sunlight to keep down heating costs, the sun will be farther south and thus lower in the sky. If well-designed, a roof overhang can shade windows from the high summer sun while allowing in light from the low winter sun. Numerous other features dependent on seasonal variations for their effectiveness are now incorporated into the design of energy-efficient structures.

Global climate change is affecting the seasons in several ways in the twenty-first century. For example, in the Northern Hemisphere, longer summer growing seasons are the result of earlier springs and later autumns. However, these conditions also increase the chances of droughts and wildfires. They are also accompanied by warmer winters with less snowfall. While some areas experience drought, others may see rainy seasons and precipitation increase. Changes in seasons can affect pollination patterns and increase the population of pests and the diseases they may carry. In the future, if mitigation efforts related to climate change are not enacted, scientists warn there may be a shift toward two main seasons instead of four. 

Principal Terms

autumnal equinox: the day that the sun passes directly over the equator in the southward direction, producing day and night of equal length and marking the beginning of autumn; in the Northern Hemisphere, the date is about September 21, and in the Southern Hemisphere, it occurs about March 21

equator: the line of latitude on Earth that is exactly halfway between the North and South Poles

monsoon: a wind system that results in an annual cycle of fair weather followed by rainy weather

perihelion: the point in a planet’s orbit at which it is closest to the sun

summer solstice: the day when the sun is directly over the Tropic of Cancer in the Northern Hemisphere; in the Southern Hemisphere, the day when the sun is directly over the Tropic of Capricorn

Tropic of Cancer: a line of latitude 23.5 degrees north of the equator; the most northerly latitude on Earth at which the noon sun passes directly overhead

Tropic of Capricorn: a line of latitude 23.5 degrees south of the equator; the most southerly latitude on Earth at which the noon sun passes directly overhead

vernal equinox: the day that the sun passes directly over the equator in the northward direction, producing day and night of equal length and marking the beginning of spring; in the Northern Hemisphere, the date is about March 21, and in the Southern Hemisphere, it is about September 21

winter solstice: the diametric opposite of the summer solstice, occurring in the Northern Hemisphere when the sun passes directly over the Tropic of Capricorn and in the Southern Hemisphere when it passes over the Tropic of Cancer

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