Climate

Nature Gallery (Earth)

Climate

Drama of the Seasons

In the autumn night sky of the northern hemishphere the constellation Orion appears earlier and stays longer as the nights lengthen. As the days continue to shorten, monarch butterflies fly south. Ocean currents off the coast of California warm the sea as the land cools under the winter skies. In the interior of continents in the northern hemishphere, the leaves of deciduous trees turn to flaming colours before being shed and animals instinctively prepare for the coming season of winter.

Six months later these events reverse. Similar changes occur in temperate regions all over the globe, alternating between the Northern and Southern hemispheres. Meanwhile, in the tropics, the seasons alternate between periods of torrential rain and dry periods. The dry season begins with an explosion of blooming plantlife and animal activity and ends with a period of wilting and waiting for the coming rainy season. The Cause of Seasons
The seasons are annual cycles caused by the angle of tilt of the Earth’s rotational axis. This tilt remains at an angle of 23°27' as the Earth orbits the sun, making one complete orbit in one year of 365.25 days. Depending on Earth’s orbital position, either the southern or northern hemishphere is tipped towards the sun, receiving more direct solar radiation. The fact that the Earth leans either towards or away from the sun has resulted in tremendous change and variation in the physical planet, plant and animal life, and the human experience. Day Length, Solstices, and Equinoxes

The Earth moves around the sun in an eliptical orbit, maintaining a constant tilt. This tilt means that one of the Earth’s hemispheres usually receives more sunlight than the other. The hemisphere nearest to the sun at either the June or December solstice receives its maximum possible sun exposure. At these times, the other hemisphere is as far from the sun as it can be, and is thrown into shadow.

The solstices represent the beginning of summer and winter, causing the daylight hours to be longest on one side of the Equator and shortest on the other. The extremes of the solstices are experienced at the poles, which receive nearly continuous daylight or continuous darkness. Six months later, when the Earth is exactly halfway around its solar orbit, the opposite conditions are experienced. Between the June and December solstices the Earth’s axis is positioned sideways towards the sun. When halfway between solstices, the sun’s rays strike the Equator perpendicularly, causing both hemispheres to briefly receive equal sunlight. Day length is then the same everywhere. These two positions are known as the equinoxes, marking the beginning of spring and autumn. The Tropics
Tropical regions experience the least variation in day length between the seasons. Their position near to the centre of the sun's exposure on Earth, means that day length here is never far from average. On the June solstice the sun's rays are perpendicular to Earth at a certain latitude which is either north or south of the equator. On this day, the sun appears to reach its midday height in the exact vertical centre of the sky. These latitudes are the geographical limits of the tropics-the equatorial regions which have a consistently warm climate. The Northern Hemisphere's tropic line is known as the Tropic of Cancer and lies at 23°27' North. The corresponding line in the southern hemisphere is the Tropic of Capricorn, and lies at 23°27' South. Not surprisingly, these angles represent the exact tilt of Earth in its orbit around the sun. If there were no tilt, the sun would always be directly above the equator, and there would be no seasons as we know them. Star Positions
As the Earth moves, so does its relationship to the stars and other astronomical bodies. Indeed, the stars and planets appear to move around Earth, although it is actually Earth that is moving. This perceived movement of the stars at night has been a major influence in the development of navigation and geography, as well as in the world’s cultures and religions. The appearance is complicated by the fact that the planet is both spinning on its own axis and orbiting the sun on another axis. A person’s view of the night sky is influenced by the seasonal progression of daylight and the night-time. For example, Orion—a winter constellation in the northern hemishphere—is situated in the daylight sky in midsummer and since the light of the sun is stronger, it cannot be seen. As the seasons progress, the night side of the planet turns toward Orion, which is then visible in the night sky. A slightly different view of the stars is therefore presented each night, and the skyscape is the opposite at the summer (June in the Northern hemisphere) and winter (December) solstices. Constellations near the horizon disappear from view entirely for one season and then gradually appear again. Comets and planets have their own complicated orbits around the sun, so their patterns of movement in the sky do not correspond to the more predictable wanderings of the stars. The position of the stars is also affected by two other factors. First, the Earth’s tilt on its axis moves slightly backwards and forwards over a period of many years, altering the celestial view. Second, the stars themselves travel through space on their own trajectories, and the constellations change their shape over many centuries. People have been trying to understand the position of the stars since long before recorded history. Constellations—visual groups of stars that resemble familiar objects—were the first attempt to classify the stars. The constellations recognized by different cultures through time depended on historical precedent, cultural factors, and geography. To locate certain stars for study and navigation, astronomers, astrologers, and navigators still use the constellations familiar to the ancient civilizations of the Mediterranean, Central America, China, and elsewhere. Although the stars have always been important to religion, navigation and agriculture were the main practical reasons for organizing them into recognizable and predictable entities. Stars that appeared to hardly move were particularly valuable in navigation, especially Polaris, the North Star of the northern hemishphere, and the constellation Octans of the southern hemisphere. These are the visible stars most closely aligned with the axis of Earth. Modern astronomy uses star charts to make star navigation more accurate. The stars appear to be spread over the inner surface of a sphere, so star charts have spherical qualities, like a globe turned inside out. They are usually organized into Northern and Southern Hemisphere views that are divided into 24 hours or radial sectors with 180-degree increments between the horizons. Charts of the equatorial skies, up to about 30° north or south of the equatorial horizon, are often represented by rectangular maps which are also divided lengthwise into 24 hours. Star charts are not static like Earth maps because they usually display seasonal views of the sky from a particular point. They can also show the entire sky irrespective of season but with some indication of the seasonal change. Climate

In temperate regions, winter is a time of short days, long nights, and cooler temperatures. Precipitation is often greater and falls more regularly than in tropical regions. Summer is a time of long days, short nights, warm temperatures, and little or no rainfall.

The seasons play a large role in determining the climate across the globe. As summer sunlight warms land masses, overlying air decreases in density and rises, before it settles over the ocean.

Ocean-cooled air is then drawn toward the land by the updraught, setting off cycles of air movement that cause major climatic disturbances.

These cycles of warming, cooling, and the resulting tumbling air movement generally subside as winter sets in and the contrast between land and ocean temperatures diminishes. These cycles are generally more pronounced in the northern hemishphere because there is a greater proportion of land surface to heat the air. Phenomena such as monsoons and hurricanes represent the extremes of such seasonal weather patterns.

Physical Effects of Seasonal Climate

As the climate changes with the seasons, it directly affects the physical Earth, as well as artificial habitats and human activity. In winter, freezing ice breaks rocks apart as it expands, helping to create soil. Similar action in the crevices of city streets and buildings exerts wear and tear on man-made structures. Heavy seasonal rain scours the terrain and summer winds raise dust, parching the earth with drought.

Modern city life is brought to a standstill by occasionally powerful winter storms, but in the far north of Russia, Canada, and Alaska, some residents await the freezing of major waterways to provide icy highways for transporting goods. Ocean Currents

Seasonal change also affects the oceans. The massive amount of water in the seas absorbs a great deal of the sun’s energy, which it then stores for a long time. The seasons do not produce the dramatic temperature fluctuations which are felt on land, but they do produce seasonal winds, that create surface currents.

Examples of this are the California current off the Pacific coast of North America and the Kuroshio current off the Pacific coast of Japan. These change the direction of their flow from clockwise in the summer to anti-clockwise in the winter. Along the Pacific coast of North America, warm surface water flows northwards in winter and cold arctic water flows southwards in summer.

The Coriolis force is produced by the Earth’s rotation. It causes the wind (or any object moving relative to the surface of the Earth) to move to the right of its expected course in the Northern Hemisphere, or to the left in the Southern Hemisphere. The result is that deep ocean water moves at right angles to the surface current. In summer, the movement is toward the land, causing downflow.

In winter, this reverses and it flows away from shore, drawing up deep water in its wake. This is known as upwelling. Seasonal mixing of deep and shallow water is an important factor in the biology of the oceans because sea life depends on recycling of the nutrients that accumulate in the deep ocean. Ocean fish harvests are often seasonal because of upwelling.

Biological Patterns

Seasonal change has profoundly influenced the survival and evolution of life on Earth. Animals and plants, especially those in temperate regions, have incorporated seasonal survival strategies into their life cycles.

Most species are diminished during winter. In temperate zones, cold weather and short days bring starvation and a constant struggle against the elements. Many animals and plants, especially the weak and old, die. Populations are naturally trimmed to leave only the fittest individuals, which has the effect of improving survival in future generations.

The leaves of plants are solar energy collectors. The leaves of herbaceous and many broad-leaved deciduous species are useless in the winter climate of temperate regions because solar radiation is minimal and temperatures are too cold for photosynthesis.

As frost settles, these plants cut off the flow of nutrients to the leaves, reabsorbing the valuable green chlorophyll and causing the leaves to turn the yellow and red colours of autumn. Finally the leaves drop off the tree. Deciduous plants survive the winter with their energy reserves efficiently stored in their roots.

Other plants use a variety of strategies to survive winter. Evergreen trees such as pines, cedars, and hemlocks retain their leaves. They have more primitive, yet successful strategies for conserving energy and tissue in cold or freezing weather. Some plants return all their energy to bulbs or corms under the soil or as seeds or spores. They are then ready to resprout when spring arrives.

As winter approaches, many mammals perform elaborate rituals and begin to look or fight for mates. Hairy animals grow thicker coats over layers of fat and the plumages of birds are renewed. Mammals such as the snowshoe hare and the arctic fox change their fur completely to camouflage with the snow. Animals in temperate climates have evolved patterns of behaviour that help them to prepare for winter. Honeybees, squirrels, and some birds frantically gather and store food, whilst bears overeat in preparation.

Many animals hibernate during the months of winter—their metabolism slows and they fall into a deep sleep, living off their summer fat stores, awakening only once or twice to sniff the air for signs of spring. Insects undergo a similar resting state known as diapause, which occurs during a particular stage of development—egg, larva, pupa, or adult, depending on the species.

In temperate zones summers can be as challenging as winters. Some plants have adapted to the extremes of summer to avoid moisture loss. Desert species, such as certain desert grasses and trees, die back to their roots or drop their leaves.

Animal Migration

Among the many seasonal habits of animals, migration is perhaps the most dramatic. Some hummingbirds migrate thousands of kilometres between continents as the seasons change. The arctic tern breeds and spends its summers in the Canadian and Alaskan tundra, before flying to the Antarctic for the winter. Chinook and sockeye salmon migrate once in their lives, when they leave their ocean habitat and return to the exact stream where they were hatched from eggs. This final journey is triggered by seasonal changes. Monarch butterflies migrate south in the autumn, leaving their summer feeding grounds in temperate North America to live in communities in Mexico that may host millions of individuals.

They travel up to 5,000 kilometres (3,100 miles). In spring they begin the first leg of the return journey, stopping to feed, mate, and lay eggs as they go. The entire circuit is completed in late summer by the second or third generation. The seasons have a profound effect on the lives of many of the Earth’s inhabitants, including human beings. For more information on Climate, go to:
El Nino Effect Glaciation Greenhouse Effect Monsoons, Tropical Storms and Tornadoes