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Instruction 5-5
Circulation Patterns in the Earth's Atmosphere and Oceans | The Relationship Between the Rotation of the Earth and the Circular Motion of Ocean Currents and Air in Pressure Centers | The Origin and Effects of Temperature Inversion | Properties of Ocean Water | Location of Deserts and Rain Forests | Features of ENSO | ||||
| Location of Deserts and Rain Forests | ||||
| CCSTD Earth Science 5.d. | ||||
As you have learned, air circulates over the Earth in set patterns. These patterns cause zones of permanent high and low pressure. A high pressure area is a section of cool air that is sinking. As the air sinks, it gets warmer, which means it can hold more moisture. This prevents the formation of clouds, which is why high pressure areas are associated with dry, clear weather. In low pressure zones like those near the Equator, warm moist air rises and does form clouds. This eventually causes rain. That's why the Tropics are so wet. And that's where most rain forests are located, which we'll tell you about in a minute. In contrast, most deserts are located under the nearly cloudless skies of the high pressure zones north and south of the Equator. When rain-spent tropical air circulates and descends from Equatorial regions into these high-pressure zones, it warms up. This means it evaporates what little moisture there is in the desert's atmosphere, so the moisture can't fall as rain. That's why the deserts are so dry. Examples of high-pressure deserts include the Sahara and Kalahari on the African continent and the Rub-al-Khali in present-day Saudi Arabia. Archeologists think that the Rub-al-Khali is the site of the famous Lost City of Arabia.
For a map of the Earth's deserts, click: Deserts
Deserts are defined by the amount of rainfall they get – or don't get – annually: less than 250mm per year. About 33% of the Earth's land surface is desert, and it's increasing. Although most deserts are located in the lower latitudes, they can be found almost everywhere on earth – even in the Antarctic. There are four major types of deserts: Hot and Dry Deserts In this type of desert, seasons are warm throughout the year and hot in the summer. Temperatures go to daily extremes since there is little humidity to block the Sun's rays. Mean annual temperatures range from 20-25 degrees Celsius. The extreme maximum ranges from 43.5-49 degrees C. Minimum temperatures can drop to as low as -18 degrees C. The soil is coarse and rocky because the finer dust and sand particles are blown away. Plants are mainly ground-hugging shrubs like cacti and short woody trees. Plant leaves are designed to conserve water and are covered with a thick outer layer called a cuticle. These plants include yucca, ocotillo and prickly pear, which bloom briefly but beautifully in the spring. The animals of the hot dry desert are mostly small nocturnal carnivores like insects, spiders, reptiles, birds and kangaroo rats. The Mojave and Sonora deserts in North America are examples of this type of desert. Semi-Arid Deserts The major semi-arid deserts include Utah and Montana's sagebrush lands as well as the Neartic realm of Newfoundland, Greenland, Russia, Europe and Northern Asia. In a semi-arid desert, the summers are long and dry, with temperatures averaging between 21-27 degrees C. Daytime temperatures normally don't go above 38 degrees C and evening temperatures are cool, around 10 degrees C. The soil can range from sandy and fine-textured to rocky. Plants are spiny, which provides some protection from their hazardous environment. Many have silvery or glossy leaves, which allows them to reflect more of the Sun's blistering light. These plants include creosote and mesquite. Animals include kangaroo rats, rabbits and skunks. Insects include ants and grasshoppers. Reptiles include lizards and snakes and birds include burrowing owls and the California thrasher (even though there's no semi-arid desert in California). Coastal Deserts Coastal deserts occur in moderately cool to warm areas, like Atacama in Chile. The cool winters in coastal deserts are followed by moderately long warm summers. The average summer temperature ranges from 13-24 degrees C. Winter temperatures are 5 degrees C or below. The soil is fine-textured with a moderate salt content. All plants have thick, fleshy leaves or stems to take in any rain that should occur, which ranges from 8 to 13 cm a year. Some animals here have developed specialized ways for dealing with the desert heat. Toads, for example, seal themselves in burrows and remain inactive for 8 or 9 months until a heavy rain occurs. Some insects lay eggs that remain dormant until environmental conditions are suitable for hatching. Other coastal desert animals include insects, coyotes, badgers, owls, eagles, lizards and snakes. Cold Deserts Cold desert may sound like a contradiction in terms – but there are a number of cold deserts on Earth, primarily in the Antarctic and Greenland. These deserts have short, moist summers and long, cold winters. They receive 15-26 cm of precipitation a year (mostly in the form of winter snow). Temperatures range from 21-16 degrees C in the summer and -2 to 4 degrees C in the winter. The soil is heavy, silty and salty. Plants are widely scattered and include spiny-leafed deciduous plants and sagebrush. Animals include jackrabbits, kangaroo rats, mice, ground squirrels and deer (in the winter). Rainforests
As we told you, rainforests are located in low-pressure zones like those at or near the Equator. A low pressure area is a section of warm air that is rising. As the air rises, it cools. This creates clouds and rain, because cool air cannot hold as much moisture as warm air. Rainforests cover only a small part of the Earth's surface – about 6% (although they used to cover twice that much territory). Yet they are home to half the species of plants and animals on Earth.
To see an excellent map of the Earth's rainforests, click: What makes a Rainforest a Rainforest? Tropical rainforests are defined in two ways. First, by where they are located. Tropical rainforests are located, not surprisingly, in the Tropics – in the Topic of Cancer in the north and the Tropic of Capricorn in the south. These areas are approximately 24 degrees north and south of the Equator. The Amazon jungle is the world's largest tropical rainforest. Other major rainforests can be found in Southern Asia, Central Africa, Australasia and Central America. The second definition of a rainforest is how much rain it receives – an astonishing 4 to 8 meters a year. New York State, by contrast, gets about 1 meter of rain a year.
The heavy vegetation of the rainforest blocks this rainfall from falling
directly Another distinctive characteristic of a rainforest is that is has no seasons – no dry or cold season of slower growth. There are four different layers in a rainforest: The Emergent Layer The tallest trees in a rainforest are called the emergents. They tower as much as 200 feet above the forest floor. Most of these trees are broad-leafed hardwood evergreens. Sunlight is plentiful here and animals abound – eagles, monkeys, bats and butterflies. The Canopy Layer This is the primary layer of the rainforest. It forms a canopy over the two remaining layers. Most canopy trees have smooth oval leaves that come to a point. Food is abundant so many animals live here, including snakes, toucans and treefrogs. The Understory Layer Little sunshine reaches this layer, so plants have to grow larger to try and reach the sun. Even so, they seldom reach heights of more than 12 feet. There are many insects in this layer as well animals like jaguars, red-eyed treefrogs and leopards. The Forest Floor It's dark down here, so few plants grow. Because of the lack of sunshine, things decay quickly. Decay that would take a year elsewhere can be complete in as little as 6 weeks. Giant anteaters rule. The Bounty of the Rainforest Rainforests are home to half of the 5-to-10 million plant and animal species on Earth. Scientists estimate that there are still 30,000 undiscovered plants on Earth and that most are in the rainforest. One fourth of the medicines available today owe their existence to plants. Seventy percent of the plants identified by the National Cancer Institute as useful in cancer treatment can only be found in the rainforest. Medicines for heart ailments, hypertension, arthritis and birth control also come from plants in the rainforest. And 99% of rainforest plants still haven't been examined for their medicinal potential. Many of our favorite foods also originated in the rainforest. These include avocados, bananas, black pepper, Brazil nuts, cayenne pepper, cashews, chocolate, cinnamon, cloves, coconut, coffee, cola, corn, eggplant, figs, ginger, guavas, jalapeños, lemons, oranges, papayas, paprika, peanuts, pineapples, rice, winter squash, sweet peppers, sugar, tomatoes and vanilla. Rubber comes from the rainforest, too. The Disappearing Rainforest Tropical rainforests are the Earth's oldest living ecosystems. Fossil records show that the rainforests of Southeast Asia have existed in more or less their present form for from 70 to 100 million years. Unfortunately, rainforests are being destroyed at a staggering rate. This is happening due to understandable population and economic pressures, but it is still a tragedy. According to the National Academy of Science, at least 50 million acres of rainforest are being lost every year. This is an area the size of England, Wales and Scotland combined. All of the primary rainforests in India, Bangladesh, Sri Lanka and Haiti are now gone. The Ivory Coast rainforests have been almost completely logged. The Philippines lost 55% of its rainforest between 1960 and 1985 and Thailand lost 45% of its rain forest between 1961 and 1985. This destruction continues. Many governmental, environmental and other organizations are trying to find solutions to the problem. Video Instruction
Now let's do Practice Exercise 5-5 (top). |
