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Mediterranean climate and vegetation

Why are Mediterranean summers hot and dry?

Because this area is less than 40° from the equator, the sun is quite high in the sky. The sun's rays are therefore concentrated and give great heat. High pressure usually dominates the area in summer. This brings long periods of settled, dry weather.

Why are winters mild and wet?

Although the sun is lower in the sky in winter, it is still high enough to give quite warm conditions. During winter, the prevailing winds blow in from the Atlantic Ocean in the west. These winds are mild and damp. When they rise over mountainous areas, relief rainfall occurs. Westerly winds also bring depressions and these result in wet, unsettled conditions.

How do Mediterranean plants adapt to summer drought?

They usually have small waxy leaves or sharp thorns that do not lose much moisture through transpiration. (Transpiration is the loss of moisture through the leaves or bark of a plant.) Trees such as the cork oak and olive have thick, protective bark. These help to stop heat getting in or moisture getting out through transpiration. Long tap roots reach far underground for water. Herbs such as lavender have short life cycles that avoid periods of drought: plants flower in the spring and lie dormant during the dry summer.

Population

Population means the number of people who live in a particular area. The population of the world doesn’t stay the same. At the moment it is growing at an increasing rate. In fact, scientists believe that the world population will increase until 2200, and then stop growing. However things are not the same in all parts of the world. At present in many Western industrial countries the population is falling. This is happening because families are small, and health conditions are good. In developing countries, on the other hand, the population is rising sharply. In Ghana, for example, most families have several children. Many children in Ghana die from illness, so it is important to have lots of children. They earn money for their parents in old age.

Rapid population growth can cause problems. The government has to provide more hospitals, schools and jobs, farmers have to produce more food and all the extra people have to have homes. In a less economically developed country, the economy can’t grow fast enough to deal with the problems. Many countries realize that they have to control population growth. Some countries believe they can encourage family planning through education and publicity. Other countries believe that they have such a serious problem that they have to take special measures to control the number of people. In China, for example, there is a ‘one child’ policy. Couples have to ask permission from the government to marry, and women have to be at least 25 years old. The couples then have to ask permission to have a child, and are allowed only one.

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In geography, a desert is an area which receives a little rain and which loses lot of its moisture through evaporation. Many polar regions can be called deserts, but most of us think of a desert as being a sandy, rocky area with not enough water. Lots of deserts consist of sand dunes or bare rock, and many are near mountain ranges, which take away the moisture from clouds. Others are far away from the sea or other water, so receive little moisture. Although we might assume that very few kinds of life live in deserts, in fact there are lots of plants, animals and insects in these regions. Many desert plants store water in their leaves or roots, and some desert plants can live for many years. Some desert animals live underground. They spend little time in the sun and only come out at night. There are lots of insects, scorpions and spiders as well as reptiles, such as snakes, lizards and tortoises, in deserts. They need to spend many hours in the sun to generate body heat, so they have little difficulty living in high temperatures. However, few of them can bear extreme sunlight, so they tend to move one area of shade to another.



Cholera

Cholera is a disease spread by dirty water, either when people drink the water, or eat food which has been washed in the water, and not properly cooked. Unfortunately, until the mid 19th century, people believed that the disease travelled through the air. In the crowded cities of 19th-century Britain, cholera spread easily. Most doctors wrongly believed that fresh air and a better diet would prevent the disease. Huge fires were usually lit in the streets to drive away the infection, and the clothes of victims were also burned. The streets were also cleaned thoroughly with the powerful chemicals. However, people continued to suffer dreadfully. In 1854, a doctor in London, John Snow, definitely proved that cholera spread through infected drinking water. He did this by carefully collecting statistics about the infection in one part of London. He managed to improve that in an area where deaths from cholera were highest, the water was entirely responsible for cholera infection. Improvements made to sewers and supplies of drinking water later led to a decrease in deaths from cholera.

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At the moment, scientists agree that the world’s climate has become warmer over the past 50 years, but they disagree about the causes. Some believe that human activities have caused climate change. They argue that for 1,000 or 2,000 years before 1850, when records began, the temperature was more or less stable. Short warm or cold periods occurred during that time, but the climate always returned to the same level. However, since the Industrial Revolution, human beings have burned more and more fossil fuels, such as coal and oil. In 1800 the atmosphere contained around 280 parts per million of carbon dioxide (CO2). Since then there has been an increase of about 31%. This extra carbon dioxide has raised the world’s temperature because of the greenhouse effect. Other scientists disagree that human activities over the past 50 years have caused global warming. They point out that volcanoes and other natural processes have always released CO2 into atmosphere, and that human activity has contributed a rise in CO2 of only three per cent. In 1999, 156 countries signed the Kyoto protocol, part of a United Nations agreement on climate change, which came into force in 2005. They agreed to reduce their emissions of CO2 and other greenhouse gases, although so far, some countries, such as the USA and Australia, have not taken any action.

 

Tropical forests

Tropical forests grow near the Equator where it is warm and wet. There are only two seasons, rainy and dry. It is normally 20-25 C in a tropical forest and there is only a drop of about 5 C in the coldest months. There is a lot of rain, on average more than 200cm per year. There is poor soil, and it is difficult for plants on the forest floor to develop, as tall trees (25-35 meters) block the light. There are many plants, birds, animals and insects. There are different kinds of tropical forests with different kinds of trees. It depends on the temperature and the amount of rainfall. In temperature forests, there are four seasons with a cold winter. It is impossible for the trees to grow all the year round in a temperature forest, and there is a growing season of only 140-200 days. There is also a greater range of temperatures, from -30 to 30 C, and it rains throughout the year (75-150 cm). As it is cooler and drier here, trees are smaller, there is more light, and the soil is richer. There are many deciduous trees in these forests, and it is common to find many animals and birds. Again, there is a range of forest types depending on the annual rainfall.


Date: 2015-12-18; view: 1602


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