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Temperature and atmosphere are the first essential factors for life on Earth. The Blue Planet has both a temperature that is livable and an atmosphere that is breathable for living things, especially for such complex living things as human beings. These two extremely different factors however have come into being as a result of conditions that turn out to be ideal for both. One of these is the distance between the earth and the sun. Earth could not be a home for life if were as near the sun as Venus is or as far from it as Jupiter: carbon-based molecules can only survive between the limits of 120 and –20°C and Earth is the only planet whose average temperatures fall within those limits. When one considers the universe as a whole, coming across a range of temperatures as narrow as this is quite a difficult task because temperatures in the universe vary from the millions of degrees of the hottest stars to absolute zero (-273°C). In such a vast range of temperatures, the thermal interval that allows life to exist is slim indeed; but the planet Earth has it. The American geologists Frank Press and Raymond Siever draw attention to the average temperatures prevailing on Earth. They note that "life as we know it is possible over a very narrow temperature interval. This interval is perhaps 1 or 2 percent of the range between a temperature of absolute zero and the surface temperature of the Sun." The maintenance of this thermal range is also related to the amount of heat that the sun radiates as well as to the distance between the earth and the sun. According to calculations, a reduction of just 10% in the sun's radiant energy would result in the earth surface's being covered by layers of ice many meters thick and that if it were to increase by a little, all living things would be scorched and die. Not only must the average temperature be ideal: the available heat must also be distributed fairly equally over the whole planet. A number of special precautions have been taken to ensure that this in fact happens. The earth's axis is inclined 23° 27'to the plane of the ecliptic. This inclination prevents overheating of the atmosphere in the regions between the poles and the equator, causing them to become more temperate. If this inclination did not exist, the temperature gradient between the poles and equator would be much higher than it is and the temperate zones wouldn't be so temperate–or livable. The rotational speed of Earth on its axes also helps keep the thermal distribution in balance. The earth makes a complete rotation once every 24 hours with the result that alternating periods of daylight and darkness are fairly short. Because they are short, the thermal gradient between the light and dark sides of the planet are quite modest. The importance of this can be seen in the extreme example of Mercury, where a day lasts longer than a year and where the difference between daytime and nighttime temperatures is almost 1,000°C. Geography also helps distribute heat equally over the earth. There is a difference of about 100°C between the polar and equatorial regions of Earth. If such a thermal gradient were to exist over a completely level area, the result would be winds reaching speeds as high as 1,000 kilometers an hour sweeping away everything in their path. Instead, Earth is full of geographical barriers that block the huge movements of air that such a thermal gradient would otherwise cause. Those barriers are chains of mountains like the one that stretches from the Pacific in the east to the Atlantic in the west, beginning with the Himalayas in China and continuing with the Taurus mountains in Anatolia and the Alps in Europe. At sea, the excess heat in the equatorial regions is transferred north and south thanks to the superior ability of the water to conduct and dissipate heat. At the same time, there are a number of auto-control systems that help keep the atmospheric temperature in balance. For example when a region heats up, the rate at which its water vaporizes increases, causing clouds to form. These clouds reflect more light back into space, preventing both the air and the surface below from getting warmer.
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