But the face of the Earth can change in a much more subtle manner, shaped by the air and water, which affect the climate and make life possible. Our atmosphere is a mix of dust and gas: 78% nitrogen, 21% oxygen and 1% rare gases, such as argon and xenon, in addition to carbon dioxide, water vapor and ozone. As we approach from space, the first layer that we reach is called the exosphere: in this region the air is so rarefied and the gravitational attraction of the Earth is so weak that atoms tend to escape into the cosmic vacuum. Below the exosphere, at about 500 kilometers from the surface of the planet, is where the thermosphere begins: here also there are very few molecules and so much so that the sky is completely black. That's because solar radiation can barely find anything on which to reflect and to scatter light. Since the air filter is almost non-existent, a living being, just like in open space, might "cook" on the side exposed to the Sun and freeze on the opposite side: in fact, the temperature in this region never exceeds -90 degrees Celsius. Continuing our descent we reach the mesosphere, a layer with a thickness of 30 kilometers in which the air is still very rarefied and the sky very dark, but the temperature increases to zero in the lowest parts. Still lower we find the denser ionosphere, where long-distance radio communications are possible: in fact, this is where short waves emitted by transmitters can bounce back to return to the Earth and be captured by receivers - and even over long distances. The next layer is called the stratosphere: 40 kilometers thick, it is the site of a strange phenomenon - it is colder at the bottom (-50 degrees) than at the top. How is this possible considering that the air seems to become hotter as it descends through the lower layers of the atmosphere? The answer lies in a strip of gas, ozone, which surrounds the Earth in the upper stratosphere. Ozone, a special form of oxygen whose molecules contain three atoms instead of two, absorbs a large amount of ultraviolet radiation emitted by the Sun, preventing such radiation from reaching the Earth's surface: and in this way is heated. The layer closest to the surface is the troposphere - the site of all the atmospheric phenomena that we are familar with. Here, the farther down we go, the air becomes denser and hotter. Heating occurs because the short wavelength radiation coming from the Sun that strikes the ground loses a part of its energy and is re-emitted with a longer wavelength in the form of heat. The higher layers of the troposphere are criss-crossed by strong winds, which blow from west to east at 400 kilometers per hour, called "jet streams": jet pilots are very familiar with these air currents and use them to fly faster, for example between the United States and Europe. Instead, a little bit lower, at an altitude of less than 10 000 meters, the winds are more moderate and the color of the sky becomes a lighter blue: here the density is higher and there is also a considerable amount of turbulence due to the continuous circulation of air masses at different temperatures and humidity which move horizontally and vertically. The greatest movements are caused by the large difference in temperature between the poles and the equator. In fact, at the lower latitudes, the air is heated, expands and tends to move towards the poles. Vice versa, near the poles, the air is cooled and "sinks", tending to move towards the equator. This phenomenon is of vital importance, because if the air circulating from the equator to the poles and back again should cease to exist, there would be a continuous increase in temperature in the tropical regions and, on the contrary, continuous cooling in the arctic and antarctic zones. However, in this way the continuous exchange of hot and cold air masses not only makes the Earth a planet with a relatively bearable climate in all regions, but it also distributes precipitations everywhere in the form of rain or snow, and therefore exchanges water with the oceans.
But the face of the Earth can change in a much more subtle manner, shaped by the air and water, which affect the climate and make life possible. Our atmosphere is a mix of dust and gas: 78% nitrogen, 21% oxygen and 1% rare gases, such as argon and xenon, in addition to carbon dioxide, water vapor and ozone. As we approach from space, the first layer that we reach is called the exosphere: in this region the air is so rarefied and the gravitational attraction of the Earth is so weak that atoms tend to escape into the cosmic vacuum. Below the exosphere, at about 500 kilometers from the surface of the planet, is where the thermosphere begins: here also there are very few molecules and so much so that the sky is completely black. That's because solar radiation can barely find anything on which to reflect and to scatter light. Since the air filter is almost non-existent, a living being, just like in open space, might "cook" on the side exposed to the Sun and freeze on the opposite side: in fact, the temperature in this region never exceeds -90 degrees Celsius. Continuing our descent we reach the mesosphere, a layer with a thickness of 30 kilometers in which the air is still very rarefied and the sky very dark, but the temperature increases to zero in the lowest parts. Still lower we find the denser ionosphere, where long-distance radio communications are possible: in fact, this is where short waves emitted by transmitters can bounce back to return to the Earth and be captured by receivers - and even over long distances. The next layer is called the stratosphere: 40 kilometers thick, it is the site of a strange phenomenon - it is colder at the bottom (-50 degrees) than at the top. How is this possible considering that the air seems to become hotter as it descends through the lower layers of the atmosphere? The answer lies in a strip of gas, ozone, which surrounds the Earth in the upper stratosphere. Ozone, a special form of oxygen whose molecules contain three atoms instead of two, absorbs a large amount of ultraviolet radiation emitted by the Sun, preventing such radiation from reaching the Earth's surface: and in this way is heated. The layer closest to the surface is the troposphere - the site of all the atmospheric phenomena that we are familar with. Here, the farther down we go, the air becomes denser and hotter. Heating occurs because the short wavelength radiation coming from the Sun that strikes the ground loses a part of its energy and is re-emitted with a longer wavelength in the form of heat. The higher layers of the troposphere are criss-crossed by strong winds, which blow from west to east at 400 kilometers per hour, called "jet streams": jet pilots are very familiar with these air currents and use them to fly faster, for example between the United States and Europe. Instead, a little bit lower, at an altitude of less than 10 000 meters, the winds are more moderate and the color of the sky becomes a lighter blue: here the density is higher and there is also a considerable amount of turbulence due to the continuous circulation of air masses at different temperatures and humidity which move horizontally and vertically. The greatest movements are caused by the large difference in temperature between the poles and the equator. In fact, at the lower latitudes, the air is heated, expands and tends to move towards the poles. Vice versa, near the poles, the air is cooled and "sinks", tending to move towards the equator. This phenomenon is of vital importance, because if the air circulating from the equator to the poles and back again should cease to exist, there would be a continuous increase in temperature in the tropical regions and, on the contrary, continuous cooling in the arctic and antarctic zones. However, in this way the continuous exchange of hot and cold air masses not only makes the Earth a planet with a relatively bearable climate in all regions, but it also distributes precipitations everywhere in the form of rain or snow, and therefore exchanges water with the oceans.
