the atmosphere

there are five layers of the earth atmosphere. i am going to tell you about all five layers.

The troposphere is the lowest layer of Earth's atmosphere and site of all weather on Earth. The troposphere is bonded on the top by a layer of air called the tropopause, which separates the troposphere from the stratosphere, and on bottom by the surface of the Earth. The troposphere is wider at the equator (10mi) than at the poles (5mi).The troposphere contains 75 percent of atmosphere's mass- on an average day the weight of the molecules in the air is14.7 lb..(sq. in.)- and most of the atmosphere's water vapor. Water vapor concentration varies from trace amounts in Polar Regions to nearly 4 percent in the tropics. Most prevalent gases are nitrogen (78 percent) and oxygen (21 percent), with the remaining 1- percent consisting of argon, (.9 percent) and traces of hydrogen ozone ( a form of oxygen), and other constituents. Temperature and water vapor content in the troposphere decrease rapidly with altitude. Water vapor plays a major role in regulating air temperature because it absorbs solar energy and thermal radiation from the planet's surface.The troposphere contains 99% of the water vapor in the atmosphere. Water vapor concentrations vary with latitudinal position(north to south). They are greatest above the tropics, where they might be as high as 3% and decrease toward the polar regions

Carbon dioxide is present in small amounts, but its concentration has nearly doubled since 1900.Like water vapor, carbon dioxide is a greenhouse gas which traps some of the Earth's heat close to the surface and prevents its release into space. Scientists fear that the increasing amounts of carbon dioxide could raise the Earth's surface temperature during the next century, bringing significant changes to worldwide weather patterns. Such changes may include a shift in climatic zones and the melting of the polar ice caps, which could raise the level of the world's oceans.

The uneven heating of the regions of the troposphere by the sun ( the sun warms the air at the equator more than the air at the poles )causes convection currents, large-scale patterns of winds that move heat and moisture around the globe. In the Northern and Southern hemispheres, air rises along the equator and subpolar ( latitude about 50 to about 70 north and south ) climatic regions and sinks in the polar and subtropical regions. Air is deflected by the Earth's rotation as it moves between the poles and equator, creating belts of surface winds moving from east to west ( easterly winds ) in tropical and polar regions, the winds moving from west to east ( westerly winds ) in the middle latitudes. This global circulation is disrupted by the circular wind patterns of migrating high and low air pressure areas, plus locally abrupt changes in wind speed and direction known as turbulence.

A common feature of the troposphere of densely populated areas is smog, which restricts visibility and is irritating to the eyes and throat. Smog is produced when pollutants accumulate close to the surface beneath an inversion layer ( a layer of air in which the usual rule that temperature of air decreases with altitude doesn't apply ), and undergo a series of chemical reactions in the presence pollutants from escaping into the upper atmosphere. Convection is the mechanism responsible for the vertical transport of heat in the troposphere while horizontal heat transfer is accomplished through advection.

The exchange and movement of water between the earth and atmosphere is called the water cycle. The cycle, which occurs in the troposphere, begins as the sun evaporates large amounts of water from the earth's surface and the moisture is transported to other regions by the wind. As air rises, expands, and cools, water vapor condenses and clouds develop. Clouds cover large portions of the earth at any given time and vary from fair weather cirrus to towering cumulus clouds. When liquid or solid water particles grow large enough in size, they fall toward the earth as precipitation. The type of precipitation that reaches the ground, be it rain, snow, sleet, or freezing rain, depends upon the temperature of the air through which it falls.

As sunlight enters the atmosphere, a portion is immediately reflected back to space, but the rest penetrates the atmosphere and is absorbed by the earth's surface. This energy is then remitted by the earth back into atmosphere as long-wave radiation. Carbon dioxide and water molecules absorb this energy and emit much of it back towards the earth again. This delicate exchange of energy between the earth's surface and atmosphere keeps the average global temperature from changing drastically changing from year to year.

The stratosphere is a layer of Earth's atmosphere. It is the second layer of the atmosphere as you go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere.

The bottom of the stratosphere is around 10 km (6.2 miles or about 33,000 feet) above the ground at middle latitudes. The top of the stratosphere occurs at an altitude of 50 km (31 miles). The height of the bottom of the stratosphere varies with latitude and with the seasons. The lower boundary of the stratosphere can be as high as 20 km (12 miles or 65,000 feet) near the equator and as low as 7 km (4 miles or 23,000 feet) at the poles in winter. The lower boundary of the stratosphere is called the tropopause; the upper boundary is called the stratopause.Temperatures rise as one moves upward through the stratosphere. This is exactly the opposite of the behavior in the troposphere in which we live, where temperatures drop with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.

The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere. Polar stratospheric clouds (PSCs) are the exception. PSCs appear in the lower stratosphere near the poles in winter. They are found at altitudes of 15 to 25 km (9.3 to 15.5 miles) and form only when temperatures at those heights dip below -78° C. They appear to help cause the formation of the infamous holes in the ozone layer by "encouraging" certain chemical reactions that destroy ozone. PSCs are also called nacreous clouds.

the mesosphere is the third layer of the atmosphere.The mesosphere is directly above the stratosphere and below the thermosphere. It extends from about 50 to 85 km (31 to 53 miles) above our planet.

Temperature decreases with height throughout the mesosphere. The coldest temperatures in Earth's atmosphere, about -90° C (-130° F), are found near the top of this layer.

The exosphere is the uppermost region of Earth's atmosphere as it gradually fades into the vacuum of space. Air in the exosphere is extremely thin - in many ways it is almost the same as the airless void of outer space.

The layer directly below the exosphere is the thermosphere; the boundary between the two is called the thermopause. The bottom of the exosphere is sometimes also referred to as the exobase. The altitude of the lower boundary of the exosphere varies. When the Sun is active around the peak of the sunspot cycle, X-rays and ultraviolet radiation from the Sun heat and "puff up" the thermosphere - raising the altitude of the thermopause to heights around 1,000 km (620 miles) above Earth's surface. When the Sun is less active during the low point of the sunspot cycle, solar radiation is less intense and the thermopause recedes to within about 500 km (310 miles) of Earth's surface.

Not all scientists agree that the exosphere is really a part of the atmosphere. Some scientists consider the thermosphere the uppermost part of Earth's atmosphere, and think that the exosphere is really just part of space. However, other scientists do consider the exosphere part of our planet's atmosphere.

Since the exosphere gradually fades into outer space, there is no clear upper boundary of this layer. One definition of the outermost limit of the exosphere places the uppermost edge of Earth's atmosphere around 190,000 km (120,000 miles), about halfway to the Moon. At this distance, radiation pressure from sunlight exerts more force on hydrogen atoms than does the pull of Earth's gravity. A faint glow of ultraviolet radiation scattered by hydrogen atoms in the uppermost atmosphere has been detected at heights of 100,000 km (62,000 miles) by satellites. This region of UV glow is called the geocorona.

The exosphere is the uppermost region of Earth's atmosphere as it gradually fades into the vacuum of space. Air in the exosphere is extremely thin - in many ways it is almost the same as the airless void of outer space.

The layer directly below the exosphere is the thermosphere; the boundary between the two is called the thermopause. The bottom of the exosphere is sometimes also referred to as the exobase. The altitude of the lower boundary of the exosphere varies. When the Sun is active around the peak of the sunspot cycle, X-rays and ultraviolet radiation from the Sun heat and "puff up" the thermosphere - raising the altitude of the thermopause to heights around 1,000 km (620 miles) above Earth's surface. When the Sun is less active during the low point of the sunspot cycle, solar radiation is less intense and the thermopause recedes to within about 500 km (310 miles) of Earth's surface.

Not all scientists agree that the exosphere is really a part of the atmosphere. Some scientists consider the thermosphere the uppermost part of Earth's atmosphere, and think that the exosphere is really just part of space. However, other scientists do consider the exosphere part of our planet's atmosphere.

Since the exosphere gradually fades into outer space, there is no clear upper boundary of this layer. One definition of the outermost limit of the exosphere places the uppermost edge of Earth's atmosphere around 190,000 km (120,000 miles), about halfway to the Moon. At this distance, radiation pressure from sunlight exerts more force on hydrogen atoms than does the pull of Earth's gravity. A faint glow of ultraviolet radiation scattered by hydrogen atoms in the uppermost atmosphere has been detected at heights of 100,000 km (62,000 miles) by satellites. This region of UV glow is called the geocorona.

Below the exosphere, molecules and atoms of atmospheric gases constantly collide with each other. However, air in the exosphere is so thin that such collisions are very rare. Gas atoms and molecules in the exosphere move along "ballistic trajectories", reminiscent of the arcing flight of a thrown ball (or shot cannonball!) as it gradually curves back towards Earth under the pull of gravity. Most gas particles in the exosphere zoom along curved paths without ever hitting another atom or molecule, eventually arcing back down into the lower atmosphere due to the pull of gravity. However, some of the faster-moving particles don't return to Earth - they fly off into space instead! A small portion of our atmosphere "leaks" away into space each year in this way.

Although the exosphere is technically part of Earth's atmosphere, in many ways it is part of outer space. Many satellites, including the International Space Station (ISS), orbit within the exosphere or below. For example, the average altitude of the ISS is about 330 km (205 miles), placing it in the thermosphere below the exosphere! Although the atmosphere is very, very thin in the thermosphere and exosphere, there is still enough air to cause a slight amount of drag force on satellites that orbit within these layers. This drag force gradually slows the spacecraft in their orbits, so that they eventually would fall out of orbit and burn up as they re-entered the atmosphere unless something is done to boost them back upwards. The ISS loses about 2 km (1.2 miles) in altitude each month to such "orbital decay", and must periodically be given an upward boost by rocket engines to keep it in orbit.

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