Earth's Atmosphere

Earth's Atmosphere

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The Earth's atmosphere is more than just the air we breathe. It's also a buffer that keeps us from being peppered by meteorites, a screen against deadly radiation, and the reason radio waves can be bounced for long distances around the planet.

The air that accomplishes all of this is composed of five major layers.

The lowest is the troposphere, which is the layer that provides most of our weather. It contains about four-fifths of the Earth's air, but extends only to a height of about 11 miles (17 kilometers) at the Equator and somewhat less at the Poles.

The name comes from a Greek word that refers to mixing. And mixing is exactly what happens within the troposphere, as warm air rises to form clouds, rain falls, and winds stir the lands below. Typically, the higher you go in the troposphere, the colder it gets.

Above the troposphere is the stratosphere. It extends to a height of about 30 miles (50 kilometers) and includes the ozone layer, which blocks much of the sun's harmful ultraviolet rays.

The stratosphere is warmer than the troposphere because of the energy from the ultraviolet light absorbed by the ozone. At its base, the stratosphere is extremely cold, about -110 degrees Fahrenheit (-80 degrees Celsius). At its top, the temperature has risen back nearly to freezing.

Next comes the mesosphere. In this layer, the air temperature drops again, down to nearly -180 degrees Fahrenheit (-120 degrees Celsius) at the top. Meteors generally burn up in the mesosphere, which extends to a height of about 52 miles (85 kilometers). This is why the Earth's surface isn't pocked with meteor craters, like the moon's.

Entering Outer Space:-
Above the mesosphere is the ionosphere. It extends to about 430 miles (690 kilometers) and is so thin it's generally considered part of outer space. The International Space Station and many satellites orbit within the ionosphere.

The ionosphere is named for the ions created within this layer by energetic particles from sunlight and outer space. These ions create an electrical layer that reflects radio waves, allowing radio messages to be sent across oceans in the days before communication satellites. Electrical displays in the ionosphere also create the auroras called the Northern and Southern Lights.

Beyond the ionosphere lies the exosphere. This tenuous portion of the Earth's atmosphere extends outward until it interacts with the solar wind. Solar storms compress the exosphere. When the sun is tranquil, this layer extends further outward. Its top ranges from 620 miles (1,000 kilometers) to 6,214 miles (10,000 kilometers) above the surface, where it merges with interplanetary space. 

 Climate
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Climate isn't the same thing as weather. Weather is the condition of the atmosphere over a short period of time; climate is the average course of weather conditions for a particular location over a period of many years.

One of the factors that influences climate is the angle of the sun's rays. In the tropics, between 23.5° N and 23.5° S, there is at least one time of year when the noontime sun is directly overhead and its rays hit at a direct angle. This produces a hot climate with relatively small temperature differences between summer and winter.

In the Arctic and Antarctic (north or south of 66.5° latitude), there are times of year when the sun is above the horizon 24 hours a day (a phenomenon known as midnight sun) and times when it never rises. Even in the summer, the sun is low enough for temperatures to be lower than in the tropics, but the seasonal changes are much greater than in equatorial regions. Interior Alaska has seen temperatures as high as 100 degrees Fahrenheit (38 degrees Celsius).

Farther from the Equator lie the temperate regions. These include the United States, Europe, China, and parts of Australia, South America, and southern Africa. They have the typical four seasons: winter, spring, summer, and fall.

Outside Influences:-
Climate is also controlled by wind, oceans, and mountains.
Winds bring moisture to land. North and south of the Equator the trade winds blow from the northeast and southeast, respectively. These winds converge in the tropics, forcing air to rise. This produces thunderstorms, humidity, and monsoons.

North and south of the trade winds, about 30° from the Equator, there is relatively little wind, and therefore little moisture blowing inland from the oceans. Also, dry air is sinking back to the surface, warming in the process. This is why many of the world's great desert regions—the Sahara, Arabia, Iran, Iraq, and chunks of Mexico—lie at the same latitude. A similar band of deserts lies to the south in Australia, South America, and southern Africa.

Mountains force wind to rise as it crosses over them. This cools the air, causing moisture to condense in clouds and rain. This produces a wet climate on the upwind side of the mountains and an arid "rain shadow" on the downwind side.

Oceans provide moisture that fuels rainstorms. They also buffer the temperature of coastal regions, regardless of latitude.

Climate Groups:-
In the early 1900s, climatologist Wladimir Köppen divided the world into five major climate groups.
Moist, tropical climates are hot and humid. Steppes and deserts are dry, with large temperature variations. Plentiful lakes, rivers, or nearby oceans give humid, midlatitude climates cool, damp winters, but they have hot, dry summers. Some of these climates are also called Mediterranean. Continental climates occur in the centers of large continents. Mountain ranges (or sheer distance) block off sources of moisture, creating dry regions with large seasonal variations in temperature. Much of southern Canada, Russia, and parts of central Asia would fall into this category. Cold, or polar, climates round out Köppen's list. A sixth region, high elevations, was later added to the classification system.
 

 Clouds
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Clouds form when humid air cools enough for water vapor to condense into droplets or ice crystals. The altitude at which this happens depends on the humidity and the rate at which temperature drops with elevation.

Normally, water vapor can only condense onto condensation nuclei—tiny particles that serve as kernels around which drops can form.

Condensation nuclei are often nothing but natural dust. But soot particles from automobile exhaust or other types of pollution can also serve the purpose. One study has found that changing levels of air pollution cause different rates of cloud formation (and rain) on weekends and weekdays, at least in humid climates with lots of cities.

Cloud Types:-
Clouds are classified into four basic categories, depending largely on the height of their bases above the ground.

High-level clouds, called cirrus clouds, can reach heights of 20,000 feet (6,000 meters) and are typically thin. They do not produce rain and often indicate fair weather. They are usually made up of ice.

Midlevel clouds form between 6,500 feet (2,000 meters) and cirrus level. They are referred to as "alto-" clouds and bear such names as altostratus or altocumulus, depending on their shape. (Altostratus clouds are flat; altocumulus clouds are puffy.) They frequently indicate an approaching storm. They themselves sometimes produce virga, which is rain or snow that does not reach the ground.

Low-level clouds lie below 6,500 feet (2,000 meters). Meteorologists refer to them as stratus clouds. They're often dense, dark, and rainy (or snowy) though they can also be cottony white clumps interspersed with blue sky.

Storm Clouds:-
The most dramatic types of clouds are cumulus and cumulonimbus, or thunderheads. Rather than spreading out in bands at a fairly narrow range of elevations, like other clouds, they rise to dramatic heights, sometimes well above the level of transcontinental jetliner flights.

Cumulus clouds are fair-weather clouds. When they get big enough to produce thunderstorms, they are called cumulonimbus. These clouds are formed by upwelling plumes of hot air, which produce visible turbulence on their upper surfaces, making them look as though they are boiling.

Just as it takes heat to evaporate water from the surface of the Earth, heat is released when water condenses to form clouds. In thunderheads, this energy can produce hail, damaging winds, lightning, torrential rain, and sometimes tornadoes.

As thunderheads reach high elevations, their tops encounter high winds that cause them to spread out sideways, earning them the nickname "anvil tops." They can reach elevations of 50,000 feet (15,000 meters).

Weather
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Weather is the state of the atmosphere at a specific time and place, with respect to temperature, precipitation, and other factors such as cloudiness. Weather is generated by many forces, some obvious, some not. Warm, humid air masses blowing in from oceans, for example, fuel rains. Sunlight heats the land, generating thermals that help produce summer thunderstorms.

Mountains and cities also affect the weather. In mountains this occurs because the wind must rise as it crosses over the ridge. This lifts the air, causing it to cool. That produces clouds, rain, or snow.

Cities, on the other hand, produce urban "heat islands" where roads, parking lots, and rooftops warm in the sun. This not only raises the city's temperature, but it can affect the weather, producing thunderstorms in some cities or altering storm tracks in others.

Predicting the Weather:-
Weather forecasting is the art of predicting what will happen in the future. In its simplest form, it's merely a matter of looking out the window to see what types of clouds are around and which way they are moving. Knowledge of local weather patterns can then allow fairly good predictions for the next 12 to 24 hours.

Professional forecasters have a wide variety of other tools. Weather stations scattered around the globe allow them to make detailed weather maps, as do satellites, which allow forecasters to see what is happening far out to sea, where there are no weather stations. Weather balloons and radar also contribute.

Nevertheless, long-run weather forecasting is notoriously difficult. That's because weather prediction involves a mathematical concept called chaos theory, in which extremely small errors in measuring today's weather conditions can snowball into large, seemingly random, errors in long-range forecasts.

It has been said, for example, that a butterfly flapping its wings today in China could produce (or prevent) tornadoes two weeks from now in Kansas. While this so-called butterfly effect is undoubtedly overstated, the basic concept is simple: Even the most minor factors can alter long-term weather forecasts.

Most weather forecasters believe that accurate forecasting more than two weeks into the future will always be impossible. Today, anything beyond five to seven days involves substantial guesswork and is often wrong.

Wind
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The energy that drives wind originates with the sun, which heats the Earth unevenly, creating warm spots and cool spots. Two simple examples of this are sea breezes and land breezes.

Sea breezes occur when inland areas heat up on sunny afternoons. That warms the air, causing it to rise. Cooler air rushes in from the ocean to take its place and presto, a wind is born. By late afternoon, a strong breeze can be blowing dozens of miles inland. A similar effect can occur near big lakes, where the wind is referred to as a lake breeze.

Land breezes come at night, when inland temperatures drop enough that the ocean is now warmer than the land, reversing the effect.

Global Patterns:-
Similar forces produce global wind patterns that affect climate. The tropics, for example, are always hot. Air rises here and spreads north and south, high above the land. Lower down, air is pulled in from the north and south. The coriolis effect, an offshoot of the Earth's rotation, makes moving air masses curve, so that the winds converging on the Equator come from the northeast in the Northern Hemisphere and the southeast in the Southern Hemisphere. These winds are called the trade winds.

Farther from the Equator, the surface winds try to blow toward the Poles, but the coriolis effect bends them the opposite direction, creating westerlies. This is why so many weather events in the United States come from the west.

At latitudes higher than about 60°, cold surface winds try to blow toward the Equator, but, like the trade winds, they are bent by the coriolis effect, producing polar easterlies.

Highs and Lows:-
Within the mid-latitudes, weather effects create high- and low-pressure zones, called highs and lows, respectively. Air moves from areas of high pressure to low pressure. As it moves, however, it spirals due to the coriolis effect, producing the shifting winds we experience from day to day, as highs and lows drift under the influence of the prevailing westerlies.

Winds reaching the center of a low-pressure area have nowhere to go but up. This causes moisture to condense into clouds, producing storms. At the center of high-pressure areas, dry air descends from above, producing fair weather.

On a smaller scale, colliding wind patterns can produce convergence, in which air also has nowhere to go but up. If one of the winds is a humid flow from a warm ocean such as the Gulf of Mexico, the result can be powerful thunderstorms and tornadoes.