The fact that the moon's surface is covered with meteorite impact craters is obvious to us today. Though the moon is not far from us, impact craters are few and far between on Earth. As it turns out, Earth has received just as many incoming meteorites as the moon, but the presence of the atmosphere has determined the fate of many of them. Small meteorites burn up in the atmosphere before ever reaching Earth. Those that do hit the surface and create an impact crater are lost to us in a different way – the craters are quickly eroded by weather generated in the atmosphere, and the evidence is washed away. The moon, on the other hand, has no atmosphere, and thus every meteor aimed at the moon hits it, and the craters have remained essentially unchanged for 4 billion years (Figure 1).
Composition of Earth's atmosphere
The early Greeks considered "air" to be one of four elementary substances; along with earth, fire, and water, air was viewed as a fundamental component of the universe. By the early 1800s, however, scientists such as John Dalton recognized that the atmosphere was in fact composed of several chemically distinct gases, which he was able to separate and determine the relative amounts of within the lower atmosphere. He was easily able to discern the major components of the atmosphere: nitrogen, oxygen, and a small amount of something incombustible, later shown to be argon.
The development of the spectrometer in the 1920s allowed scientists to find gases that existed in much smaller concentrations in the atmosphere, such as ozone and carbon dioxide. The concentrations of these gases, while small, varied widely from place to place. In fact, atmospheric gases are often divided up into the major, constant components and the highly variable components, as shown in Table 1 and Table 2.
|Neon, Helium, Krypton
Table 1: Constant Components. Proportions remain the same over time and location.
|Carbon dioxide (CO2)
|Water vapor (H2O)
|Sulfur dioxide (SO2)
|Nitrogen oxides (NO, NO2, N20)
Table 2: Variable Components. Amounts vary over time and location.
Although both nitrogen and oxygen are essential to human life on the planet, they have little effect on weather and other atmospheric processes. The variable components, which make up far less than 1 percent of the atmosphere, have a much greater influence on both short-term weather and long-term climate. For example, variations in water vapor in the atmosphere are familiar to us as relative humidity. Water vapor, CO2, CH4, N2O, and SO2 all have an important property: They absorb heat emitted by Earth and thus warm the atmosphere, creating what we call the "greenhouse effect." Without these so-called greenhouse gases, the Earth's surface would be about 30 degrees Celsius cooler – too cold for life to exist as we know it. Though the greenhouse effect is sometimes portrayed as a bad thing, trace amounts of gases like CO2 warm our planet's atmosphere enough to sustain life. Global warming, on the other hand, is a separate process that can be caused by increased amounts of greenhouse gases in the atmosphere.
In addition to gases, the atmosphere also contains particulate matter such as dust, volcanic ash, rain, and snow. These are, of course, highly variable and are generally less persistent than gas concentrations, but they can sometimes remain in the atmosphere for relatively long periods of time. Volcanic ash from the 1991 eruption of Mt. Pinatubo in the Philippines, for example, darkened skies around the globe for over a year.
Though the major components of the atmosphere vary little today, they have changed dramatically over Earth's history, about 4.6 billion years. The early atmosphere was hardly the life-sustaining blanket of air that it is today; most geologists believe that the main constituents then were nitrogen gas and carbon dioxide, but no free oxygen. In fact, there is no evidence for free oxygen in the atmosphere until about 2 billion years ago, when photosynthesizing bacteria evolved and began taking in atmospheric carbon dioxide and releasing oxygen. The amount of oxygen in the atmosphere has risen steadily from 0 percent 2 billion years ago to about 21 percent today.
Measuring the atmosphere
We now have continuous satellite monitoring of the atmosphere and Doppler radar to tell us whether or not we will experience rain anytime soon; however, atmospheric measurements used to be few and far between. Today, measurements such as temperature and pressure not only help us predict the weather, but also help us look at long-term changes in global climate (see our Temperature module). The first atmospheric scientists were less concerned with weather prediction, however, and more interested in the composition and structure of the atmosphere.
The two most important instruments for taking measurements in Earth's atmosphere were developed hundreds of years ago: Galileo is credited with inventing the thermometer in 1593, and Evangelista Torricelli invented the barometer in 1643. With these two instruments, temperature and pressure could be recorded at any time and at any place. Of course, the earliest pressure and temperature measurements were taken at Earth's surface. It was a hundred years before the thermometer and barometer went aloft.
While many people are familiar with Ben Franklin's kite and key experiment that tested lightning for the presence of electricity, few realize that kites were the main vehicle for obtaining atmospheric measurements above Earth's surface. Throughout the 18th and 19th centuries, kite-mounted instruments collected pressure, temperature, and humidity readings; unfortunately, scientists could only reach up to an altitude of about 3 km with this technique.
Unmanned balloons were able to take measurements at higher altitudes than kites, but because they were simply released with no passengers and no strings attached, they had to be retrieved in order to obtain the data that had been collected. This changed with the development of the radiosonde, an unmanned balloon capable of achieving high altitudes, in the early 1930s. The radiosonde included a radio transmitter among its many instruments, allowing data to be transmitted as it was being collected so that the balloons no longer needed to be retrieved. A radiosonde network was developed in the United States in 1937, and continues to this day under the auspices of the National Weather Service.
Temperature in the atmosphere
Through examination of measurements collected by radiosonde and aircraft (and later by rockets), scientists became aware that the atmosphere is not uniform. Many people had long recognized that temperature decreased with altitude – if you've ever hiked up a tall mountain, you might learn to bring a jacket to wear at the top even when it is warm at the base – but it wasn't until the early 1900s that radiosondes revealed a layer, about 18 km above the surface, where temperature abruptly changed and began to increase with altitude. The discovery of this reversal led to division of the atmosphere into layers based on their thermal properties.
The lowermost 12 to 18 km of the atmosphere, called the troposphere, is where all weather occurs – clouds form and precipitation falls, wind blows, humidity varies from place to place, and the atmosphere interacts with the surface below. Within the troposphere, temperature decreases with altitude at a rate of about 6.5° C per kilometer. At 8,856 m high, Mt. Everest still reaches less than halfway through the troposphere. Assuming a sea level temperature of 26° C (80° F), that means the temperature on the summit of Everest would be around -31° C (-24° F)! In fact, temperature at Everest's summit averages -36° C, whereas temperatures in New Delhi (in nearby India), at an elevation of 233 m, average about 28° C (82.4° F).
At the uppermost boundary of the troposphere, air temperature reaches about -100° C and then begins to increase with altitude. This layer of increasing temperature is called the stratosphere. The cause of the temperature reversal is a layer of concentrated ozone. Ozone's ability to absorb incoming ultraviolet (UV) radiation from the sun had been recognized in 1881, but the existence of the ozone layer at an altitude of 20 to 50 km was not postulated until the 1920s. By absorbing UV rays, the ozone layer both warms the air around it and protects us on the surface from the harmful short-wavelength radiation that can cause skin cancer.
It is important to recognize the difference between the ozone layer in the stratosphere and ozone present in trace amounts in the troposphere. Stratospheric ozone is produced when energy from the sun breaks apart O2 gas molecules into O atoms; these O atoms then bond with other O2 molecules to form O3, ozone. This process was first described in 1930 by Sydney Chapman, a geophysicist who synthesized many of the known facts about the ozone layer. Tropospheric ozone, on the other hand, is a pollutant produced when emissions from fossil-fuel burning interact with sunlight.
Above the stratosphere, temperature begins to drop again in the next layer of the atmosphere called the mesosphere, as seen in the previous figure. This temperature decrease results from the rapidly decreasing density of the air at this altitude. Finally, at the outer reaches of Earth's atmosphere, the intense, unfiltered radiation from the sun causes molecules like O2 and N2 to break apart into ions. The release of energy from these reactions actually causes the temperature to rise again in the thermosphere, the outermost layer. The thermosphere extends to about 500 km above Earth's surface, still a few hundred kilometers below the altitude of most orbiting satellites.
Pressure in the atmosphere
Atmospheric pressure can be imagined as the weight of the overlying column of air. Unlike temperature, pressure decreases exponentially with altitude. Traces of the atmosphere can be detected as far as 500 km above Earth's surface, but 80 percent of the atmosphere's mass is contained within the 18 km closest to the surface. Atmospheric pressure is generally measured in millibars (mb); this unit of measurement is equivalent to 1 gram per centimeter squared (1 g/cm2). Other units are occasionally used, such as bars, atmospheres, or millimeters of mercury. The correspondence between these units is shown in the table below.
|millimeters of mercury
|760 mm Hg
Table 3: Correspondence of atmospheric measurement units.
At sea level, pressure ranges from about 960 to 1,050 mb, with an average of 1,013 mb. At the top of Mt. Everest, pressure is as low as 300 mb. Because gas pressure is related to density, this low pressure means that there are approximately one-third as many gas molecules inhaled per breath on top of Mt. Everest as at sea level – which is why climbers experience ever more severe shortness of breath the higher they go, as less oxygen is inhaled with every breath.
Though other planets host atmospheres, the presence of free oxygen and water vapor makes our atmosphere unique as far as we know. These components both encouraged and protected life on Earth as it developed, not only by providing oxygen for respiration, but by shielding organisms from harmful UV rays and by incinerating small meteors before they hit the surface. Additionally, the composition and structure of this unique resource are important keys to understanding circulation in the atmosphere, biogeochemical cycling of nutrients, short-term local weather patterns, and long-term global climate changes.
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