32 - The Structure of the Atmosphere
The atmosphere is composed of 78% nitrogen (N2), 21% oxygen (O2), 0.9% argon (Ar), 0.04% carbon dioxide (CO2), and other trace gases. Water vapor can be up to 0.25% of the atmosphere by mass, but it is not considered a well-mixed regular constituent as its concentration varies greatly with time and location. It is water vapor's dynamic nature within the atmosphere as well as its high heat capacity and strength as a greenhouse gas which make it such an important factor in global circulation and weather.
We divide the atmosphere into five layers. The heights listed are not static, or absolute: rather they are typical values, obtained from years of observations via balloons, radiosondes and satellites. (They also vary by latitude, being oblate spheroids like the earth, but with even higher eccentricities: these average values are actually the polar values. The equatorial values are greater.)
1) 0-12 km (7.5 mi, 39,000 feet): troposphere. (From Greek, "turning sphere".) Most weather happens within this layer, the closest to the earth, where pressure and density are highest. Topography and radiative effects from the earth and oceans have major effects on air temperature and humidity. Air pressure is, simply, the weight of all overlying air on a given volume. Typical air pressure at sea level is 14.7 psi (pounds per square inch), or 1013 mbar (millibars) which we term "1 atmosphere". Pressure decreases with altitude.
Since gas is compressible (upcoming in the future post on the ideal gas law), increased air pressure means increased density. (The troposphere contains about 80% of the atmosphere's total gas by mass.) Higher air density in the troposphere means higher heat capacity and greater ability to sustain life. Almost all water vapor exists in the troposphere. At the top of this layer is a transition called the tropopause.
2) 12-50 km (7.5-31 mi): stratosphere. Contains the ozone (O3) layer which blocks 97-99% of incoming ultraviolet light from the sun. Air pressure is 1/1000th of an atmosphere at the stratopause, at the top of the stratosphere. Temperatures in the stratosphere tend to be well below freezing, but increase toward the top to nearly 0 deg C.
3) 50-80 km (31-50 mi): mesopshere. Some ice crystals at this altitude form thin clouds, called "noctilucent", visible at dawn and dusk. This layer is too high for jets to operate but too low--with too great air density--for orbital craft. At the top of this layer is the mesopause.
4) 80-500 km (50-310 mi): thermosphere (also known as the exobase). Temperatures in this region can reach 1500 deg C, but the air is so sparse that the actual heat content is very low. No clouds form at this layer. At the top is the thermopause. (Note: the ionosphere, 50-1000 km (30-620 mi) in altitude, is created by the sun's radiation breaking molecules in the atmosphere and creating charged atoms, or ions--but only during daylight hours. Sometimes at night, this ionizing effect is visible in the outer reaches of the atmosphere, creating the aurora borealis and aurora australis.)
5) 500-10,000 km (310-6,200 mi): exosphere. Outermost layer of the earth's atmosphere. Contains high proportions of hydrogen (H2), helium (He) and some nitrogen (N2). It is at this altitude that the aurora borealis and australis usually occur. Many man-made satellites orbit in this region.
Tomorrow: the ideal gas law.
Be well!
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