Complex systems nonuniform sampling

The atmosphere itself is difficult to study, largely because it is not uniform in composition, temperature, or density (Table 4.1). One reason for this nonuniformity is the effect of solar radiation, which causes different chemical reactions at different altitudes (Box 4.1). The density of air also varies with altitude, partly because the temperature varies, but also as a result of the compressibility of air, its ability to be squashed into a smaller volume. The weight of air above compresses the air at sea level, causing it to be denser than the air at higher altitudes. The air outside an airplane cruising at 10 km is only 25% as dense as air at sea level. To simplify their task, scientists initially study samples of gases that are uniform in composition and density and only after uniform gases are understood do they turn their attention to more complex systems like the atmosphere. 

The inherent difficulty in the measurement of the complex dynamic moduli of viscoelastic materials is emphasized by the results of this paper. The agreement among the shifted modulus data as measured by different systems is limited by several difficulties (1) measurement inaccuracies of the instruments, (2) differences in the data reduction techniques used to apply the time-temperature superposition principle and propagation of shift curve errors and, (3) nonuniformity of the test samples. 

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