INFERRING CLOUDS FROM RAOB

Rawinsondes, which penetrate cloud systems, reflect, to some extent (primarly in the humidity trace), the vertical distribution of clouds. If the humidity element were perfect, there would usually be no difficulties in locating cloud layers penetrated by the instrument. Because of the shortcomings in the instrument, however, the relationship between indicated humidity and cloud presence is far from definite, and art empirical interpretation is necessary. Nevertheless, rawinsonde reports give valuable evidence that, when compared with other data, aids greatly in determining a coherent picture of stratiform and frontal cloud distributions. Their value in judging air mass cumulus and cumulonimbus distribution is negligible. INFERRING CLOUDS FROM RAOB Theoretically, we should be able to infer from the humidity data of RAOBs the layers where the rawinsonde penetrates cloud layers. In practice, the determination that can be made from temperature and dewpoint curves are often less exact and less reliable than desired. Nevertheless, RAOBs give clues about cloud distribution and potential areas of cloud formation. These clues generally cannot be obtained from any other source. DEWPOINT AND FROST POINT IN CLOUDS The temperature minus the dewpoint depression yields the dewpoint, which is defined as the temperature to which the air must be cooled at a constant vapor pressure for saturation to occur. The FROST POINT (that is, the temperature to which the air has to be cooled or heated adiabatically to reach saturation with respect to ice) is higher than the dewpoint except at 0°C, where the two coincide. In the graph shown in figure 4-15, the difference between dewpoint and frost point is plotted as a function of the dewpoint itself. In a cloud with the temperature above freezing, the true dewpoint will coincide closely with the true air temperature, indicating that the air between the cloud droplets is practically saturated. Minor discrepancies may occur when the cloud is not in a state of equilibrium (when the cloud is dissolving or forming rapidly, or when precipitation is falling through the cloud with raindrops of slightly different temperature than the air); but these discrepancies are very small. In the subfreezing portion of a cloud, the true temperature is between the true dewpoint and the true frost point, depending on the ratio between the quantities of frozen Figure 4-15.-Difference between frost point and dewpoint as a function of the dewpoint. and liquid cloud particles. If the cloud consists entirely of supercooled water droplets, the true temperature and the true dewpoint will, more or less, coincide. If the cloud consists entirely of ice, the temperature should coincide with the frost point. Therefore, we cannot look for the coincidence of dewpoint and temperatures as a criterion for clouds at subfreezing temperatures. At temperatures below -12°C, the temperature is more likely to coincide with the frost point than the dewpoint. The graph shown in figure 4-15 indicates that the difference between the dewpoint and frost point increases roughly 1°C for every 10°C that the dewpoint is below freezing. For example, when the dewpoint is –10°C, the frost point equals –9°C; when the dewpoint is –20°C, the frost point is –18°C; and when the dewpoint is –30°C, the frost point is –27°C. Thus, for a cirrus cloud that is in equilibrium (saturated with respect to ice) at a (frost point) temperature of –40°C, the correct dewpoint would be –44°C, (to the nearest whole degree). We can state, in general, that air in a cloud at temperatures below about –12°C is saturated with respect to ice, and that as the temperature of the cloud decreases (with height), the true frost point/dewpoint difference increases. Any attempt to determine the height of cloud layers from humidity data of a RAOB is, there fore, subject to error. It is possible to overcome 4-14