To a first-order approximation, the general shape of the abundance profile of a condensable species may be determined from an equilibrium cloud condensation model (ECCM) by considering a parcel of deep air that is lifted right up through the atmosphere without mixing with surrounding air. At first the mixing ratio remains fixed at its deep level, but at a certain level it becomes equal to the saturated volume mixing ratio (v.m.r.) and thus the gas starts to condense. Moving the parcel to higher altitudes—and thus lower temperatures—more and more of the gas condenses to form aerosols and thus the mixing ratio profile follows the saturated v.m.r. curve. At the tropopause, the temperature stops decreasing with height and instead starts to rise again. However, if we assume that cloud particles condensed at lower altitudes are not carried with the parcel, but instead fall through the atmosphere, then the v.m.r. cannot rise again above the tropopause by re-evaporation of the aerosols and instead remains fixed at the tropopause value. Hence, the tropopause acts as a "cold trap" to molecules that condense in the troposphere and limits stratospheric abundances. The altitude where the deep fixed v.m.r. meets the saturated v.m.r. curve determines the base level of the condensed cloud. Clearly if the deep v.m.r. is higher then the cloud base pressure is higher and vice versa.
In reality, the mixing of rising air parcels with descending dry air, both vertically and horizontally, means that the v.m.r. profile derived from spatially averaged remotely sensed data is usually substantially subsaturated even in areas of localized rapid convection. Hence, while this model is useful for estimating the approximate level of the cloud bases, and thus where the volatile species start to condense, it does not model well the rate of decrease of gas abundance with height. In addition, the technique gives no indication on the vertical extent or optical thickness of the cloud. These depend on two things: (1) the rate of uplift, or vertical mixing; and (2) the rate of formation of cloud aerosols and their size, which governs how quickly they fall back down through the atmosphere towards warmer regions where they may again evaporate. Thus, accurately estimating the vertical distribution of cloud particle density (sometimes expressed in terms of a cloud scale height, analogous to the pressure scale height mentioned earlier) and cloud particle size distribution, both of which are vital parameters to know if remote-sensing observations are to be used to interpret cloud structure, is an extremely difficult microphysical problem.
As air is uplifted from the deep atmosphere, in addition to the observable clouds of material such as water and ammonia, a number of rather exotic layers may form at deeper pressures. For example, in Jupiter's atmosphere, clouds of magnesium silicates are predicted to condense near 2,000 K, and clouds of silver and gold are predicted to condense in thin layers near 1,000 K!
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