Water vapor supercooling

Toon and Tolbert (1995) suggest that if Type I PSCs are primarily ternary solutions rather than crystalline NAT, the higher vapor pressure of HN03 over the solution would in effect distill nitric acid from Type I to Type II PSCs, assisting in denitrification of the stratosphere. This overcomes the problem that if Type II PSCs have nitric acid only by virtue of the initial core onto which the water vapor condenses, the amount of HN03 they could remove may not be very large. The supercooled H20-HN03 liquid layer observed by Zondlo et al. (1998) clearly may also play an important role in terms of the amount of HNO, that can exist on the surface of these PSCs. 

Weak detonations are believed to represent the condensation shocks observed in supersonic wind tunnels [12], [51]. Supercooled water vapor in a supersonic stream has been observed to condense rapidly through a narrow wave. The amount of liquid formed is so small that the equations for purely gaseous waves are expected to apply approximately. Since a normal shock wave would raise the temperature above the saturation point (thus ruling out the ZND structure, for example), and the flow is observed to be supersonic downstream from the condensation wave, it appears reasonable to assume that condensation shocks are weak detonations. This hypothesis may be supported by the fact that unlike chemical reaction rates, the rate of condensation increases as the temperature decreases. Proposals that weak detonations also represent various processes occurring in geological transformations have been presented [52]. 

Frequently, both kinds of adiabats must be employed to follow the behavior of a parcel of air. A parcel of dry air may rise by its own buoyancy, or may be pushed up by orographic lifting, which occurs when winds meet mountains and are deflected upward. The temperature of the air parcel follows the dry adiabat until water vapor condensation is incipient with further cooling, condensation can occur. If it is assumed that supercooling (a nonequilibrium situation in which air cools below the dew point without condensation) does not occur, the parcel of air then moves upward following the corresponding wet adiabat. Conditional stability refers to conditions under which dry,... 

Physico-chemical ways of achieving metastability of the initial system are usually related to changes in temperature, pressure (less often), and composition of solvent [10]. The supersaturation (supercooling) of water vapor is the reason for certain meteorological phenomena (cloud formation). The formation of disperse systems upon changes in temperature is the key for the preparation of all polycrystalline materials in metallurgy. Here control of... 

One may effectively control the stability of atmospheric aerosols by spraying concentrated solutions of hygroscopic substances, such as calcium chloride, or solid substances, such as silver iodide and solid carbon dioxide. These substances cause condensation of water vapor (or the formation of small ice crystals in supercooled clouds), and result in precipitation. Analogous means can be used to dissipate fog. 

FIGURE 17.27 Pressure-temperature phase diagram for water. The dashed line corresponds to supercooled water and its metastable equilibrium with water vapor. 

Ice Nuclei Ice particles can be formed through a variety of mechanisms. All of these require the presence of a particle, which is called an ice nucleus (IN). These mechanisms are (1) water vapor adsorption onto the IN surface and transformation to ice (deposition mode), (2) transformation of a supercooled droplet to an ice particle (freezing mode), and (3) collision of a supercooled droplet with an IN and initiation of ice formation (contact mode). 

From equation (X), the temperature of supercooling and the interfacial tension required for spontaneous nucleation to occur can be estimated. At a degree of supersaturation of 3 and 4, the time for spontaneous nucleation to occur in supercooled water vapor (3) is 1000 years and 0.1 seconds, respectively. 

Solutions, such as of sodium acetate in water or of water vapor in air, can also be supercooled. Rain or snow sometimes do not fall from air... 

In Figure 11-1 the lines below and to the left of the hydrate/ice formation line represent a meta-stable equilibrium between water vapor in the gas phase and supercooled liquid water. The actual equilibrium with solid ice or hydrate is at a lower water content. The effect is depicted in Figure 11-3, which also extends the water content scale of Figure 11-1 down to 0.1 lb water/MMscf. The data on equilibrium water contents in the 0.1 to 1.0 lb water/MMscf range are necessary for the design of the recently developed superdehydration processes. Water content data down to as low as 0.001 Ib/MMscf are plotted by Buck-lin et al. (1985). Such extremely low values are of interest in the design of natural gas turboexpander plants. 

Supercooled (metastable) water vapor commonly occurs in the atmosphere if dust particles are not present to begin condensation to the liquid. Sometimes small particles, such as tiny crystals of silver iodide, are released from airplanes in an attempt to begin condensation. This process is called cloud seeding. At a certain location, water vapor at 25°C has a metastable partial pressure of 32.0 torr. The equilibrium value at this temperature is 23.756 torr. Consider the air that is present to be the surroundings, and assume it to remain at equilibrium at 25°C. A tiny particle is added to begin condensation. Calculate AS, A//, and ASgurr per mole of water that condenses. State any assumptions. 

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