Clouds supercooled water

Pollution can cause opposite effects in relahon to precipitation. Addition of a few particles that act as ice nuclei can cause ice particles to grow at the expense of supercooled water droplets, producing particles large enough to fall as precipitation. An example of this is commercial cloud seeding with silver iodide particles released from aircraft to induce rain. If too many particles are added, none of them grow sufficiently to cause precipitation. Therefore, the effects of pollution on precipitation are complex. 

One item produces up to 25 grams of effective nuclei per minute for 4% minutes. A single gram of silver iodide can form billioas of small ice crystals when sprayed into a cloud holding supercooled water droplets. 

This happened because of our investigation of the rate of nucleation of ice in deeply supercooled water. Previous laboratory studies of the freezing of water occurred in substantially warmer water and were blind to the phase of ice obtained. We studied water undergoing nucleation at roughly the temperature of nucleation in cirrus clouds, I believe. I understand that what happens in cirrus clouds has an important effect on the climate. Moreover, we showed directly that the ice first nucleated was the metastable cubic ice, not the ordinary hexagonal ice. Atmospheric scientists had inferred that result from indirect evidence. Previously it had not been possible to carry out experiments like ours in the laboratory, which is why our work attracted the attention of atmospheric scientists. 

Water is probably the most important and the most intensely studied substance on Earth. It is the solvent of life and it is also of vital importance in many aspects of our existence, ranging from cloud microphysics to its key role as a solvent in many chemical reactions. The familiar process of water freezing is encountered in many natural and technologically relevant processes. In this contribution, we discuss the applicability of the methods of computational chemistry for the theoretical study of two important phenomena. Namely, we apply the molecular dynamics (MD) simulations to the study of brine rejection from freezing salt solutions and the study of homogeneous nucleation of supercooled water. 

FIGURE 17.26 Average frequencies of appearance of supercooled water, mixed phase, and ice clouds as a function of temperature in layer clouds over Russia (Boronikov et al. 1963). 

An ice crystal in a cloud of supercooled water droplets grows by accretion into a rimed crystal, a graupel particle, or a hailstone. Water drops can grow by an analogous process of drop-droplet collection, as previously discussed (Section IV.B). Theoretical treatments of the accretion process are much more complex than those for drops, because they must consider a variety of complex shapes (e.g.. Fig. 8) changing with time during the accretion process, the release of latent heat at the crystal surface by droplet freezing, and the density of droplets accreted on the surface of the ice particle since the density depends on the characteristics of the ice particle (these densities vary fi om 0.1-0.91 g cm ). 

Water in its several forms is the substance most essential to life on earth. Some of its crystalline forms are stable in certain temperature-pressure ranges and others are metastable. Although the stable form of water at sufficiently low temperatures is crystalline, inside this stable phase, water can also exist in liquid form. When this occurs, water is said to be supercooled. Supercooled water occurs naturally in the form of small droplets in clouds. If liquid water is cooled quickly enough, the crystalline phase can be bypassed and a noncrystalline (amorphous) solid, that is, a glass, is the result. This amorphous glass phase of water is polymorphic, that is, it can exists in two different forms. Glassy water is undoubtedly the most common form of water in the universe. Scientists puzzle over the anomalous properties of glassy water when it is cooled it becomes more compressible, when compressed it is less viscous, and when cooled sufficiently, it expands. 

However, one finds that, in cooling a liquid below its freezing point, the liquid may not always turn into solid phase at the freezing point. In fact, in some cases, such as water, even at around -40°C, liquid water does not turn into a solid phase. It stays in what is called a supercooled state. A major phenomena is the freezing of supercooled clouds. However, if certain so-called nucleating agents are used, then the clouds would turn into liquid droplets (and form rain). The nucleation process is a surface phenomena and is observed in transitions from... 

Distortion of the particle size during the sampling process is a concern in the use of this probe on an aircraft. Compressional heating due to deceleration of the particles may distort the size distribution, because evaporation of water from aerosol particles reduces their diameters. Likewise, particle sizes can be reduced by use of a heater, incorporated into some models of this probe, to prevent icing when supercooled clouds are being flown through. One study (88) indicated that the probe heater removes most of the water from aerosol particles sampled at relative humidities of 95%. Thus, size distributions of aerosol particles measured with the probe heater on correspond to that of the dehydrated aerosol. These results were confirmed by a later study (90) in which size distributions of aerosols measured with a nonintrusive probe were compared to size distributions measured with a de-iced PCASP probe. Measurement of the aerosol size distribution with the probe heater on may be an advantage in certain studies. 

If particles (or ions) are already present in a supersaturated vapor, nucleation will take place preferentially on these particles at supersaturations far smaller than for the homogeneous vapor. In this case, nucleation takes place heterogeneously on the existing nuclei at a rate dependent on the free energy of a condensate cap forming on or around the nucleus. Heterogeneous nuclei always occur in the earth s atmosphere. They are crucial to the formation of water clouds and to the formation of ice particles in supercooled clouds. 

Collection of supercooled liquid water in clouds is simple, using only a plate or screen exposed to RAM air the water is later melted and stored prior to analysis (6 ). Collection of frozen cloud particles is a little more problematical since the liquid water content can be low, and individual particles are more subject to bounce-off during impactive collection. Collection of snow particles aboard the aircraft is most difficult of all due to the low aerodynamic diameter exhibited by these particles in RAM air streams. Successful methods for the collection of snow and ice clouds are still in an active stage of development. 

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. 

Atmospheric observations indicate that water readily supercools, and water clouds are frequently found in the atmosphere at temperatures below 0°C. Figure 17.26 shows that supercooled clouds are quite common in the atmosphere, especially if cloudtop temperature is warmer than — 10°C. However, the likelihood of ice increases with decreasing temperature, and at —20°C only about 10% of clouds consist entirely of water drops. At these low temperatures, ice particles coexist with water drops in the same cloud. 

Polar stratospheric clouds have been classified into two broad types, so-called Type I and Type II (Table 4.1). Type I PSCs have been further subdivided into Type la and Type Ib. Type la PSCs have traditionally been identified as crystals of nitric acid trihydrate, HNO, 3 H2O, denoted NAT, that form once temperatures fall below about 195 K. Type lb PSCs consist of supercooled ternary solutions of HNO3/H2SO4/H2O, also forming at about the same temperature threshold. Type II PSCs are largely frozen water ice, nonspherical crystalline particles, that form at temperatures below the ice frost point. The ice frost point, for example, at 3 X 10 Torr H O is 191 K. Despite the above classification, the composition of PSCs is still uncertain (Toon and Tolbert, 1995). 

---