Supercooled clouds

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. 

Sattler B, Puxbaum H, Psenner R (2001) Bacterial growth in supercooled cloud droplets. Geophys Res Lett 28 239-242... 

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. 

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. 

Although recognized for hundreds of years, artificial weather modification was not applied before the turn of the 20 century. It started by simply introducing artificial CCN to the cloud layers by guns, later rockets and aircrafts. By end of the 1940s the introduction of crystallization nuclei (e. g. Agl, dry ice and many others) affected the ice phase processes in supercooled clouds and larger hygroscopic particles (artificial precipitation embryos about 30 pm in diameter) to stimulate collision-coalescence processes in warm clouds. About 10 countries applied different techniques with variable success. 

Supercooled cloud Cloud composed of liquid water droplets at temperatures below 0°C. 

Fogs and low clouds often pose hazards around airports. The concept for seeding these clouds is to introduce large salt particles or ice nuclei to sweep out the cloud droplets, thus clearing an area temporarily. Some success has been achieved, particularly in supercooled clouds. However, the clearing is often of short duration, and operations can be fairly expensive. 

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