Cold clouds have tops which are colder than freezing, but often consist of liquid water drops, not ice, even though temperatures are between 0°C and -38°C. In that case, the droplets are described as supercooled, remaining liquid for want of suitable solid particles on which ice can start. Such particles are called ice nuclei, and have a crystal structure like that of ice. They may consist of dust particles, typically between 0.1 pm and several microns in size. Certain clay minerals are particularly effective. Or the ice nuclei may be bacteria or fungi released by plants, soil and the open sea. (Such biological nucleation is also involved in forming frost—Section 3.6). Few ice nuclei are active in creating crystals above -4°C, but at lower temperatures the supercooled droplets become increasingly indifferent to the kind of nuclei on which they will freeze, so the number acceptable increases tenfold for each 4 K of cooling. Various kinds of ice nuclei require cooling to different activation temperatures to become effective, e.g. -3°C for bacteria from leaf mould but around -10°C for many clay particles. At temperatures below -38°C, crystals form automatically without the need of any nuclei at all. This is called spontaneous nucleation, where the natural tendency towards freezing is so great that it overcomes the need of preliminary infection by a foreign body.
The best nuclei are ice crystals themselves, so freezing of a few supercooled droplets leads to rapid freezing of those nearby. This process is known as glaciation. It occurs in three ways— either because of some larger droplets (which have more chance of containing an ice nucleus), or through natural seeeding, whereby ice crystals from a high cloud fall into a lower supercooled cloud, as is common near warm fronts (Chapter13 ). Or, thirdly, glaciation may occur by ice multiplication, often by the splintering of ice crystals. For instance, a freezing supercooled droplet may eject tiny droplets which solidify instantaneously. This occurs less in continental air, where cloud droplets are small on account of numerous cloud condensation nuclei.
The fact that supercooled cloud droplets often occur in nature shows that ice nuclei are scarce, unlike cloud condensation nuclei (Section 8.2). The availability of useful ice nuclei varies greatly with time and location. Air subsiding from the clean upper troposphere has very few nuclei, which may explain the regular occurrence of supercooled droplets near the top of stratiform clouds, whereas ice crystals are found lower down.
Raindrops are created relatively quickly in a cold cloud, because ice crystals develop rapidly amongst supercooled droplets by means of the Bergeron—Findeisen process (Note 9.C). The crystals grow as snowflakes, whose large surface facilitates collecting more ice crystals and droplets. In addition, the snowflakes become sticky as they descend near the freezing level, so they aggregate into large flakes, which melt into large raindrops, creating heavy rain. A study of a storm in Victoria showed a linear relationship between cloud-top temperature and the rainfall rate, e.g. no rain at 10°C but 0.45 mm/h at -15°C.
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