getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(16)

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Consequences of the above are the following:
●  The range of OAT for icing is: slightly positive °C, down to -40°C; but severe icing rarely occurs below -12°C. This can be translated into altitudes: at mid latitudes, altitudes where severe icing is most likely to occur are around FL 100 down to the
27
ground.

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Due to varying temperature conditions around the airframe, a slightly positive OAT

does not protect from severe icing.

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Accumulation of ice (icing) occurs on the .penetrating. or protruding parts of the airframe: nose, wing or fin, or tailplane leading edges, engine intakes, antennas, hinges, etc.

●  
On ground, in addition to all types of precipitation (all types of snow, freezing rain), the full airframe may get covered with frost. That almost systematically occurs overnight, if the sky is clear and temperature gets around 0° C or below.

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Most of the time, icing conditions do not last for long in the sky. This is why it is unsafe to rely only on pilot reporting (PIREP or absence of PIREP) to detect icing conditions.

A1.2 Meteorology at a glance
Supercooled water can be found in many clouds in the atmosphere, provided the temperature is below and not too far from freezing. Largely convective clouds like big Cumulus or Cumulonimbus are good suppliers. Apart from the possible effects of hail, Cumulonimbus are a special icing threat, because, contrary to all other icing clouds, icing conditions can be met outside the cloud body, for example under the anvil. Anvils often generate freezing drizzle or freezing rain. The precipitation under an anvil can lead to severe icing. At tropical latitudes, this may happen at such high altitudes and outside air temperatures that no icing should normally be expected. But the good news is that it should not last for long. A good operational precaution would be to avoid flying under the root of the anvil when you turn around a Cumulonimbus.
Layers of stratiform clouds, absolutely regardless of their thickness, can exhibit high quantities of freezing drops, including freezing drizzle. That is because, in spite of their stratus appearance, they include some limited but continuous convective activity, which makes it an ideal location for generating freezing drizzle (have you met turbulence in a stratus? The answer must be: Sometimes, yes!).
Meteorology provides a classification of supercooled water drops according to their diameter in microns or drop size (one micron = 1 μm = one thousandth of a millimeter):
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0 to 50 μm: standard supercooled droplets. They stay aloft and make clouds.

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50 to 500 μm: freezing drizzle. They sink extremely slowly and generate curious ice shapes.

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500 to 2000μm: freezing rain. Fall down and lead to clear ice.

Cloud physics show that no cloud is ever made of a single drop size. One cloud can be unequivocally described by its spectrum of droplets. It is considered that most common supercooled clouds contain a spectrum of droplets between 0 and 50μm, which culminates around 20μm. When freezing drizzle is present, the spectrum is not deeply changed, but another peak, smaller, shows at about 200μm, with very few drops in between. However, the large majority of the water content remains within the lower part of the spectrum (i.e. large droplets are much fewer than the 20μm ones).
　
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