Understanding Icing
Airframe icing in cloud depends primarily on the constitution of the cloud and the air temperature. There is a risk of ice accretion so long as supercooled droplets are present in clouds. On the other hand, if the cloud particles are all frozen, ice accretion is unlikely to occur as solid particles do not adhere to the aircraft.
It is only in parts of clouds, where the temperature has fallen to a value less than about -40 deg C that one can feel reasonably sure that only solid particles exist. However, on rare occasions, some supercooled water droplets may be present among the ice crystals at these low temperatures. Hence, there remains a small risk of icing.
A cloud which contains both liquid droplets and ice crystals may have some regions in which droplets predominate. In other portions of the cloud the concentration of ice crystal particles may exceed that of the liquid particles. The severity of the icing may therefore vary over short distances.
The rates of accretion are greatest in convective clouds, but the total accumulation is usually mitigated by the structure and by the isolated nature of these clouds. However, in cases where the clouds become widespread and difficult to avoid, the duration of exposure to severe icing is increased. This is most likely to occur in hilly country, with moist, unstable conditions, or in the vicinity of fronts.
Basic Requirements for Icing
Icing requires sub-zero temperatures of both the air and the aircraft and the presence of supercooled water droplets. Such droplets are common in stable cloud down to about -15 deg C, due to the rarity of ice-forming nuclei in the atmosphere. Below that temperature, larger droplets freeze and ice crystals become dominant around -20 deg C. Most supercooled water disappears between -30 and -40 deg C. Water vapour will not produce any significant icing but only hoarfrost, through sublimation. Likewise ice crystals and snow will only produce icing to the extent that they are first melted by warmer air or plane.
The greater the liquid water content of the cloud, the greater the chance of icing as long as the droplets are supercooled.
Droplet size will affect the rate of capture by aircraft, and the type of icing. Larger droplets will freeze more slowly and tend to spread out into clear icing.
Aircraft factors... an aircraft traveling at high speed will collect ice more rapidly since the faster it travels the more droplets it hits.
Orographic cloud (lifted by terrain) has a higher risk of icing which is likely to be of the clear type and more intense than for a non-orographic cloud. The effect is felt on the windward side and above the mountain crest, but dangerous icing is also possible in the thick lee wave clouds downwind.
Large bodies of water have a potential effect on icing, through their heat and moisture fluxes. This is particularly significant in the cold season, over and downwind from open water. If marked convection or freezing drizzle results, icing could be moderate or worse. Otherwise rime icing is more likely, but it could be of moderate intensity due to the abundant moisture and heat supply.
A moist, unstable airmass, sufficiently cool for most cloud to extend above the freezing level, presents the greatest icing hazard. In mid-latitudes, icing is most prevalent in winter because the freezing level is lower and cloud more abundant. this maximum shifts to spring and fall in the higher latitudes because the very cold air supports little moisture, essentially in the ice crystal form. Summertime icing can be very hazardous in convective cloud, but otherwise is much less frequent or extensive.
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