Next we have pack snow — Page 291, Lesson 280

I want to walk you through the rest of the icing chapter, starting with a rare but important type: rain ice. Rain ice is rare over the UK, but it is common in winter over North America and Central Europe. The factors that affect its formation are summarised in Figure 16.3, which is on screen now. Next we have pack snow. Pack snow is icing that is due to a mixture of supercooled water droplets and snow. It can block air intakes and other aircraft openings. Normally, the effects are slight. Then we come to hoar frost. Hoar frost is a white crystal deposit that appears similar to frost on the ground. It occurs in clear air — that is an important distinction, because most other icing types require visible cloud. Hoar frost will form if the airframe temperature is below 0°C and the ambient temperature is lowered to saturation level. Water vapour in contact with the airframe is converted directly to ice crystals without becoming liquid first — that process is called sublimation. This process requires the presence of another type of ice nucleus, the sublimation nucleus. Their composition is usually inorganic, for example volcanic dust, clay, or soil particles. There are two situations where hoar frost can occur. First, on the ground. This usually occurs at night and is similar to the frost which forms on a car. It must be cleared before take-off for three reasons: skin friction will increase the take-off run; windscreens will be obscured; and radio interference will be caused by ice on aerials. Second, in flight. Hoar frost can occur in flight in two cases: if a rapid descent is made from a very cold region to a warm moist layer, or if a climb is made from a temperature below 0°C through an inversion. The icing is not severe. The effects can be overcome by flying in a region where the temperature is above 0°C, or by flying faster to increase the kinetic heating. Now let's move to the factors affecting the severity of icing. There are two main factors. The first is the size of the supercooled water droplets. This depends on cloud type and temperature. Here is how it breaks down. For moderate to heavy clear ice, supercooled water droplets can only be large in cumulus, cumulonimbus, and nimbostratus clouds, and then only when temperatures are in the general range 0°C to -20°C. For light to moderate rime ice, in layer clouds small supercooled water droplets are present from 0°C to -20°C. For light rime ice, in layer clouds supercooled water droplets are smaller below -20°C. Supercooled water droplets are also small in cumulus, cumulonimbus, and nimbostratus from -20°C to -40°C. At -40°C and below, supercooled water droplets are very small and icing is usually negligible. The second factor is the concentration of supercooled water droplets. The concentration of water droplets is higher in heap clouds because the up currents are stronger. Hence cumulus and cumulonimbus clouds have a high concentration of supercooled water droplets, and this causes the icing to be moderate to severe. There is always a greater concentration of droplets near the base of the cloud where it is warmest. Icing severity by cloud types tends to be as follows. Cumulus and cumulonimbus — moderate to severe. Nimbostratus — moderate to severe. Stratocumulus — light to moderate, but may be severe in mountainous areas. Altocumulus, altostratus, and stratus — light. Cirrus, cirrostratus, and cirrocumulus — nil or trace.