Icing

• When a supercooled liquid droplet strikes an object that is at a subfreezing temperature, that object serves as the ice nucleus, and any supercooled liquid coming into contact with it will freeze quickly. The colder the object or the smaller the droplet, the more quickly the ice will form. These near-instant freezing droplets usually create rime ice. Because it freezes instantly, rime ice tends to trap air within its structure, which gives it a milky appearance, and often makes it more pliable than other forms of ice.
• Clear ice commonly forms at temperatures between 2° C (35° F) and -10° C (14° F), and often poses a greater danger than rime ice. The reasons are that clear ice is more difficult to see, it can appear as a "wet" surface and it changes the shape of the airfoil. And because it takes time to freeze, it can extend to surfaces that are not protected by anti-ice or deice systems. While rime can form at warmer temperatures, it is most likely in temperatures below -15° C (5° F), and it tends to be limited to the leading edges of airfoils and around protruding surfaces.
• In the intervening temperature range of -10° C (14° F) to -15° C (5° F), pilots are more likely to experience mixed icing – a combination of rime and clear ice. Mixed ice can form quickly, and become very thick and rough.

Source: ProPilot

Source: Aeronautical Information Manual, Tble 7-1-8

1. Nearly all aircraft icing occurs in supercooled clouds. Liquid droplets are present at outside air temperatures below 0°C (32°F) in these clouds. At outside air temperatures close to 0°C (32°F), the cloud may consist entirely of such droplets, with few or no ice particles present. At decreasing temperatures, the probability increases that ice particles will be found in significant numbers along with the liquid droplets. In fact, as the ice water content increases, the liquid water content tends to decrease since the ice particles grow at the expense of the water particles. At temperatures below about -20°C (-4°F), most clouds are made up entirely of ice particles.
2. The general rule is that the more ice particles and the fewer liquid droplets that are present, the less ice accumulation on the airframe. This is because the ice particles tend to bounce off an aircraft surface, while the supercooled droplets freeze and adhere. As a result, ice accumulation is often greatest at temperatures not too far below 0 °C (32°F), where liquid water content can be abundant, and is usually negligible at temperatures below about -20°C (-4°F).
3. An exception to the general rule just stated may be made for surfaces heated by a thermal ice protection system (or by aerodynamic heating near the stagnation point of an aircraft component at speeds in excess of perhaps 250 knots). For such surfaces, ice particles may melt upon impact and then run back to colder aft regions and refreeze.
4. The higher liquid water content associated with temperatures near freezing is not the only concern. Tests have shown that at outside air temperatures near freezing, the total air temperature may be above freezing. The result is no ice accretion near the stagnation, but the refreezing of the water running back on the airfoil, causing runback ice accretions, possibly behind the protected areas. The formation of a ridge is possible. Pilots should be vigilant at total temperatures (TAT) between -5°C (23°F) and +2°C (35°F). Pilots should know whether the temperature instrument they are reading in the cockpit is outside air temperature (OAT) or TAT.
5. The greater the liquid water content of the cloud, the more rapidly ice accumulates on aircraft surfaces. The size of the droplets also is important. Larger droplets have greater inertia and are less influenced by the airflow around the aircraft than smaller droplets. The result is that larger droplets will impinge on more of the aircraft surface than smaller droplets.

Source: AC 91-74A, ¶3-1.

Source: Aeronautical Information Manual, Tble 7-1-9

Source: AC 00-45G, Table 3-9

• Stratus clouds, sometimes called layer clouds, form a stratified layer that may cover a wide area. The lifting processes that form them are usually gradual, and so they rarely have exceptionally high liquid water contents. Icing layers in stratus clouds with a vertical thickness in excess of 3,000 feet are rare, so a change of altitude of a few thousand feet may take the aircraft out of icing.
• Cumulus clouds, which often form because of vigorous convection, can have high liquid water content. If an aircraft traverses them, the icing can be rapid. Because they tend to be of limited horizontal extent, it may be possible to avoid many of them. Because of the vertical development of cumulus clouds, icing conditions can be found in layers thousands of feet in depth, but with much less horizontal development than in stratus clouds. This class of clouds includes the cumulonimbus, or thunderstorm, clouds. Updrafts in such clouds can be great and result in very large liquid water contents. Thus, a large icing threat can be added to the other excellent reasons to stay out of such clouds. The thunderhead anvil can spread out from the core for several miles and is composed mainly of ice crystals. These crystals will not adhere to unheated surfaces when they hit, but they may melt on a heated surface, run back, and refreeze. The ice content in the anvils can be high, and ingestion of the ice crystals has resulted in uncommanded thrust reductions.
• Orographic clouds form when moist air is lifted by flowing up the side of a mountain. As the parcel of air is lifted, it cools and forms a cloud. Such clouds can contain a large volume of water, and in some cases, large droplets.
• Wave clouds, recognized by their "wavy" tops, can have high liquid water contents. Continued flight along a wave may result in airframe icing.
• Cirrus clouds, found at very high, cold altitudes, are composed entirely of ice particles. Flight through these clouds should not result in structural icing, although the possibility exists for runback icing from the refreezing of particles that melted on thermally or aerodynamically heated surfaces.

Source: AC 91-74A, ¶3-2.

[The figure] shows how ice often affects the coefficient of lift for an airfoil. Note that at very low angles of attack, there may be little or no effect of the ice on the coefficient of lift. Thus when cruising at a low angle of attack (AOA), ice on the wing may have little effect on the lift. However, note that the maximum coefficient of lift (CLmax) is significantly reduced by the ice, and the AOA at which it occurs (the stall angle) is much lower. Thus when slowing down and increasing the AOA for approach, therefore, the pilot may find that ice on the wing that had little effect on lift in cruise now causes stall to occur at a lower AOA and higher speed. Even a thin layer of ice at the leading edge of a wing, especially if it is rough, can have a significant effect in increasing stall speed. For large ice shapes, especially those with horns, the lift may also be reduced at a lower AOA as well. Depending on the airfoil section, the lift loss may even be larger if ice accretes behind areas normally protected, such as due to large drop impingement and runback.

Source: AC 91-74A ¶4-2.a.