Structural icing is one of the most insidious hazards in instrument meteorological conditions. Unlike turbulence or low visibility, ice can accumulate silently and invisibly, steadily degrading the very surfaces that keep an aircraft airborne. Understanding why ice forms — the physics of supercooled water — is the first step toward recognizing when you are in danger and making sound go/no-go decisions.
This article covers the complete picture of structural icing as described in the FAA Aviation Weather Handbook: how supercooled water behaves, the three types of structural ice, the environmental factors that control icing severity, and the operational traps that catch pilots off guard.
The Physics of Supercooled Water
Most pilots know that water freezes at 0 °C (32 °F), but that rule applies to bulk water with impurities and container walls to act as freezing nuclei. Tiny, pure water droplets suspended in the atmosphere behave very differently. Surface tension inhibits the formation of ice crystals, so these droplets can remain liquid at temperatures far below the normal freezing point. Pure water droplets suspended in the air will not freeze spontaneously until they reach −40 °C — at that extreme temperature, freezing occurs even without nuclei. Water in this below-freezing yet still-liquid state is called supercooled water.
The smaller and purer a droplet is, the more likely it is to remain supercooled. This matters because cloud droplets are typically very small and remarkably pure. However, supercooled water is not limited to tiny cloud droplets. Supercooled Large Drops (SLDs) — droplets with diameters greater than 50 microns — can exist in freezing drizzle (roughly 50–500 microns) and freezing rain (greater than 500 microns). SLDs represent a far more dangerous icing threat, as explained below.
The liquid water composition of a cloud changes with temperature in a predictable way. Between 0 °C and −10 °C, clouds consist mainly of supercooled liquid water droplets. Between −10 °C and −20 °C, liquid droplets and ice crystals coexist. Below −20 °C, icing potential decreases substantially as clouds become increasingly dominated by ice crystals, which do not readily stick to an airframe, though supercooled liquid water can still persist at these colder temperatures depending on conditions. The practical implication: nearly all structural icing is reported between 0 °C and −20 °C.
One critical exception to the general decline in icing risk below −20 °C: strong vertical currents inside cumulonimbus clouds can loft supercooled water droplets to very high altitudes where temperatures approach −40 °C. This is why flying near or in convective clouds can expose an aircraft to icing at altitudes and temperatures where it would not normally be expected.
Supercooled water is inherently unstable. Any significant agitation — including the aerodynamic disturbance caused by an aircraft flying through a cloud — can trigger rapid freezing. This is precisely why structural icing occurs: the airframe provides both the mechanical disturbance and the cold surface onto which supercooled droplets freeze on contact.
The Three Types of Structural Icing
Structural icing is the ice that accumulates on the outside of the airplane when supercooled water droplets strike and freeze on the airframe. The FAA identifies three distinct types, each with different characteristics, hazard levels, and environmental conditions that favor their formation.
Rime Ice
Rime ice is rough, milky, and opaque. It forms when small supercooled water droplets strike the aircraft and freeze instantaneously. The rapid freezing traps air bubbles within the ice structure, giving it a porous, brittle, white or milky appearance. Rime ice is the most frequently reported type. It tends to grow forward into the airstream from the leading edges of wings and other exposed surfaces, building into a rough, irregular shape. Rime icing is favored by colder temperatures (generally below −15 °C), lower liquid water content (LWC), and small droplet sizes. Although rime ice appears less dramatic than clear ice, its jagged, rough texture disrupts the smooth airflow over the airfoil and degrades lift.
Clear (Glaze) Ice
Clear ice — also called glaze ice — is glossy, transparent, or translucent. It forms when large supercooled water droplets strike the aircraft and only a small portion of each drop freezes immediately on impact. The remaining liquid water spreads or smears over the surface before it gradually freezes. Because this process is slow, very few air bubbles are trapped, producing ice that is denser and less opaque than rime ice. Clear ice is favored by temperatures warmer than −10 °C, higher liquid water content, and larger droplet sizes.
Clear ice is generally considered the more hazardous type for several reasons. It tends to form horns near the top and bottom of the airfoil's leading edge, creating a shape that dramatically disrupts airflow over a much larger area than rime ice does. Because it is clear, it may be difficult for the pilot to detect visually. It can also spread beyond the protected area of deicing or anti-icing equipment, leaving residual ice on unprotected surfaces.
Supercooled Large Drops (SLD) — A Special Clear-Ice Hazard
SLD icing is a particularly dangerous subset of clear icing. Because freezing drizzle and freezing rain droplets are so large, they do not freeze immediately on contact with the leading edge. Instead, they flow aft along the airfoil surface before freezing, producing a lumpy, uneven, textured ice deposit that can extend well behind the leading edge — often beyond the reach of deicing boots or other protective equipment. Even small residual ice deposits aft of the protected zone can act like a spoiler, disrupting airflow over a large portion of the wing, reducing lift, and in extreme cases causing flow separation and aileron activation that produces dangerously unstable flight conditions.
Mixed Ice
Mixed ice is exactly what the name implies: a combination of rime and clear ice that forms when an aircraft encounters small-scale variations in temperature, liquid water content, and droplet size within the same cloud system. Viewed from the side, mixed ice appears as alternating layers of opaque and relatively clear ice. Its hazard profile is similar to clear ice — it can form horns, spread beyond protected surfaces, and cause significant disruption to airflow. It is also more difficult to remove than rime ice alone.
Why Structural Icing Matters Operationally
Even a thin, smooth coating of ice on an airfoil can reduce lift and increase drag dramatically. Rough or irregular ice shapes — horns, ridges, or SLD deposits aft of the protected zone — are especially destructive because they trigger turbulent airflow and boundary layer separation over a large percentage of the wing. The net effect is an increase in stall speed, a reduction in control effectiveness, and an increase in fuel burn and drag. In the most severe cases, the aircraft may become uncontrollable.
Beyond the wing, ice can accumulate on propellers, disrupting their efficiency and causing dangerous vibration; on antennas, affecting communications; on pitot tubes, causing false airspeed readings; and on windshields, destroying forward visibility.
Key Numbers and Rules
- 0 °C to −10 °C: Clouds consist mainly of supercooled liquid water — high icing potential.
- −10 °C to −20 °C: Mixed liquid droplets and ice crystals — still significant icing potential.
- Below −20 °C: Icing potential decreases substantially as ice crystals dominate, though supercooled liquid can still persist, except in strong convection.
- −40 °C: The absolute physical lower limit for supercooled water; spontaneous freezing occurs.
- Icing temperature range: Nearly all structural icing is reported between 0 °C and −20 °C.
- SLD droplet sizes: Freezing drizzle roughly 50–500 microns; freezing rain greater than 500 microns.
- Rime ice tendency: Generally below −15 °C; clear ice tendency above −10 °C; mixed ice in between.
- High airspeed effect: Aerodynamic skin friction heating at high airspeeds reduces icing potential, though the Aviation Weather Handbook does not specify a single universal airspeed threshold.
- Supercooled Large Drop (SLD) danger: Ice forms aft of leading-edge protection — residual ice acts as a spoiler.
The Cold-Soak Hazard
Temperature alone does not determine whether icing occurs on the ground or during descent into warmer air. An airframe that has been operating in a cold environment for an extended period — especially one with flush-mounted wing fuel tanks — can remain colder than 0 °C for a significant time even after the ambient temperature rises above freezing. This cold-soak effect means that moisture in warmer air can freeze on contact with a sub-zero airframe surface, even though the ambient temperature appears safe. Pilots must know the specific cold-soak characteristics of their aircraft and not rely solely on ambient temperature when assessing icing risk on arrival or departure.
Icing Severity Factors — What Controls How Bad It Gets
The FAA ranks the meteorological factors that drive icing type and severity in order of importance: supercooled liquid water content (SLWC), temperature, and droplet size. Aircraft type, design, and airspeed are also significant non-meteorological variables. SLWC is highest in cumuliform clouds and lowest in stratiform clouds, though in most reported icing encounters SLWC is relatively low. Droplet size primarily matters when drops exceed standard cloud droplet dimensions — as in freezing drizzle and freezing rain — because large drops impact farther aft on the airfoil, bypassing leading-edge protection. Aircraft airspeed increases the rate at which droplets impact the surface, accelerating ice accumulation, but the skin heating from aerodynamic friction at high speeds partially offsets this effect.
Common Test Traps
- Assuming −20 °C is always safe: Strong convection can carry supercooled water to altitudes where temperatures approach −40 °C — icing is possible even at very cold temperatures near cumulonimbus clouds.
- Confusing rime and clear ice hazard levels: Clear ice is generally more hazardous because it forms horns, spreads beyond deicing equipment, and is harder to see — not rime ice, even though rime is more frequently reported.
- Ignoring SLD icing sources: Freezing rain and freezing drizzle produce SLDs that form ice aft of leading-edge protection — deicing boots alone will not prevent SLD-related flow separation.
- Forgetting cold-soak: An airframe can be below 0 °C even in above-freezing air — fuel-tank wing surfaces are particularly vulnerable.
- Overlooking ice crystals vs. liquid drops: Ice crystals alone (below −20 °C, outside of convection) generally do not cause structural icing because they do not adhere to the airframe the same way supercooled liquid drops do.