Structural icing is one of aviation's most insidious hazards because it can build quickly, degrade aircraft performance significantly, and sometimes form in places the pilot cannot easily see or where deicing equipment cannot reach. To manage that risk effectively, a pilot must understand not only how ice forms, but also where it is most likely to form and what type of ice to expect. The answer depends heavily on the cloud environment: stratiform clouds, cumuliform clouds, and frontal systems each present a distinct icing profile driven by differences in liquid water content, droplet size, temperature, and vertical motion.
The foundation of all structural icing is supercooled liquid water. Pure water suspended as tiny droplets can remain liquid at temperatures well below 0 °C — down to -40 °C — because surface tension inhibits the formation of ice crystals. The smaller and purer the droplet, the more likely it is to be supercooled. When a moving airframe contacts these supercooled droplets, the mechanical agitation triggers rapid or gradual freezing on the aircraft surface. The resulting ice type — rime, clear, or mixed — depends on how fast that freezing occurs, which in turn depends on temperature, supercooled liquid water content (SLWC), and droplet size.
How Supercooled Water Behaves With Temperature
The FAA Aviation Weather Handbook (FAA-H-8083-28B) describes a clear temperature-based gradient of cloud composition. Between 0 °C and -10 °C, clouds consist mainly of supercooled water droplets — the most icing-favorable zone. Between -10 °C and -20 °C, supercooled liquid droplets coexist with ice crystals, meaning icing is still possible but SLWC is decreasing. Below -20 °C, clouds are generally composed entirely of ice crystals, which by themselves do not accrete on airframes the way liquid droplets do. The practical result is that most icing occurs at temperatures between 0 °C and -20 °C, with the majority of reports concentrated in the upper part of that range, and icing is most common in the lower altitudes between the freezing level and roughly 20,000 feet. The only hard physical lower limit is -40 °C, below which droplets freeze spontaneously even without ice nuclei.
An important exception to the cold-cloud rule is the cumulonimbus. Strong vertical currents can loft supercooled water droplets to extreme altitudes where temperatures reach -40 °C, creating icing hazards at heights where it would not otherwise be expected. This makes deep convection uniquely dangerous from an icing standpoint in addition to its turbulence and lightning threats.
Icing in Stratiform Clouds
Stratiform clouds — including stratus, altostratus, and nimbostratus — are characterized by gentle, widespread lifting with relatively low vertical velocities. This produces clouds with low SLWC but extensive horizontal coverage. A flight through a stratiform layer may expose an aircraft to icing conditions for hundreds of miles with little variation in intensity.
Because SLWC is low and temperatures in the icing zone tend to be on the colder side of the 0 to -20 °C range, stratiform clouds predominantly produce rime ice. Rime forms when small supercooled droplets strike the airframe and freeze almost instantaneously. Air becomes trapped in the rapid freeze, producing rough, milky, opaque ice that grows forward into the airstream from leading edges. While rime is brittle and generally easier to remove with deicing equipment, its jagged texture disrupts laminar airflow and can degrade lift and increase drag. Because the ice accumulates over a long exposure in stratiform clouds, total accumulation can still be significant even at low rates.
Stratiform clouds associated with warm fronts are a particularly common icing environment because the frontal lifting is broad and gentle, producing extensive layers of clouds and precipitation over large areas. Nimbostratus associated with warm fronts frequently extends from near the surface to 20,000 feet or higher, potentially exposing an aircraft to prolonged icing on climb or descent.
Icing in Cumuliform Clouds
Cumuliform clouds — cumulus, cumulonimbus, and towering cumulus — are driven by strong convective updrafts that produce the highest SLWC values of any cloud type. More liquid water is available per unit volume because active updrafts continuously replenish the supply of supercooled droplets. The droplets in vigorous convective clouds also tend to be larger than in stratiform clouds.
The combination of high SLWC and warmer temperatures in the lower to middle portion of convective clouds favors the formation of clear (glaze) ice. Clear ice forms when large supercooled droplets strike the airframe and only a portion freezes immediately; the rest flows back along the surface before gradually freezing. This process traps few air bubbles, producing dense, glossy ice that can be difficult to see and can form characteristic horns at the top and bottom of the airfoil's leading edge. These horns create a zone of separated, turbulent airflow that is far larger than the physical footprint of the ice itself, causing dramatic aerodynamic penalties. Clear ice is harder to remove and can spread beyond the protected envelope of deicing or anti-icing equipment.
In the upper levels of cumulonimbus clouds, where updrafts carry supercooled water to extreme altitudes, icing can occur at temperatures that would normally produce only ice crystals in stratiform clouds. Because cumulonimbus clouds also carry severe turbulence and lightning, pilots are instructed to avoid them entirely; the icing hazard is one of many reasons.
Icing in Frontal Clouds
Frontal systems combine elements of both stratiform and cumuliform environments and introduce an additional, especially severe hazard: Supercooled Large Drops (SLDs). SLDs are defined as supercooled water droplets with diameters greater than 50 microns — this includes freezing drizzle (50–500 microns) and freezing rain (greater than 500 microns). They form most commonly in the warm-frontal icing environment, where rain falls from a warm layer aloft and passes through a subfreezing layer below before reaching the aircraft.
SLDs are uniquely dangerous because their large size allows them to cross the aerodynamic streamlines around the wing and impact the airfoil aft of the leading edge — beyond the reach of most deicing boots and anti-icing systems. The resulting ice is lumpy, uneven, and textured, resembling frosted bathroom glass. Even a thin layer of SLD ice on the lower or upper surface of the airfoil can seriously disrupt its aerodynamic properties. In extreme cases, residual SLD ice acts as a spoiler, promoting flow separation bubbles that can travel along the wing and inadvertently deflect the ailerons, creating dangerously unstable flight characteristics. Pilots should treat any report of freezing rain or freezing drizzle as a severe icing condition requiring immediate avoidance or escape.
Cold fronts can also produce embedded convective cells within the frontal cloud mass, combining the high SLWC of cumuliform clouds with the large-area coverage of stratiform clouds. The result is highly variable icing that can shift rapidly from light rime to heavy clear or mixed ice within short distances.
The Three Ice Types and Where They Fit
Rime ice is most common in stratiform clouds, colder temperatures (generally between about -10 °C and -20 °C), low SLWC, and small droplets. It is rough, opaque, and brittle, growing forward from leading edges. Clear (glaze) ice is most common in cumuliform clouds and frontal systems with large droplets, higher SLWC, and warmer subfreezing temperatures (generally above -10 °C). It is dense, transparent, forms horns, and is the most aerodynamically hazardous. Mixed ice forms in environments with small-scale variations in temperature, SLWC, and droplet size — typical of frontal zones — and presents challenges similar to clear ice, including spreading beyond deicing equipment coverage.
Key Numbers and Rules
- 0 °C to -10 °C: Clouds are mostly supercooled liquid droplets — highest icing risk per unit of SLWC.
- -10 °C to -20 °C: Mixed supercooled droplets and ice crystals — icing still possible.
- Below -20 °C: Generally all ice crystals; icing unlikely except in deep convection.
- -40 °C: Absolute lower limit — droplets freeze spontaneously regardless of nuclei.
- Most common icing altitudes: Between the freezing level and roughly 20,000 feet, with icing more concentrated at lower altitudes within that range.
- Most common icing temperatures: Between 0 °C and -20 °C, with most reports concentrated toward the warmer portion of that range.
- SLD definition: Droplets greater than 50 microns; freezing drizzle 50–500 microns, freezing rain greater than 500 microns.
- Highest SLWC: Cumuliform clouds; lowest SLWC: stratiform clouds.
- Rime general range: Temperatures roughly -10 °C to -20 °C; clear ice warmer than -10 °C; mixed in between.
- Airspeed factor: Airframe icing risk generally diminishes above roughly 300 knots due to aerodynamic heating.
- Cold-soak risk: Airframes — especially those with flush fuel tanks — can remain below 0 °C even after entering above-freezing air.
Common Test Traps
- Assuming cold means safe: Temperatures below -20 °C generally mean ice crystals only and low icing risk — but strong cumulonimbus updrafts can sustain supercooled droplets all the way to -40 °C. Do not assume extreme altitude equals no icing in convective clouds.
- Confusing rime and clear ice hazard levels: Rime is more commonly reported, but clear ice is more hazardous. Clear ice forms horns, is harder to see, and can spread beyond deicing equipment coverage — rime is brittle and more easily shed.
- Overlooking SLD aft-of-boot icing: A common misconception is that activated deicing boots eliminate the icing threat. SLDs impact aft of the boots; the residual ice remains and can be as aerodynamically disruptive as a spoiler.
- Ignoring stratiform exposure time: Low SLWC in stratiform clouds may seem benign, but prolonged exposure over hundreds of miles can produce significant total ice accumulation even at a slow rate.
- Cold-soak in above-freezing air: Students often assume that OAT above 0 °C means no icing. A cold-soaked airframe — particularly one with flush fuel tanks — can remain below freezing for a significant period after entering warmer air, creating icing conditions even on the ground after landing.