Why is freezing drizzle more dangerous than regular icing from clouds?

Freezing drizzle contains Supercooled Large Droplets (SLD) with diameters between 40 and 200 microns — far larger than typical cloud droplets. These large droplets have enough momentum to flow aft past the leading edge before freezing, depositing lumpy, uneven ice beyond the reach of standard deicing equipment. The residual aft ice acts like a spoiler, severely disrupting airflow and reducing lift across a large portion of the airfoil.

At what temperatures does structural icing most commonly occur?

According to the FAA Aviation Weather Handbook, almost all structural icing occurs between 0 °C and −20 °C, with about half of all reports concentrated between −8 °C and −12 °C. The peak altitude of occurrence is near 10,000 ft MSL, with roughly half of all incidents between 5,000 and 13,000 ft. The absolute lower temperature limit for icing is −40 °C, below which liquid droplets cannot exist.

Can an aircraft pick up ice when the outside air temperature is above freezing?

Yes. If an aircraft has been flying in a cold environment, its airframe — especially wings with flush-mounted fuel tanks — can remain cold-soaked below 0 °C even after the aircraft descends into air with a positive outside air temperature. In that situation, moisture in the warmer air can freeze on contact with the still-cold airframe surfaces, producing structural ice despite above-freezing OAT. Pilots should be aware of this risk after extended cold-altitude flight.

Supercooled Large Droplets (SLD) and Freezing Drizzle Icing

Structural icing is a well-known hazard to flight, but not all icing is equally dangerous. Among the various forms pilots may encounter, ice formed from Supercooled Large Droplets (SLD) stands apart as especially threatening. SLD icing is associated with freezing drizzle and freezing rain — precipitation types that produce water droplets far larger than ordinary cloud droplets. Because these large droplets behave differently on impact with an airfoil, the resulting ice forms in places and shapes that can defeat standard deicing equipment and dramatically degrade aircraft performance.

Understanding SLD icing requires first grasping the physics of supercooled water, the spectrum of structural ice types, and the meteorological conditions that generate the most severe accumulations. This article draws on FAA Aviation Weather Handbook FAA-H-8083-28B, Chapter 20 to provide a thorough treatment of the subject for pilots, students, and anyone preparing for FAA knowledge or practical tests.

The Physics of Supercooled Water

Pure water suspended in the atmosphere does not freeze the moment the temperature drops below 0 °C. Surface tension on tiny droplets actually inhibits the freezing process, allowing liquid water to persist at temperatures well below the normal freezing point. In fact, pure, very small water droplets can remain liquid down to −40 °C — the absolute physical lower limit at which spontaneous freezing without ice nuclei occurs. This phenomenon is called supercooling.

Cloud composition changes markedly with temperature. Between 0 °C and −10 °C, clouds consist mainly of supercooled liquid water droplets — prime icing territory. Between −10 °C and −20 °C, liquid droplets coexist with ice crystals, producing mixed conditions. Below −20 °C, clouds are generally composed entirely of ice crystals, which by themselves do not adhere to airframes the way liquid water does. However, strong vertical currents — such as those inside a cumulonimbus — can loft supercooled water to great heights where temperatures approach −40 °C, extending the icing envelope well above the typical range.

The key practical fact: supercooled water freezes almost instantly when it is sufficiently agitated. An aircraft penetrating a cloud or precipitation layer of supercooled droplets provides exactly that agitation — the impact of the droplets on the airframe triggers freezing, and structural ice begins to build.

Types of Structural Icing

Structural icing — ice that adheres to the outside of the aircraft — is broadly divided into three types: rime, clear (glaze), and mixed. Understanding these types is essential context for appreciating why SLD is uniquely hazardous.

Rime Ice

Rime ice is the most frequently reported icing type. It forms when small supercooled droplets strike the airframe and freeze instantaneously. The rapid freezing traps air bubbles, producing ice that is rough, milky, opaque, and porous. Rime ice grows from the leading edges forward into the airstream, maintaining a relatively defined shape. While its jagged texture disrupts airflow and degrades aerodynamic performance, it tends to stay close to the leading edge where most anti-ice and deice protection is designed to work. Rime ice favors colder temperatures (generally colder than −15 °C), lower liquid water content (LWC), and smaller droplets.

Clear (Glaze) Ice

Clear ice forms when large supercooled droplets do not freeze all at once on impact. Only a small fraction of each droplet freezes immediately; the remaining liquid water flows or smears rearward along the airfoil surface before gradually freezing. Because freezing is slow, few air bubbles are trapped, creating ice that is glossy, dense, translucent, and difficult to see. Clear ice tends to form characteristic horns near the top and bottom of the leading edge, generating a region of turbulent, separated airflow considerably larger than that caused by rime ice. Clear icing conditions favor temperatures warmer than approximately −10 °C, higher LWC, and larger droplets.

Mixed Ice

Mixed ice is, as the name implies, a combination of rime and clear ice. It forms when an aircraft flies through regions with small-scale variations in temperature, LWC, and droplet size — conditions that might alternate between rime- and clear-ice-forming environments over distances of tens of kilometers or less. Viewed from the side, mixed ice shows alternating layers of opaque and clearer ice. Mixed ice can form horns and irregular shapes, is harder to remove than rime ice, and can spread beyond deicing equipment onto unprotected surfaces.

Supercooled Large Droplets: Definition and Behavior

SLDs are supercooled water droplets with diameters larger than 40 microns. This distinguishes them sharply from typical cloud droplets, which are usually 10–20 microns in diameter. SLD encompasses two precipitation types:

Freezing drizzle: droplets with diameters of approximately 40 to 200 microns
Freezing rain: droplets with diameters greater than 200 microns

Because of their larger mass and momentum, SLD droplets do not simply follow the airstream around an airfoil the way smaller cloud droplets tend to. Instead, they cross the curving streamlines and strike the airfoil surface farther aft — sometimes well behind the leading edge. Once they hit the surface, they behave like clear ice droplets: a portion freezes immediately, but the remaining water continues to flow rearward before finally freezing.

The resulting ice has a distinctive character: lumpy, uneven, and textured, sometimes compared to obscure or pebbled bathroom glass. It is a form of clear icing, but its aft location and irregular texture make it far more dangerous than standard leading-edge clear ice.

Why SLD Icing Is Especially Dangerous

The central danger of SLD icing is that it forms aft of the protected zone. Standard anti-icing and deicing equipment — pneumatic boots, heated leading edges, weeping wing systems — is engineered to protect the leading edge and a short distance behind it. SLD ice accumulates beyond that protection, where it cannot be removed in flight.

Even a small amount of residual ice on the lower and upper surfaces of an airfoil can significantly reduce its lift-generating capability. The rough, uneven ice acts much like a spoiler — a device literally used to intentionally reduce lift and increase drag when slowing an aircraft. In severe cases, the turbulence and flow-separation bubbles generated by aft ice can travel along the airfoil and inadvertently deflect the ailerons, producing dangerously unstable handling characteristics. This is not merely a performance problem; it is a control problem.

Because SLD conditions are associated with freezing rain and freezing drizzle, they often occur beneath warm fronts or near temperature inversions, where warm air overrides a cold surface layer. These meteorological setups can produce SLD over wide geographic areas at relatively low altitudes — exactly where general aviation aircraft operate most of the time.

Key Numbers and Rules

SLD droplet size threshold: greater than 40 microns in diameter (freezing drizzle: 40–200 microns; freezing rain: >200 microns).
Supercooling limit: pure water droplets can remain liquid down to −40 °C; below that, spontaneous freezing occurs regardless of nuclei.
Peak icing temperature range: approximately 0 °C to −20 °C, with about half of all icing reports between −8 °C and −12 °C.
Peak icing altitude: near 10,000 ft MSL; roughly half of all icing encounters occur between 5,000 and 13,000 ft.
Rime ice temperature regime: generally colder than −15 °C.
Clear ice temperature regime: generally warmer than −10 °C.
Mixed ice temperature regime: approximately −10 °C to −15 °C (overlap zone).
Airspeed effect: icing risk generally diminishes at higher airspeeds due to aerodynamic heating of the airframe skin, though the FAA Aviation Weather Handbook does not cite a specific knot threshold for this effect.
Cold-soaked airframe: aircraft that have been flying in cold air can retain airframe temperatures below 0 °C even after descending into above-freezing air, making icing possible on the ground or in positive-OAT conditions — especially for aircraft with wing-mounted fuel tanks.

Meteorological Factors Driving Icing Severity

The FAA ranks the meteorological factors controlling icing type and severity in order of importance: (1) supercooled liquid water content (SLWC), (2) temperature (altitude), and (3) droplet size. Aircraft type, design, and airspeed are important non-meteorological modifiers.

SLWC — the mass of liquid water per unit volume of air — determines how much water is available to freeze on the airframe. Cumuliform clouds have the highest SLWC; stratiform clouds have lower values. High SLWC combined with temperatures just below freezing produces the most rapid and dangerous ice accumulation. Droplet size generally plays a secondary role unless the droplets are large enough to be classified as SLD, at which point their aft-impacting behavior elevates droplet size to a primary safety concern.

Common Test Traps

Assuming deicing boots protect against all icing: Standard deicing equipment protects the leading edge area only. SLD ice forms aft of that zone, where it cannot be removed in flight. Boots do not solve an SLD problem.
Confusing rime and clear ice hazards: Rime ice is rough and opaque but stays near the leading edge. Clear ice and SLD ice are denser, harder to see, form irregular shapes (including horns and aft deposits), and are more aerodynamically disruptive.
Ignoring above-freezing OAT as a guarantee of safety: A cold-soaked airframe can remain below 0 °C even when the outside air temperature is slightly above freezing, especially with wing-mounted fuel tanks. Structural icing can still occur.
Underestimating low-altitude SLD exposure: SLD events (freezing rain/drizzle) occur at relatively low altitudes beneath warm fronts. Pilots may assume icing risk disappears after descending out of clouds, but SLD can be encountered in clear air below the cloud base during freezing rain.
Misremembering the −40 °C rule: The −40 °C limit applies to spontaneous freezing of pure water droplets without ice nuclei. It does NOT mean icing is impossible above that altitude — strong updrafts can carry supercooled water to those heights. It means that below −40 °C, no liquid water can exist, so structural icing cannot occur.