What Is Ice Accretion?

Ice accretion is the growth of an ice layer on an exposed surface when supercooled water droplets, freezing precipitation, or wind-driven spray strike that surface and freeze onto it. The term covers both the physical process and the resulting ice shape, and it belongs to the branch of meteorology concerned with how atmospheric liquid water interacts with solid bodies. Engineers study accretion because the added mass, the changed surface roughness, and the altered geometry all degrade the performance of the object underneath. Aircraft wings, overhead conductors, wind turbine blades, ship superstructures, radio masts, and meteorological sensors are all affected.

Accretion requires liquid water at a temperature at or below freezing. Cloud droplets a few tens of micrometers across can remain liquid well below 0 degrees Celsius, often down to about -20 degrees Celsius, and they freeze on contact with a solid nucleus. The rate and character of the ice that forms depend on air temperature, wind speed, liquid water content, droplet size distribution, and the size and shape of the collecting object.

Icing Regimes and Accretion Physics

Icing is conventionally divided into two limiting regimes. In dry growth, droplets freeze essentially on impact, trapping air and producing rime, a white opaque deposit with densities of roughly 200 to 600 kilograms per cubic meter. In wet growth, the latent heat released by freezing cannot be carried away fast enough, so a water film spreads along the surface before solidifying. That produces glaze, a clear deposit approaching the density of solid ice at roughly 900 kilograms per cubic meter and adhering far more strongly. Freezing rain and freezing drizzle almost always yield glaze.

The quantitative treatment rests on a mass balance and an energy balance at the surface. Collection efficiency, the fraction of droplets in the swept volume that actually strike the body rather than following the airflow around it, rises with droplet inertia and falls with body size, which is why thin wires and small leading edges collect ice efficiently while thick structures collect it poorly. Runback water that refreezes downstream of the impingement zone builds the horn-shaped glaze formations that are hardest to predict.

Aircraft Icing

Aviation drives much of the measurement and modeling work. NASA has operated an Icing Research Tunnel at its Cleveland laboratory since 1944, and the icing research program at NASA Glenn developed much of the test data and analysis behind current ice protection practice, along with the LEWICE and GlennICE accretion codes. Ice on a wing raises drag, reduces maximum lift, and can shift the stall angle by several degrees, effects that NASA documents in its work on airframe icing and its aerodynamic penalties. Supercooled large droplets, which impinge aft of protected surfaces, received sustained attention after several loss-of-control accidents in the 1990s. Because measured ice shapes are geometrically complex, researchers now feed laser scans of real accretions into high-fidelity simulations, as in large-eddy simulations of an iced NACA 23012 airfoil.

Icing of Structures and Power Systems

For fixed structures, the design question is load rather than aerodynamics. ISO 12494 defines ice classes and reference ice loads for masts, towers, antennas, cables, and wind turbines, while IEC 60826 governs ice loading on overhead transmission lines. Accreted ice increases vertical load and enlarges the wind-exposed area, and asymmetric shedding can set conductors into the low-frequency, high-amplitude oscillation known as galloping. Wind turbine blades lose power output and gain fatigue load when iced, and shed fragments create a throw hazard. Detection relies on vibrating-probe and impedance sensors, heated anemometers, and increasingly on power-curve deviation analysis.

Applications

Ice accretion analysis has applications in a range of fields, including:

• Aircraft certification and ice protection system design
• Overhead transmission and distribution line loading
• Wind energy siting and blade de-icing in cold climates
• Marine engineering, where spray icing affects vessel stability
• Weather forecasting and aviation icing advisories
• Instrument design for polar and high-altitude observation