Surface 0 °CReaches the ground as -Liquid at ground 0 %Most melted 0 %Fall time 0 minFPS 0

Time-lapse and compressed height: the 4 km column is drawn 14 units tall and particles fall about 150 times faster than real. Colours on the back wall are the air temperature (white line = 0 °C). White flake = ice, blue-white = partly melted, blue drop = liquid, glassy bead = refrozen ice pellet.

Warm Nose Sounding: Melting Layer, Refreezing, Sleet and Freezing Rain Lab

Whether winter precipitation arrives as snow, rain, ice pellets or freezing rain depends on the temperature of the air it falls through. Build a sounding and follow one particle down.

Temperature profile

Falling particle

Time

Live readouts

Sounding and particle

Liquid fraction of the particle

Regime map: surface temperature against nose temperature

Experiment: how deep a warm layer is needed?

Sweeps the warm-layer depth from 0 to 2500 m with all other settings as they are now and plots the most melted fraction and the liquid fraction reaching the ground.

Independent numerical checks

Controls

Drag / one finger: orbit; wheel / pinch: zoom.
Surface temperature below 0 gives a cold layer at the ground; the warm nose is a layer aloft that rises above 0 °C.
Effective density: new snow about 80 to 150, graupel about 300 to 500, solid ice 917 kg per cubic metre. Denser particles melt more slowly.

The science

Sounding. Above the warm nose the air cools at 6.5 K per km. The nose is a triangle of peak temperature T_n and depth Δz. Below it, a cold layer warms linearly from T_s to 0 °C. If T_s is above 0 the whole column up to the nose top is warm.

Heat exchange. A falling particle of diameter D exchanges heat with the air at Q = Nu π k D (T_air - T_p), with Nu = 2 + 0.6 Re^1/2 Pr^1/3 (Ranz-Marshall, Re = vD/ν) or Nu = 2 without ventilation. Ice below 0 °C warms with time constant τ = m c_i / (Nu π k D). At 0 °C the heat goes into melting (or out of freezing): df/dt = Nu π k D T_air / (m L_f), f = liquid fraction, L_f = 3.34 × 10⁵ J/kg.

Melt time. For Nu = 2 and constant air temperature ΔT above zero a particle melts completely in t = ρ L_f D² / (12 k ΔT). Small, low-density flakes melt fast, solid ice pellets slowly.

Fall speed. Ice: v = √(4 g D ρ / (3 C_d ρ_air)) with C_d = 2.8 (aggregate flakes, illustrative). Liquid: Gunn-Kinzer fit v = 9.65 - 10.3 e^{-0.6 D_w} (D_w in mm, water-equivalent diameter). A partly melted particle blends the two by f.

Outcomes. Never melted: snow. Melted a little and refrozen: ice pellets (sleet). Fully melted and falling into air below 0 °C: freezing rain (supercooled drops, no ice seed left to freeze them; they freeze on contact). Fully melted over a warm surface: rain. Partly melted over a surface just above 0: wet snow. The page also shows the positive area (warm nose, K·m) and negative area (surface cold layer): the first is the heat available to melt, the second the cold available to refreeze.

Honest simplifications. Spheres of fixed effective density (real flakes change shape and density as they melt), no heat from condensation or evaporation (wet-bulb effects), no riming, no collisions, no nucleation of supercooled drops (a drop that is fully liquid stays liquid), a straight-line sounding, no wind or updrafts. Real ice-pellet and freezing-rain forecasts need a full sounding and more physics. All numbers are illustrative.

Models built with Blender (flake, drop, pellet, cloud, house, pine, car, street slab) and rendered with Three.js.