Snow Line Elevation: Freezing Level & Rain/Snow Phase Transition (2D)

This 2D companion turns the same falling-snow scene the 3D version renders decoratively into a real atmospheric-science instrument: a side panel exposes sea-level temperature, environmental lapse rate and relative humidity, which together locate the freezing level and — via a wet-bulb-depression estimate — the true snow line below it, drawn as horizontal bands across a mountain slope. Each falling particle checks its own altitude against that line every frame and switches from a slow, swaying snowflake to a fast, straight raindrop the instant it crosses, building a real snowpack only on the terrain above the line while everything below runs off, so raising the humidity, warming the air, or shrinking the mountain visibly eats into the snow cover instead of just changing a color.