hail, hailstones, small cubes
霰 centers on frozen precipitation in the form of small, hard pellets: hail or hailstones. The sense of small cubes is a natural extension of the shape and size of hailstones.
霰 combines 雨 (rain) with 散 (scatter), suggesting scattered frozen rain or hail. The component 散 likely contributes both sound and meaning.
Rain 雨 scatters 散 down as small, hard pellets: that's hail. Picture a storm cloud scattering tiny ice cubes.
For セン, imagine a hailstone hitting a metal sign with a sharp 'sen' sound. The clink of ice on metal cues セン.