What does HLSTO stand for?

Hailswones

Hailstones

A pellet of hail. Hailstones may be spheroidal, conical, or generally irregular in shape. The spheroidal stones often exhibit a layered internal structure, with layers of ice containing many air bubbles alternating with layers of relatively clear ice. These probably correspond to dry growth and wet growth and are called rime and glaze, respectively.