Hailstone-generating storm system over the Livingston Range (3 September 2008) (Oberlin Bend-Garden Wall viewpoint, just northwest of Logan Pass, Glacier National Park, northwestern Montana, USA)
Storm clouds over the Livingston Range from the storm system that spawned the hailstones on the spruce tree branches shown in this album, Oberlin Bend-Garden Wall viewpoint, just northwest of Logan Pass, Glacier National Park, northwestern Montana, USA (3 September 2008).
Hailstones are scarce, ephemeral, polycrystalline, concentrically layered, monomineralic rocks of meteoric origin. They are composed of the mineral ice (hydrogen oxide, H2O). Ice has a low melting temperature for a mineral (= 0˚ Celsius/Centrigrade; = 32˚ Fahrenheit; = 273˚ Kelvin). As a result of this, rocks (hailstones, firn, glacial ice) and sedimentary deposits (snow) consisting of ice are ephemeral, except at very high elevations (mountain tops) and in polar to near-polar facies. Hailstones form in many thunderstorms and can reach the land surface before melting. They range in size from about half-a-centimeter to >20 cm (very large hailstones, including the largest on record (see: www.wunderground.com/hurricane/2010/hail_sdrecord.jpg), are not really hailstones, but are aggregate hailstones, formed by ice cementing many smaller hailstones together). Hailstones form spherical to subspherical to irregularly-shaped masses.
The term "cryometeorites" has been applied to hailstones by some people. Considering that they have a meteoric origin, and do not originate from outer space, this term is inappropriate and is rejected here.