Temperature profiles and phase transitions
Rain, snow, sleet, and freezing rain are all ice or liquid water, but which form depends entirely on the vertical temperature structure. In a cloud, water droplets freeze and aggregate into ice crystals, which grow heavier and fall. If they pass through a warm layer before reaching the ground, they melt into rain. If the warm layer is shallow and the air below is freezing, they become sleet: partially melted particles that refreeze. If liquid rain falls on a surface below freezing, it forms freezing rain, coating everything in ice.
The difference between rain and snow is the thickness and location of the warm layer. A deep warm layer near the surface produces rain; a thin or absent warm layer produces snow. This is why the same air mass can produce rain in valleys and snow on mountains: the troposphere cools with altitude, so high elevations experience deeper cold layers.
Hail formation and severe weather
Hail requires violent updrafts that keep ice particles aloft in freezing air long enough to accumulate multiple layers. Hailstones grow by collecting supercooled cloud droplets as they cycle up and down in the updraft. The largest hail occurs in intense supercell thunderstorms where updrafts exceed 30 m/s. These stones can reach tennis-ball size before they finally become too heavy to support and fall.
Frozen precipitation impacts agriculture, infrastructure, and transportation severely. Hail damage costs billions annually worldwide. Climate change is expected to increase the frequency of environments favorable for extreme precipitation events, though the precise regional effects remain uncertain. Understanding precipitation types is crucial for emergency preparedness and assessing climate vulnerability.