Severe Weather 101

Hail: Types of Frozen Precipitation

Hail is only one of many types of frozen precipitation.

Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into ice. To be considered hail, the frozen precipitation pieces must have a diameter greater than 5mm (.20”).

Graupel (a.k.a. soft hail or snow pellets) are soft small pellets of ice created when supercooled water droplets coat a snowflake.

Sleet (a.k.a. ice pellets) are small, translucent balls of ice, and smaller than hail. They often bounce when they hit the ground.

Snow forms mainly when water vapor turns to ice without going through the liquid stage. There is no thunderstorm updraft involved in either of these processes.

What we do: NSSL was a leader and major contributor to the scientific and engineering development of dual-polarized weather radar technology installed on all NWS radars. Dual-polarization radar can clearly identify rain, hail, snow, or ice pellets inside the clouds.

NSSL scientists are developing algorithms that will produce estimates of whether the precipitation is falling in liquid or frozen form, or if the precipitation is reaching the ground. NSSL's Hydrometeor Classification Algorithm (HCA) uses dual-polarization technology to automatically sort between ten types of radar echoes including big raindrops and hail. This helps the forecaster quickly assess the precipitation event and better forecast how much will fall.

NSSL's Meteorological Phenomena Identification Near the Ground (mPING) project also collects data on types of precipitation. Volunteers are invited to submit reports of what is actually falling to the ground at their location. This data is used to refine radar algorithms that detect hail and other frozen precipitation.