page_title: Hail
section_title: Accumulations
hierarchical_section_title: Hail / Accumulations
caption_attribution_description: English: Hand holding hail in a strawberry patch
context_page_description: Hail is a form of solid precipitation. It is distinct from ice pellets, though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets fall generally in cold weather while hail growth is greatly inhibited during cold surface temperatures.
  Unlike other forms of water ice such as graupel, which is made of rime, and ice pellets, which are smaller and translucent, hailstones usually measure between 5 mm and 15 cm in diameter. The METAR reporting code for hail 5 mm or greater is GR, while smaller hailstones and graupel are coded GS.
  Hail is possible within most thunderstorms as it is produced by cumulonimbus, and within 2 nmi of the parent storm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm and lowered heights of the freezing level. In the mid-latitudes, hail forms near the interiors of continents, while in the tropics, it tends to be confined to high elevations.
  There are methods available to detect hail-producing thunderstorms using weather satellites and weather radar imagery.
context_section_description: Narrow zones where hail accumulates on the ground in association with thunderstorm activity are known as hail streaks or hail swaths, which can be detectable by satellite after the storms pass by. Hailstorms normally last from a few minutes up to 15 minutes in duration. Accumulating hail storms can blanket the ground with over 2 inches (5.1 cm) of hail, cause thousands to lose power, and bring down many trees.  Flash flooding and mudslides within areas of steep terrain can be a concern with accumulating hail.
  Depths of up to 18 in (0.46 m) have been reported. A landscape covered in accumulated hail generally resembles one covered in accumulated snow and any significant accumulation of hail has the same restrictive effects as snow accumulation, albeit over a smaller area, on transport and infrastructure. Accumulated hail can also cause flooding by blocking drains, and hail can be carried in the floodwater, turning into a snow-like slush which is deposited at lower elevations.
  On somewhat rare occasions, a thunderstorm can become stationary or nearly so while prolifically producing hail and significant depths of accumulation do occur; this tends to happen in mountainous areas, such as the July 29, 2010 case of a foot of hail accumulation in Boulder County, Colorado. On June 5, 2015, hail up to four feet deep fell on one city block in Denver, Colorado.  The hailstones, described as between the size of bumble bees and ping pong balls, were accompanied by rain and high winds.  The hail fell in only the one area, leaving the surrounding area untouched.  It fell for one and a half hours between 10 p.m. and 11:30 pm. A meteorologist for the National Weather Service in Boulder said, "It's a very interesting phenomenon.  We saw the storm stall. It produced copious amounts of hail in one small area. It's a meteorological thing."  Tractors used to clear the area filled more than 30 dump-truck loads of hail.
  Research focused on four individual days that accumulated more than 5.9 inches (15 cm) of hail in 30 minutes on the Colorado front range has shown that these events share similar patterns in observed synoptic weather, radar, and lightning characteristics, suggesting the possibility of predicting these events prior to their occurrence. A fundamental problem in continuing research in this area is that, unlike hail diameter, hail depth is not commonly reported. The lack of data leaves researchers and forecasters in the dark when trying to verify operational methods. A cooperative effort between the University of Colorado and the National Weather Service is in progress. The joint project's goal is to enlist the help of the general public to develop a database of hail accumulation depths.
caption: Hand holding hail in a strawberry patch
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