Hailstreaks

Stanley A. Changnon Jr. Illinois State Water Survey, Urbana

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Abstract

Unique detailed hail and rain data from dense networks operated in central Illinois during 1967 and 1968 have been used to define and study in detail hailstreaks and their associated rainfall. A hailstreak is an area of continuous hail with temporal coherence and is considered an entity of hail generated within a thunderstorm. The average hailstreak represents a fast-moving, short-lived, and relatively small phenomenon. Eighty per cent of all hailstreaks had areas <16 mi2 and hail impact energy values <0.1 ft-lb ft−2, but areas extremes were 0.9–788 mi2 and energy extremes sampled were 0.0001–12.6 ft-lb ft−2. A hail-producing system in a 1600 mi2 area normally produced five hailstreaks with an average separation distance of 15 mi. Cold fronts and unstable air mass conditions led in the production of hall systems and hailstreaks, respectively. Hailstreaks produced by different synoptic weather conditions differed considerably. Point durations of hailstreaks averaged 3 min, 43% occurring in the 1500–1900 CDT period. Eighty-eight per cent of all hailstones had diameters ≤¼ inch and only 1% were ≥1 inch. A 1-ft2 area in a hailstreak normally experienced 24 hailstones. Hailstreaks occurred in all locations and stages of age of their associated rain cells, but their preferred location was along the major axis and in the mature stage. Normally, a rain cell produced only one hailstreak, although 20% produced four or more. However, 52% of the rain cells in hail-producing systems did not produce hail. The average rainfall in a hailstreak was 0.19 inch, with an average point rainfall rate of 0.63 inch hr1. Rainfall produced throughout the network areas during hail-producing systems accounted for 38% of the 1967 network warm season total and 57% of the 1968 total.

Abstract

Unique detailed hail and rain data from dense networks operated in central Illinois during 1967 and 1968 have been used to define and study in detail hailstreaks and their associated rainfall. A hailstreak is an area of continuous hail with temporal coherence and is considered an entity of hail generated within a thunderstorm. The average hailstreak represents a fast-moving, short-lived, and relatively small phenomenon. Eighty per cent of all hailstreaks had areas <16 mi2 and hail impact energy values <0.1 ft-lb ft−2, but areas extremes were 0.9–788 mi2 and energy extremes sampled were 0.0001–12.6 ft-lb ft−2. A hail-producing system in a 1600 mi2 area normally produced five hailstreaks with an average separation distance of 15 mi. Cold fronts and unstable air mass conditions led in the production of hall systems and hailstreaks, respectively. Hailstreaks produced by different synoptic weather conditions differed considerably. Point durations of hailstreaks averaged 3 min, 43% occurring in the 1500–1900 CDT period. Eighty-eight per cent of all hailstones had diameters ≤¼ inch and only 1% were ≥1 inch. A 1-ft2 area in a hailstreak normally experienced 24 hailstones. Hailstreaks occurred in all locations and stages of age of their associated rain cells, but their preferred location was along the major axis and in the mature stage. Normally, a rain cell produced only one hailstreak, although 20% produced four or more. However, 52% of the rain cells in hail-producing systems did not produce hail. The average rainfall in a hailstreak was 0.19 inch, with an average point rainfall rate of 0.63 inch hr1. Rainfall produced throughout the network areas during hail-producing systems accounted for 38% of the 1967 network warm season total and 57% of the 1968 total.

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