SECTION 4: RISK ASSESSMENT
Average Diameter
Corresponding Household Object
3.00 inch 4.00 inch 4.5 inch
Teacup
Grapefruit
Softball
Source: National Weather Service
The Tornado and Storm Research Organization (TORRO) has further described hail sizes by their typical damage impacts. Table 4.57 describes typical intensity and damage impacts of the various sizes of hail. Table 4.57 – Tornado and Storm Research Organization Hailstorm Intensity Scale
Intensity Category Hard Hail Potentially Damaging Significant
Diameter (mm)
Diameter (inches)
Size Description
Typical Damage Impacts
5-9
0.2-0.4 0.4-0.6
Pea
No damage
10-15
Mothball
Slight general damage to plants, crops
16-20 21-30
0.6-0.8 0.8-1.2
Marble, grape Significant damage to fruit, crops, vegetation
Severe
Walnut
Severe damage to fruit and crops, damage to glass and plastic structures, paint and wood scored Widespread glass damage, vehicle bodywork damage Wholesale destruction of glass, damage to tiled roofs, significant risk of injuries Bodywork of grounded aircraft dented, brick walls pitted
Severe
31-40
1.2-1.6
Pigeon’s egg > squash ball Golf ball > Pullet’s egg Hen’s egg Tennis ball > cricket ball Large orange > softball
Destructive
41-50
1.6-2.0
Destructive
51-60
2.0-2.4
Destructive
61-75
2.4-3.0
Severe roof damage, risk of serious injuries
Destructive
76-90
3.0-3.5
Severe damage to aircraft bodywork
Super Hailstorms Super Hailstorms
91-100
3.6-3.9
Grapefruit
Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open Extensive structural damage. Risk of severe or even fatal injuries to persons caught in the open
>100
4.0+
Melon
Source: Tornado and Storm Research Organization (TORRO), Department of Geography, Oxford Brookes University Notes: In addition to hail diameter, factors including number and density of hailstones, hail fall speed and surface wind speeds affect severity.
The average hailstone size recorded between 1998 and 2017 in Wake County was a little over 1” in diameter; the largest hailstone recorded was 2.75”, recorded on Septemb er 1, 2017. This storm resulted in a recorded $10 million in property damage per NCEI. The largest hailstone ever recorded in the U.S. fell in Vivian, SD on June 23, 2010, with a diameter of 8 inches and a circumference of 18.62 inches. Impact: 1 – Minor Hailstorms frequently accompany thunderstorms, so their locations and spatial extents coincide. Wake County is uniformly exposed to severe thunderstorms; therefore, the entire planning area is equally exposed to hail which may be produced by such storms. However, large-scale hail tends to occur in a more localized area within the storm. Spatial Extent: 2 – Small
Historical Occurrences Thunderstorm Winds
Between January 1, 1998 and December 31, 2017, the NCEI recorded 324 separate incidents of thunderstorm winds, occurring on 172 separate days. These events caused $2,956,000 in recorded
Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019
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