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Hailstorm Characteristics
Hailstone characteristics:
- Random aspects of hailstones include size, shape, density,
and fall pattern.
- Very large hailstones often have irregular shapes and can have
knobs or spikes.
- Larger hailstones generally are fewer than small hailstones when both are present in a storm.
- Hailstone trajectories and speeds are affected by size of the stones and the wind in the hailstorm.
- Impact energy from a hailstone increases exponentially with
hailstone size (see kinetic energy Table 1).
- Perpendicular impacts impart the most energy; glancing blows transmit less energy.
| Table 1 |
| HAIL SIZE (IN) |
TYPICAL DAMAGE THRESHOLD |
| 1/2 |
FEW, IF ANY, ROOFS DAMAGED. Bushes and trees-
leaves stripped; crops- damaged. Oxidation coatings on paint,
wood, metal- spattered. Thin elastomeric coatings on polyurethane
foam roofs- cracked or broken. Thin aluminum vents, fins on
air conditioning units, lead sleeves on soil stacks, window
screens, aluminum awnings- dented.
|
| 3/4 |
THRESHOLD SIZE FOR DAMAGE TO ROLL ROOFING AND
DETERIORATED ASPHALT COMPOSITION SHINGLES, ESPECIALLY WHERE
UNSUPPORTED. Painted wood surfaces, deteriorated gray-black
slates (especially at corners)- chipped. Most aluminum vents,
flashings, valleys, siding- dented.
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| 1 |
THRESHOLD SIZE FOR DAMAGE TO MOST LIGHTWEIGHT
ASPHALT COMPOSITIONSHINGLES. Thin and/or deteriorated wood shingles,
shakes- occasionally punctured or cracked. Single-pane windows,
thin skylight shells- cracked or broken.
|
| 1-1/4 |
THRESHOLD SIZE FOR DAMAGE TO MOST HEAVY WEIGHT
ASPHALT COMPOSITION SHINGLES, WOOD SHINGLES, AND OLDER MEDIUM
SHAKES. Automobile body metal- dented; galvanized metal vents-
dented. Older plastic skylights- cracked or broken.
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| 1-1/2 |
THRESHOLD SIZE FOR DAMAGE TO CLAY TILE, SLATE,
MEDIUM SHAKES, AND MODIFIED BITUMEN SINGLE-PLY MEMBRANES. Automobile
body metal- extensive denting.
|
| 1-3/4 |
THRESHOLD SIZE FOR DAMAGE TO HEAVY SHAKES AND
CONCRETE TILES . Metal vents- caved in. Bare spots and blisters
on deteriorated built-up roofs- bruised or punctured.
|
| 2 |
THRESHOLD SIZE FOR DAMAGE TO JUMBO SHAKES, WELL-SUPPORTED
UNBALLASTED BUILT-UP ROOFING, AND UNBALLASTED EPDM.
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| 2-1/2 |
THRESHOLD SIZE FOR DAMAGE TO WELL-SUPPORTED BALLASTED
BUILT-UP ROOFING, BALLASTED EPDM, AND METAL PANELS.
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Notes
1. Threshold is the smallest hail size at
which damage can occur.
2. These guidelines are for hard hail ice hailstones that strike
the impacted material in relatively good weathered condition
(unless noted) perpendicularly to it surface.
3. These general guidelines apply in most circumstances, but
there are exceptions. Determining factors include material properties,
deteriorated condition, and underlying support.
4. This information is based on testing and field experience.
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Hailstones fall during some thunderstorms, with hailstones 3/4-inch diameter or larger being one of the National Weather Service (NWS) criteria for a severe thunderstorm. The central portion of the United States is one of the parts of the world where large hail is most frequent (see Figure 1).
Point Frequency for 3/4" diameter or larger hail
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| Figure 1 |
Hailstone Formation Process:
Hailstones are "cycled" from lower to upper regions of a thunderstorm (cumulonimbus) cloud until the storm updrafts are no longer able to suspend the weight of the hailstones (Figure 2). A layer of ice accretes onto the hailstone each time it is carried upward and downward through the thunderstorm cloud. Milky, white-colored rings of less dense ice are formed high in the cloud where super-cooled water droplets freeze and entrap bubbles of air. Transparent rings of ice are formed low in the cloud nearer the freezing level in the atmosphere where greater quantities of liquid water are available (Figure 3).
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| Figure 2 |
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| Figure 3 |
Hailstorm Path:
In typical severe thunderstorms, the swath of hail will contain the largest hailstones in only a portion of the path, with smaller pea- to marble-sized stones found throughout the swath (Figure 4). At a given property location, falling hail will leave marks or dents on many different surfaces or materials. This can provide information on the relative size(s), quantities, and direction of the hailfall (Figure 5).
Typical Hail Swath
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| Figure 4 |
Hailstones Hitting Various Surfaces
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| Figure 5 |
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