Hail: Sizing It Up!

hail size and the direction from which it came can be obtained. Examining a mechanical unit’s aluminum coils also assists in determining the same information. Hail: Sizing It Up! By Vickie Crenshaw and Jim D. Koontz Photo 1, Aluminum Coil Determining the size of hail impacting a roof following a storm event can be a challenging task. Correlating the size of hailstones to “splash marks” or indentations on metal surfaces, or both, is the subject of this article. To study this phenomenon, various common roofing metals were impacted with ice spheres cast in the laboratory. Introduction Hail sizes are often compared to everyday known objects, Table 1. Hail Size Description Hail Size 0.25 inch Pea Size 1.75 inch Golf Ball Size 0.50 inch Mothball Size 2.00 inch Hen Egg Size 0.75 inch (Severe Criteria) Dime/Penny Size 2.50 inch Tennis Ball Size 0.88 inch Nickel Size 2.75 inch Baseball Size 1.00 inch Quarter Size 3.00 inch Teacup Size 1.25 inch Half Dollar Size 4.00 inch Grapefruit Size 1.50 inch Walnut or Ping Pong Ball Size 4.50 inch Softball Size Description Photo 2, Hail Damage, Roof Exhauster Hail hitting the top and sides of metal surfaces leave evidence of impact in the form of marks as surface oxides and airborne deposited debris are disturbed. Hail striking these surfaces dislodges the oxides and particulates, leaving visible outlines or fingerprints (Photo 3) known as “splash marks.” These “splash marks” supply additional information about hail size and direction. Table 1. – Hail Size Chart, NWS Tampa Bay, Florida1 When examining roofs after hail events, debates often occur about hail stone sizes. Nationally recognized standards for determining the size of hail after a weather event are nonexistent. Without a video camera on the roof or an official meteorological station near the roof, obtaining accurate hail information can be difficult. Information about hail events is obtained from web sites such as HailTrax , Compu-Weather and the National Climatic Data Center (NCDC). They report hail events compiled from meteorological data, spotters in the field, law enforcement personnel, and other sources. The report often lists the longitude and latitude of the location and the size of the hailstones observed. Other documenting sources include newspaper articles that report the hail event and damage. Police reports provide similar information. This subjective information is useful but not always accurate. Splash Marks and Dents Hail damage in the form of dents is often found on mechanical units, aluminum coils (Photo 1), exhauster covers (Photo 2), aluminum roof vents, edge metal, and coping. From these metal surfaces, information indicating Photo 3, Splash Marks Hail Velocities and Impact Energy Various organizations or testing agencies each have test standards or methods for determining impact or hail resistance of materials. Some include the American Society of Testing and Materials (ASTM), Factory Mutual Research Corporation (FMRC), Underwriters Laboratories (UL) and the National Institute of Standards and Technology (NIST), formerly National Bureau of Standards (NBS). Industry test methods developed for impacting roof targets with ice spheres include the NBS Building Science Series 23, "Hail Resistance of Roofing Products" and FMRC Class Number 4473, “Specification Test Protocol for Impact Resistance Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls.” Previous researchers using ice spheres to evaluate hail damage include Rigby, 1952 , Laurie, 1960 , Greenfeld, 1969, Hairston, 1972 , Koontz, 1988 , 1991 , Morrison, 1999 and Crenshaw/Koontz, 2001 . Currently the NBS Series No. 23, FMRC 4473, FMRC 4470 , ASTM D3746 and UL2218 use kinetic energy in the calculations. The NBS Series 23, FM 4470 and FM 4473 address hail resistance of roofing materials. The UL 2218 and ASTM methods address impact resistance. The NBS Series No. 23 and FMRC 4473 employ laboratory cast ice spheres to substitute for hailstones while the other impact test methods use steel darts or balls. Recent industry debate about employing momentum versus kinetic energy in calculations is noted but not addressed in this research. Mathey concluded kinetic energy at impact is a suitable criterion since work required to stop a moving object is equal to its kinetic energy. An earlier hail researcher, J.A.P. Laurie, derived hail sizes and correlating kinetic (impact) energies of hail in the 1960’s. Laurie graphed the relationship between terminal velocity, hail diameter, and the approximate kinetic (impact) energy, Table 2. Laurie developed this information from data collected by Bilham and Relf in prior research. Diameter inches 1-1/4 1-1/2 1-3/4 2-1/2 2-3/4 cm (2.5) (3.2) (3.8) (4.5) (5.1) (6.4) (7.0) (7.6) Terminal Velocity ft/s mi/hr (m/sec) (22.3) (25.0) (27.4) (29.6) (32.0) (35.7) (37.8) (39.6) to a preset value, and a ballistics timer measured the spheres’velocities. Constructing the simulated hail in silicone molds in two stages permits the expansion of the ice without cracking. Weighing the mass of water into each mold provided consistency of the spheres’ masses and diameters. Ice spheres were formed at 10?F. The known mass and velocity of the sphere allowed for an accurate determination of the kinetic energy. The spheres were propelled at a variety of supported and unsupported metal surfaces.