Hailstorm Data Sources and Hail Characteristics
With the advent of Spring, we are entering the climatological “hail season” for the central and southern United States, and you should expect to start getting calls to make roof inspections related to hailstorms and potential hail-caused damage. We want to assist you in preparation by discussing the various sources for hailstorm data, and how to compare that with the information you can obtain during your site inspections. Some hail data is from free governmental sources, and some is from private sources including maps or lists from third-party meteorological consulting firms that typically are fee-based.
Governmental Data Sources
The climatological data from the federal government is under the umbrella of NOAA (National Oceanic and Atmospheric Administration). Severe weather reports are obtained by local offices of the National Weather Service (NWS) through eyewitness reports by individuals, trained storm spotters, or emergency management officials; media and social media reports; official NOAA recording stations, and occasionally the reports of observation teams dispatched by the NWS (NWS teams are mainly used to document tornado tracks). The severe weather reports can be found at three types of NOAA websites:
NWS offices: http://www.srh.noaa.gov/
Local Storm Reports issued by individual NWS offices. This data is considered preliminary and generally only remains available for a limited amount of time, often less than one week.
Storm Prediction Center (SPC): http://www.spc.noaa.gov/
Nationwide reports added on a near real-time basis and listed as “daily” reports that start/stop at 6:00 a.m. Central Time. The archive of daily reports is retained permanently, but this data (regardless of age) is considered preliminary is it has not undergone quality control and may have errors and omission.
National Climatic Data Center (NCDC): http://www.ncdc.noaa.gov/stormevents
Nationwide reports searchable over a specified date range after selecting the state and county of interest. The NCDC database has undergone quality control and is considered “final” data. The data is usually several months behind the current date and the site will not have information on the most recent storms. The “Event Details” listed may provide additional information about the storm or if any property damage was reported.
The local NWS offices forward their “Local Storm Reports” to the SPC for posting during the storm events, and then the NWS offices prepare monthly severe weather summaries to the NCDC for inclusion into the Storm Event Database. The current NWS “severe” criteria for entry in the database for hailstones is 1.0 inch diameter or larger (although sometimes hail sizes as small as 0.75 inch diameter are listed). The hail reports are generally listed as “point locations”, although often the geo-codes (latitude and longitude) provided with the reports are not exact because a database with listing of the latitude/longitude at the center of cities is used to create the geo-codes.
Non-Governmental Data Sources
There are numerous private or educational institution websites that will provide links to hailstorm or severe weather reports, although for the most part, these sites will be routing you to the above-listed NOAA information or re-packaging it in some way. One organization that provides a different non-governmental source of eyewitness hail reports is COCORAHS (Community Collaborative Rain, Hail, and & Snow Network), http://www.cocorahs.org/. The volunteer spotters of this nationwide network can report hail of any size, and searches can be made by state or county for user-defined date ranges.
The final data sources for hail information we will discuss are third-party maps or lists based on radar imagery. There are numerous firms that offer maps of individual storms or provide “site-specific” estimates of maximum hail size of a single storm or a date range. It is important to note that actual hailstone sizes can be larger or smaller than those listed, and even occurrence of hail at the site is not guaranteed. It should be understood that several factors can influence the accuracy of the estimated hail sizes and the proximity of the estimated hail to the location of interest when analyzing radar signatures. All service providers of this kind of report use the data obtained from the same radar signatures; however, the output from different providers of the same storm can be quite different. The service providers attempt to process the data through proprietary algorithms to determine whether hail fell and the size of that hail. Meteorological researchers and NOAA personnel actively study this methodology, and the NWS uses similar algorithms for predicting the occurrence of severe hail (at a county level), although published studies have revealed widely varying success in determining maximum hailstone size, and no peer-reviewed study has indicated accuracy of determining hailstone size at a specific address. Simply put, the radar data is not fine enough to be directly measuring individual hailstone sizes. As such, these reports are not a substitute for site-specific observations.
Hail Data From Site Inspection
The data that can be observed during a site inspection regarding hailstone quantity, direction of hail fall, hailstone size range, and estimated maximum hailstone size at a particular location is greater and more complete than can be obtained from the data sources listed above. Also, it is prudent to ask the building owner or site contact if there are any photographs or video of the actual hailstones or hailstorm event. Depending on the quality of the images and video, this can provide useful information on the hail sizes, hailstorm duration, and hail fall direction.
At the inspection site, various surfaces and materials including utility boxes, air-conditioning units, fences, windows, siding, gutters/downspouts, fascia, plastic and metal vents and roof appurtenances, and the roofing materials can be inspected for spatter marks, dents, and other forms of damage related to hail impact. Painted surfaces and exposed materials often form a layer of oxidation or become covered with dirt, grime, algae, or other organic materials that can be cleaned away when impacted by hail, resulting in spatter marks. Spatter marks are temporary markings left by removal of surface oxides, grime, organic growths, etc. caused by hail impacts that can provide an approximate hailstone size and direction of hail fall.
Since spatter marks tend to fade from oxidized surfaces after a year or two, they provide a helpful temporary record of recent hailstorms. Although the visibility and longevity of spatter marks can vary based on the material and amount of oxides and organic materials removed, harder hailstones can remove a greater amount of surface material and tend to produce a “crisper” edge to the spatter marks with greater contrast, while spatter marks from softer hailstones show a greater scattering of material from the hailstone breaking apart upon impact. Harder (or larger) hailstones also would produce deeper dents in metal than smaller and softer hailstones. Dents produced in light-gauge metals when impacted by hail leave a permanent record of hailstones that have struck exposed surfaces over the years. Spatter marks and dents can be evaluated to help determine the approximate size of hail at a location and provide insight regarding the time passed since passage of a hailstorm. Determining the age of a dent in metal can sometimes be challenging, but dents that contain spatter marks or have a shiny surface from removal of grime and oxides would be indications of recent denting. Accumulation of hardened grime and organic growths in a dent generally takes a considerable length of time. Examining vertical surfaces such as siding, sides of mechanical units, and fences for hail-caused dents and spatter marks can provide information about the direction of hail fall.
Hail Impact Forces and Threshold Sizes
Impacts from larger hailstones result in higher forces (impact energies) than impacts by smaller hailstones because larger hail is more massive and falls at higher velocities than does smaller hail. Also, harder (frozen solid) hailstones transmit their energy over smaller areas than do softer hailstones of the same size, because softer hailstones tend to break apart upon impact. Consequently, impact forces from harder hailstones result in higher material stresses than impacts from softer hail. For these reasons, harder and larger hailstones are more damaging to roofing materials than smaller and softer hail. Of these two parameters, the size of hail has much more influence on the maximum possible force at impact and accordingly, the ability to damage roof coverings. Hail damage thresholds listed in the HCI courses, Haag publications, and published research papers by Haag personnel are for hailstones that are at the upper range of density and hardness striking perpendicularly to roofing material of average quality and thickness. Therefore, not all hailstones that are of threshold size or larger will result in damage due to variations in density, hardness, angle of impact, and material quality or thickness.
Note that the hail data sources as discussed above do not provide any specific information about quantity, hardness, direction, or size range of the hailstones for a particular location. At most, the direction of hailstorm movement can inferred by mapping the hail reports with the time or by looking at the shape of the hail swaths (with radar-based maps). However, these characteristics of hail fall can be determined and documented during a thorough inspection of your inspection site, and will provide support for your roof inspection findings whether you find hail-caused damage to the roof covering or not.
Richard F. Herzog P.E., Meteorologist, RRC, and Haag Principal Engineer (04/2015)
Richard Herzog’s primary areas of consulting are Roofing Systems, Building Envelope Systems, Evaluation of Wind Damage to Structures, Construction Defect Evaluations, Meteorological Investigations, Development of Hail Analysis Software, and Alternative Dispute Resolution. He serves as a primary advisor in the creation of many Haag Education seminars and products. See his profile here.