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Two very different kinds of evidence get grouped together under the general label of weather verification, and it helps to understand what separates them before relying on either. Radar data is a remote, algorithmic estimate of what is happening in the atmosphere, generated continuously across wide areas. Ground reports are individual human observations, submitted after the fact by spotters, emergency managers, or members of the public, covering a specific point at a specific time.
Radar measures conditions aloft, often thousands of feet above the surface, and infers what is happening at ground level through reflectivity patterns and polarization signatures. That inference has known weaknesses. Hail can melt partially on its way down depending on the depth of warm air it passes through, wind can carry stones laterally from where the radar beam sampled them, and the algorithm’s output is a statistical estimate rather than a physical count of stones. Wind speed estimates from radar carry comparable uncertainty, since radar measures motion within the storm rather than a gust at a rooftop. The farther a property sits from the radar tower generating the scan, the higher above the ground the beam is sampling by the time it reaches that location, which can further widen the gap between what the radar recorded and what actually reached the roof.
Ground reports solve some of these problems by recording an actual observation at a specific location, but they introduce different limitations. Reporting density varies enormously, concentrated near population centers, along major roads, and around trained spotter networks, while rural stretches of Minnesota and similar states can go unreported even during a significant event. A report also depends on an individual’s estimate of hail size or wind speed, which introduces the ordinary variability of human perception, and reports are sometimes filed with imprecise timing or location detail.
Volunteer observation networks such as the Community Collaborative Rain, Hail, and Snow Network, generally known by the shorthand CoCoRaHS, add a more standardized layer of ground reporting in many areas, since participants follow consistent measurement protocols and submit reports through a structured system rather than an informal call or online form. Even with that structure, coverage still depends on where volunteers happen to live, which means a network like this improves reliability within its coverage area without eliminating the broader geographic gaps present in ground reporting generally.
Because each data type compensates for some of the other’s weaknesses, a sound weather analysis typically uses both together rather than relying on either exclusively. Radar establishes the broader pattern and timing of a storm’s passage, while ground reports, where available, anchor that pattern to confirmed conditions at specific points. When the two sources agree closely, the resulting picture of a storm’s timing, path, and severity carries more weight than either source would on its own; when they diverge, that divergence is itself useful information, prompting a closer look at what might explain the difference before drawing a firm conclusion. An appraiser or analyst reviewing weather evidence in a claim is looking at how well these two sources corroborate one another for the property and date at issue, which informs the physical damage assessment without resolving any separate coverage question tied to that assessment.
This article is general education about how the appraisal process commonly works. It is not legal advice, and specific procedures can vary by state and policy.
Russ Lis is a working property insurance appraiser and umpire based in Minnesota, serving clients nationwide. Contact Appraisal Resolution.