What a Hail Report Can and Can't Tell You
Weather of Record
The roof is the record
We'll start with the most important thing we know about hail data. What we find on site generally serves as the real record of truth. The weather data is a supporting piece.
That may sound strange coming from a site that puts weather data in front of people. But it's the reason the data is worth trusting. A hail report is a generalized product, built largely from modeled radar images and reports people on the ground happened to send in.
When everything lines up, the weather data shows hail of a certain size on a certain date. The marks on the property fit that size, on the surfaces that faced the storm. Often, the weather data shows that an event happened somewhere in the area. What we find on and around the house then gives a far more accurate picture of conditions at that property.
A report showing large hail nearby doesn't mean a roof covering is damaged. It goes both ways: a report showing nothing nearby doesn't mean nothing happened there.
So why bother with the data? Because it's one of the puzzle pieces, and a useful one, as long as you know its limits.
What a hail report actually is
A hail report is a record of what the radar and the weather reporting network saw near an address on a given date. It isn't a photo of your roof or a measurement taken in your yard.
Most hail reports pull from two main sources:
- Radar estimates. Weather radar scans storms all day, every day. Computer models turn those scans into an estimate of how large the hail may have been across a grid of squares on the map.
- Ground reports. These are real sightings sent to the National Weather Service by trained storm spotters, emergency managers, and members of the public. Each one carries a place, a time, and usually a size.
A good report puts those together for one address and one date. It shows how far the nearest estimate or sighting was from the house. It shows how big the hail was thought to be, and when it fell. A solid report also names the source of each number. Data from government sources such as NOAA can be traced and checked. A number with no named source can't.
What radar sees, and what it doesn't
Radar doesn't watch a hailstone hit the ground. It sends out a beam, and the beam bounces off whatever is floating in the storm: rain, ice, hail, all mixed together. The radar measures how strongly that signal comes back, at several heights inside the storm.
A strong return high up in the cold part of the storm tends to mean ice is present. A computer model turns those readings into an estimate of the largest hail likely to reach the ground. That estimate is what shows up on most radar-based hail maps.
So the size on a radar hail map is inferred. It's an educated estimate built from what the storm looked like from miles away. It is not a verified hailstone that fell at any particular property. Actual hailstone size at a given property can vary for several reasons:
- Hail can melt as it falls. Stones can shrink on the way down, especially on a warm day, so what lands may be smaller than the estimate.
- Distance from the radar. The beam travels in a straight line while the earth curves away beneath it. The farther the storm is from the radar, the higher the beam passes. The higher it passes, the less it sees near the ground.
- Wind. Strong winds aloft can carry stones some distance from where the radar saw them aloft.
- Grid size. Each square on the map gets one value. Everything inside that square, whether it's a cornfield or a subdivision, gets the same number.
None of this makes radar wrong. It makes radar what it is: a strong, consistent tool for showing where hail was likely, and its possible size. It's an estimate, not gospel.
The honest limits
Every tool has limits. Here are the ones we watch for.
The report covers a general area, not a specific property
A hail report describes a patch of ground, and your property sits somewhere inside it. Hail rarely falls like an even blanket. It tends to come down in bands and streaks, called swaths. Stone size can change from one end of a swath to the other. Two homes a short walk apart can see different stone sizes, different amounts, or different wind behind them. One house may sit in the heart of a swath while another sits on the edge, or just outside it.
Ground reports are sparse
A ground report only exists if someone was there, noticed the hail, judged the size, and took the time to send it in.
Hail that falls on farmland at night may leave no ground reports at all. The same hail falling on a busy town in the afternoon may leave a dozen. Same hail, very different public record.
That's why the absence of a report is not the absence of the event. A blank map near your address means nobody reported hail there. It doesn't mean hail didn't fall.
Sizes are often eyeballed
Many ground reports compare stones to everyday objects: a penny, a quarter, a golf ball. That's honest and useful, but it's a quick judgment. It's often made through a window, and often of the biggest stone the person saw. Treat it as a good-faith observation, not a lab measurement.
Timing and place are rounded
Radar takes its pictures one sweep at a time. Ground reports log a time and place as best the reporter can. So a report may put a storm a mile or two off, or several minutes early or late. That's normal, and it's one more reason a report narrows things down rather than settling them.
Why it's still worth having
With all those limits, why bother? Here's why:
Memory fades fast. A year later, most people remember "that big one in the spring," not the date, or whether there were two storms that month. A report pins down the date, the time, and the distance, from public records that are hard to dig through on your own.
It shows the recorded history. A roof that has weathered fifteen years has likely seen more than one storm. A good report lists the recorded events near the address over time, not just the one you're asking about. It's a starting point for the roof's service life, limited to what was recorded.
It narrows the search. Say hail of a certain size was estimated within a mile of the house on a certain date. That tells you which sizes and which dates are worth a close look. Storm motion on radar can help show which direction the hail likely came from, and which sides of the house most likely faced it. It doesn't answer the question, but it points you at the right part of the roof and the right day on the calendar.
Used this way, the data frames the question. The site is where we answer it.
Two different questions
A hail report and an on-site inspection answer two different questions.
The weather data tells you what probably happened in the general area, often across several square miles. The roof and collateral surfaces (gutters, vents, siding, and other exposed surfaces around the house) show what the property itself recorded.
Neither one, alone, finishes the job. When the size on the report, the marks on those surfaces, and the date all fit, the puzzle pieces make a clear picture. When one piece doesn't fit, that's worth a closer look. Weather data can be a key piece of the puzzle, but what we find on site generally serves as the real record of truth.