Territory Intelligence · Data note
One inch, and what the hail record actually holds.
Illinois carries 12,000 hail reports on NOAA's severe-weather archive, the first dated 3 March 1955 and the most recent 27 July 2026. Of those, 60.5% logged stones of one inch or larger. That one share drives how most exterior contractors read a storm season, so it is worth knowing exactly what it counts, what it never counted, and why it has moved twice across seventy-two years of record for reasons that have nothing to do with the weather.
12,000Illinois hail reports on fileNOAA SPC · 1955–2026
60.5%Reports logging hail 1.00" or largerNOAA SPC
653Cook County hail reportsNOAA SPC
244Separate Cook County hail daysNOAA SPC
01 · The threshold
One inch is a rule, not a measurement.
Ask five roofers where the one-inch line comes from and you will get five answers, most of them involving shingle impact testing. We are not going to assert that lab history here, because we cannot source it cleanly. What we can source is this: the National Weather Service's published criterion for a severe thunderstorm is hail at least one inch in diameter, quarter size. The forecasting rule and the sales rule are the same number.
That convergence is convenient and slightly circular. NWS calls a storm severe at one inch, warns on it, and the public reports that come back cluster around the events it warned about. So the archive is not a neutral census of Illinois hail. It is a record of what the warning system asked people to notice.
Within it, severity falls off fast. 7,262 of the 12,000 Illinois reports reached 1.00" or larger. 3,046 reached 1.50", which is a quarter of the file. Only 652 ever reached two inches, and 101 reached three.
The read for contractors
Treat one inch as a filter on where to spend attention, never as a statement about a building. The file says hail was reported near a point on a date. It does not know your prospect's roof exists, and your mail should not pretend otherwise.
Illinois reports by size classNOAA SPC · n=12,000
02 · What a size really is
The archive knows twenty-five sizes, and mostly uses four.
Across 12,000 Illinois reports there are exactly 25 distinct values in the hail-size field. Four of them account for 10,285 reports, 85.7% of the file: 1.00" (3,689), 0.75" (3,105), 1.75" (1,866) and 0.88" (1,625).
Those four are not calipers. They are the reference objects on the NWS spotter chart: quarter, penny, golf ball, nickel. The 0.88 is a seven-eighths nickel rounded by whoever keyed it. A hail report is a person under an awning naming a coin, and the archive faithfully preserves the coin.
The floor matters as much as the vocabulary. Nothing smaller than 0.75" appears anywhere in seventy-two years of Illinois record. Pea and mothball hail fall on this state constantly. The archive was simply never built to hold them, which is the first reason a raw report count means less than it looks like it means.
The read for contractors
A 1.00" report and a 0.88" report may describe identical stones seen by two people with different pocket change. Do not build a campaign boundary on a tenth of an inch. Build it on whether a township has a repeated storm history at all.
Most-recorded hail sizes, IllinoisNOAA SPC · reports
03 · The reporting artifact
The one-inch share has moved twice. Neither move was weather.
Split the Illinois file into eras and the headline share stops behaving. In 1955 through 1989, 69.9% of reports were an inch or bigger. Through the 1990s and 2000s that fell to 49.9%. It climbed back to 59.5% in the 2010s, and in 2020 through 2026 it stands at 86.0%.
Nothing in that curve is a hail trend. The early archive only holds storms big enough that somebody thought to phone them in, so it skews large. Then the spotter network, chasers, and eventually phones filled the middle with small-hail reports, and the share fell. Lately the collection has tightened back toward the severe criterion, and the small stuff has thinned out again.
Volume tells the same story louder. The 1950s left 59 Illinois reports on file. The 2000s left 4,098. Illinois did not get seventy times more hail; it got telephones, spotter training, and a lot more people looking up. Report density follows observers, and observers follow population.
The read for contractors
Never compare a raw report count from one decade against another, and be suspicious of any vendor that shows you a rising hail line as evidence of rising demand. Compare within a window, or compare storm days rather than reports.
Share of reports ≥ 1.00", by eraNOAA SPC · Illinois
04 · Cook County
653 reports, 244 days, 69 of 72 years.
Cook County holds 653 Illinois hail reports, more than any other county in the file. Read that rank carefully. A county total in this archive counts observations, not storms, so it reflects how many people were outside and willing to call it in as much as how often hail fell. We have not adjusted these counts for population, and nobody should treat the ranking as a hail-frequency league table.
444 of Cook's reports, 68.0%, logged an inch or larger. 133 reached 1.50". Twenty-four reached two inches. The largest stone ever recorded in the county's file is 2.75", baseball size.
Collapse those reports into distinct dates and the picture gets usable: 244 separate hail days across 72 calendar years, roughly 3.4 a year. Only three years produced no Cook report at all (1973, 1979 and 1981), and every year since 1991 has produced at least one report of an inch or more, a run of 36 consecutive years.
Recent years look dramatic until you check the denominator. From 2022 through July 2026 Cook logged 111 reports and 110 of them were an inch or larger, a 99.1% severe share that is a recording convention rather than a change in storms. Storm days are the honest series, and they are flat to down: 58 Cook hail days in 1997–2006, 66 in 2007–2016, 46 in 2017–2026.
The read for contractors
Cook is a repeat-hail county, not a rare-event county. Three-plus hail days a year, sustained for seven decades, is the case for treating storm history as a standing territory input instead of a fire drill you run twice a decade.
Cook County hail, by decadeNOAA SPC · county FIPS 031
| Decade | Reports | Hail days | ≥ 1.00" |
|---|---|---|---|
| 1955–59 | 8 | 8 | 3 |
| 1960s | 22 | 20 | 17 |
| 1970s | 20 | 16 | 15 |
| 1980s | 19 | 15 | 13 |
| 1990s | 41 | 24 | 29 |
| 2000s | 209 | 71 | 102 |
| 2010s | 206 | 62 | 139 |
| 2020s* | 128 | 28 | 126 |
*2020s runs through 27 July 2026 and is a partial decade.
05 · Seasonality
April makes the headlines. June makes the days.
By raw report count, April is Cook County's biggest hail month: 130 reports against May's 112 and June's 127. Count separate hail days instead and the order flips. June carries 64 of Cook's 244 hail days, 26.2% of them, well clear of May's 41 and April's 34.
The gap is arithmetic. April in Cook averages 3.8 reports per hail day and June averages 2.0. April's total is carried by a few enormous multi-report days; the single largest in the county's record is 4 April 2023, with 27 reports and a maximum stone of 1.75". June's total comes from many separate, smaller events. Statewide the same inversion holds: 2,739 Illinois reports fall in April, but June owns the most distinct hail days at 392.
Either way the season is short and front-loaded. April through June holds 56.5% of Cook's reports and 63.1% of Illinois's. From September to February the record thins to almost nothing, and December has produced exactly one Cook hail day in seventy-two years of record.
This is the one timing signal a permit file cannot give you. Permits record when work was pulled, which is months of adjusting and scheduling downstream of the sky. The hail archive records the sky.
The read for contractors
Plan crews and media against the days curve, not the reports curve. A month with many small hail days spreads inbound demand across weeks; a month carried by one huge day concentrates it into a few days of phones, and your capacity plan should know which kind of month it is looking at.
Cook County: separate hail days by monthNOAA SPC · 1955–2026 · n=244 days
06 · Doctrine
Say “hail reported.” Stop the sentence there.
Storm chasing is a race to a footprint you heard about on the scanner. Storm-cohort work is different: you take a township that the archive shows has taken repeated one-inch-plus hail across decades, you cross it with aging roof stock and with the homes that already pulled a permit, and you work it over seasons instead of over a weekend. The record supports the second discipline. It cannot support the first, and it certainly cannot support what most storm copy says.
Here is the boundary in plain terms. A NOAA hail report is a person's observation, logged at a latitude and longitude that describes roughly where the observer was, on a date. It is not a gauge reading. It is not a survey. It does not know the address next door, and it has nothing at all to say about the condition of any individual building.
So the safe sentence looks like this: NOAA logged 444 hail reports of one inch or larger in Cook County since 1955, across 244 separate hail days. Every clause there is checkable against a public file, and a regulator reading your mail piece can check it too. The unsafe sentence is any version that quietly converts a report into a condition claim about somebody's house. That is not a stylistic preference. Damage assertions you cannot substantiate are where contractor licensing complaints and consumer-protection actions actually come from, and the phrasing discipline is the cheapest license insurance in the trade.
The same rule governs how we build. Territory reads stay aggregate, at township and neighborhood level. Storm history is an input to ranking, never an output that names a home.
The read for contractors
Write your storm copy so that every factual clause traces to a public file you could hand over. If a sentence needs an inspection to be true, it belongs in the conversation on the roof, not in the mailbox.
Cook County reports by size classNOAA SPC · n=653
Cook, last five archive years2022–2026 · through 27 Jul 2026
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Method & sources
Every figure on this page is computed from the TerritoryX copy of the NOAA Storm Prediction Center severe-weather hail archive, filtered to state = 'IL', which is the whole table: 12,000 rows spanning event dates 1955-03-03 through 2026-07-27. Cook County figures use county_fips = 31, the county portion of FIPS 17031, stored as an integer in this table. The archive combines the SPC's 1955–2025 warehouse file with SPC daily storm reports for 2026, so the current year is preliminary and will be revised. 109 rows carry no county code and are therefore absent from every county-level figure while remaining in the Illinois totals.
Percentages of report counts use all 12,000 rows as the denominator; the hail-size field has no null or empty values in this table, so no rows are excluded from the size distributions. “Hail days” counts distinct event dates rather than reports, because a single storm generates many reports and the report count reflects how many people observed it.
The one-inch severe-thunderstorm criterion and the hail-size reference objects (0.75" penny, 0.88" nickel, 1.00" quarter, 1.75" golf ball) come from the National Weather Service's published definitions, an external source, and are labelled as such. We make no claim about shingle impact-testing standards, which we did not verify. Everything else here is our own computation.
The archive is a record of human observation, not a gauge network. Report density tracks population, spotter training, and reporting practice at least as much as it tracks storms, which is why the era comparisons above are presented as reporting artifacts. All figures are aggregate. No individual property is identified anywhere on this page, and nothing here is a claim about the condition of any specific building.
NOAA SPC · hail archive 1955–2026
NWS · severe criteria & size chart (external)
TerritoryX · aggregation & analysis
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