Frost dates for Ann Arbor, Michigan
Michigan · first & last freeze, from NOAA 1991–2020 climate normals
In a typical year, the last spring freeze around Ann Arbor comes near May 5 and the first fall freeze near October 10. One year in ten, a freeze still arrives after May 20 in spring — and one year in ten, the first fall freeze has already landed by September 28. NCEI's own growing-season statistic (days between freezes, percentiled per year) puts the median at 157 days.
Freeze-date probabilities, three thresholds
| Threshold | Last spring — 9 in 10 years past by | Last spring — typical (median) | Last spring — 1 in 10 years later than | First fall — 1 in 10 years by | First fall — typical (median) | First fall — 9 in 10 years by |
|---|---|---|---|---|---|---|
| 36°F — near-freezing (frost can occur) | May 1 | May 15 | May 29 | September 18 | October 1 | October 12 |
| 32°F — freezing | April 21 | May 5 | May 20 | September 28 | October 10 | October 24 |
| 28°F — hard freeze | April 10 | April 24 | May 11 | October 7 | October 23 | November 6 |
Measured at Ann Arbor Se, MI (USC00200228), 3.0 mi from the city's census reference point, elev. 832 ft. NOAA/NCEI 1991–2020 normals. Read each date column as odds over the 30-year record — e.g. "9 in 10 years past by" means only one spring in ten saw a later freeze than that date.
Frost-free windows (computed)
| Window (32°F ends) | Opens | Closes | Length |
|---|---|---|---|
| Cautious (1-in-10 freeze risk at each end) | May 20 | September 28 | 131 days |
| Typical (median freeze dates) | May 5 | October 10 | 158 days |
| Gambler's (9-in-10 freeze risk at each end) | April 21 | October 24 | 186 days |
Pure arithmetic on the freeze-date percentiles above — the "cautious" window opens on the date only 1 year in 10 still sees a later spring freeze, and closes on the date by which only 1 year in 10 has already frozen.
About this city's numbers
- Census entity: Ann Arbor city, Michigan — official 2024 population estimate 122,925.
- Weather station: Ann Arbor Se, MI (USC00200228), 3.0 mi from the city's census-gazetteer reference point at elevation 832 ft. Frost dates are that station's history — neighborhoods vary.
- Data pulled 2026-09-05; normals period 1991-2020 (replaced by NOAA roughly once a decade).
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