THE WEATHER BEAT · FROM THE ROCK SHOP · OCT 1
Clear moon, frost soon: the old saying in tonight's frost advisories, and the shirt that carries it
Grand Forks wrote the old proverb into its own frost advisory tonight. Here is why a clear moon means frost, and the Rock Shop shirt that says so.
Key points
- Frost advisories from the National Weather Service offices in Grand Forks, Duluth and Pueblo run into Friday morning, Oct. 2. Grand Forks wrote the reason in plain words: "Clear skies and light winds will result in frost formation."
- That is the old saying "clear moon, frost soon." On a clear, still night the ground loses its heat to the open sky and nothing sends it back.
- The Rock Shop's Clear Moon, Frost Soon shirt carries the saying. The shop is how the forecast on this site stays free.
Tonight's frost, in the Weather Service's own words
There is an old line that farmers and gardeners have used for generations: clear moon, frost soon. Tonight the Weather Service put it into an official product. The Grand Forks office covers the Red River Valley of North Dakota and Minnesota. Its frost advisory says temperatures "will drop into the 32 to 36 range by sunrise Friday morning," and then gives the cause: "Clear skies and light winds will result in frost formation."
The saying isn't about the moon. A bright moon only tells you the sky is clear. All day the ground soaks up sunshine, and all night it gives that heat back to the sky. Clouds act like a lid and send some of it back down. Wind stirs the coldest air near the ground in with milder air above it. Take away both and the coldest air settles right where the grass grows.
That's why frost can form when the advisory says 34 or 36. The official reading is taken several feet above the ground. On a clear, calm night the grass and the car roof can run colder than that. The Weather School lesson on [morning frost](/journal/explainers/morning-frost) has a drawing of the two kinds of night side by side. It also explains why the low spot in the yard frosts first: cold air is heavier and drains downhill.
Grand Forks has company tonight. Duluth's advisory covers northern Minnesota and northwestern Wisconsin, from Brainerd to Ashland, with lows as low as 32 to 34 from midnight to 9 a.m. CDT Friday. In Colorado, Pueblo's advisory covers the San Luis Valley around Alamosa, with lows in the lower 30s from 3 a.m. to 8 a.m. MDT. Pueblo says sensitive plants "may be killed if left uncovered." This desk wrote about the [frost advisories in four offices](/journal/beat/northeast-frost-advisories-growing-season-line-2026-09-23/) on Sept. 23. The frost line keeps moving south as October goes on.
| Office | Area | Low in the product | When (Friday, Oct. 2) |
|---|---|---|---|
| NWS Grand Forks ND | Red River Valley and nearby, ND and MN, including Grand Forks and Cass counties | 32 to 36 | midnight to 9 a.m. CDT |
| NWS Duluth MN | Crow Wing, Pine, Aitkin, Cass, Carlton and South St. Louis, MN; Price and Burnett, WI | as low as 34 | midnight to 9 a.m. CDT |
| NWS Duluth MN | Itasca, northern Aitkin, northern Cook and Lake, MN; Ashland, Bayfield, Douglas, Iron, Sawyer and Washburn, WI | as low as 32 | midnight to 9 a.m. CDT |
| NWS Pueblo CO | San Luis Valley below 8,500 feet | lower 30s | 3 a.m. to 8 a.m. MDT |
*Source: NWS Frost Advisories issued Oct. 1, 2026, by Grand Forks (1:37 p.m. CDT), Duluth (12:03 p.m. CDT) and Pueblo (12:01 p.m. MDT), as read at 9:30 p.m. EDT.*
The shirt
The Rock Shop sells a shirt that says it: Clear Moon, Frost Soon. It's for the person who checks the sky before bringing in the tomatoes, and for anyone who likes a saying that turns out to be physics. It suits the shoulder weeks of fall, the first cold mornings, and an October walk under a clear sky. The shop is how this site pays for itself. Buying from it keeps the forecast free, with no ads and no paywall.
This is an evening summary, not emergency information. For live warnings see weather.gov and your local officials.
The rock's live forecast for the places in this story
More from the Journal
Follow the Journal: RSS · get it by email