NOAA's freeze-date normals are published at six temperature thresholds: 36, 32, 28, 24, 20 and 16 degrees Fahrenheit. The date is when the overnight minimum temperature at the weather station first or last reaches that threshold or colder. Our products show 36, 32, 28 and 24 degrees.
The thresholds in plain terms
- 32F is the freezing point of water and the number most people mean by "frost". It is the default in our products.
- 36F is a few degrees warmer. Weather services sometimes mention frost possibilities at temperatures in the mid-30s because the air near the ground can be colder than the air at the thermometer's height. A station at 36F is not a freeze, but it is a useful early signal.
- 28F and 24F are colder: the dates are later in fall and earlier in spring. Many gardeners treat them as the dates by which hardier plants may be affected, but how a plant responds depends on the plant, how long the cold lasts and how it has been hardened off. Check the guidance for your own crops.
How people use them
- Choose the threshold that matches the plants you are timing. Tender summer crops are usually timed against 32F; cold-hardy crops can often be sown by a 28F or 24F date.
- The gap between thresholds shows how fast the season turns. A short gap between 32F and 24F dates tells you winter arrives abruptly; a long gap means a slow, gradual change.
- Use the same threshold and the same probability level for the spring and fall dates when you compute a freeze-free season, so the comparison is fair.
A reminder about the data
The threshold dates are historical statistics (1991-2020) from one weather station. Frost can form on a clear, still night even when the station reading stays a few degrees above freezing, and your own yard may be warmer or colder than the station. The dates are not a forecast or a guarantee. Use them to plan, and use the current weather forecast to decide.
Want the dates for your own station? See the Garden Calendars workbook and regional packs, or the free sample PDF. Next guide: How to find the nearest NOAA station for your garden.
Please note. Historical 1991-2020 averages from NOAA; actual frost dates vary year to year; not a forecast or a guarantee; not agricultural advice. Planting windows are generic offsets from historical dates; follow your seed packet, your local extension office and the current weather forecast. This product is AI-assisted, so errors are still possible; please report them to gb1@mail.grokbot.com.
Source: Palecki, M., Durre, I., Applequist, S., Arguez, A., Lawrimore, J. (2021). U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), version 1.0.1. NOAA National Centers for Environmental Information. doi:10.25921/4ek7-fk11. Quietcog is independent and is not affiliated with, endorsed by or sponsored by NOAA or the National Weather Service. Quietcog processed the station files; any processing errors are ours.
Support: gb1@mail.grokbot.com.