Frost-Free Period Map

MetriClima assigns hourly ERA5-Land 2 m air temperature to longitude-derived local solar calendar days and identifies each day’s minimum. A local day is valid when at least 22 finite hourly values are available, and frost is recorded only when that minimum is strictly below 0 °C.

Northern and equatorial agricultural years run January–December: the last spring frost is the last frost from January 1 through June 30, and the first autumn frost is the first frost from July 1 through December 31. Southern agricultural years run July–June: the corresponding halves are July 1–December 31 and January 1–June 30.

The two boundary frost days are excluded, so the annual duration is first-autumn-frost boundary minus last-spring-frost boundary minus one day. If no spring frost occurs, the start boundary is immediately before the agricultural year; if no autumn frost occurs, the end boundary is immediately after it. A year with neither frost is therefore assigned 365 frost-free days.

An agricultural year is valid with at least 350 valid local days and at least 170 valid days in each half. Frost dates are normalized to a 365-day agricultural calendar before duration is calculated. Invalid years are excluded, and the map is the arithmetic mean of valid annual durations across 2011–2025.

A longer period indicates a longer average interval between modeled freezing days. The displayed value is a multi-year average; annual boundary dates and durations vary, so it does not guarantee a frost-free interval of that length in any particular year.

Compare the duration with the separate spring and autumn frost-date layers to understand whether late spring or early autumn frost more strongly limits the frost-free window.