Average annual extreme minimum temperature

The average annual extreme minimum temperature is the formal basis of USDA zones: for each weather station, take the coldest reading of each winter, average them across 30 years (1991–2020 for the current map). Zone numbers are these averages rounded into 10°F bands. It is a winter-survival metric, not a frost-date metric — a zone says how cold your coldest night gets, not when frosts fall.

See also: Plant hardiness zone