Vorstperioden

Vorstperioden is a term used to describe the portion of the year during which frost occurs, i.e., when nighttime temperatures fall to or below the freezing point. In practice it is used to refer to both the autumn onset of frost and the spring return of above-freezing temperatures, as well as the overall period when frost is likely. A common way to define it is by the first frost date in autumn and the last frost date in spring, with the length of the period measured as the number of days between these dates. Another common measure is frost days, the number of days with minimum temperatures below 0°C. Some definitions distinguish harder frosts, such as -2°C or lower, which pose greater risks to crops and vegetation.

Geographical variation is large. The frost period tends to be shorter near the equator and at low elevations and longer in high latitudes and mountains. Local factors such as altitude, proximity to water, wind patterns, humidity, and urban heat islands can modify frost risk within a region. Frost is also influenced by broader climate patterns and air masses, which can shift from year to year.

Impacts and uses: frost affects agriculture by damaging buds, flowers, and young crops, and farmers often employ frost protection measures such as coverings, irrigation, or heaters. Frost also influences forestry, horticulture, infrastructure, and energy demand for heating. In agriculture planning, frost calendars and risk assessments help determine sowing dates, crop choice, and protection strategies. Climate change is altering frost patterns, often extending the growing season in some regions while increasing the likelihood of late spring or early autumn frosts in others. Related concepts include the growing season and frost dates.