Heat and cold — frost days

Definition

The frost days index is defined as the number of days in a year with a daily minimum temperature below 0 °C. Variations of this index can limit the counting of frost day occurrences to particular seasons (e.g. the growing season or the spring months). A closely related index, ‘ice days’, uses a daily maximum instead of minimum temperature.

More information - Index factsheet (ETC/CCA Technical Paper): Frost days

Relevance

This index is widely applicable in several sectors, including agriculture, infrastructure and transport. The index proposed here can easily be computed and interpreted because of its simple definition and the availability of the input data required.

Assessments of the risk of late spring frosts for agricultural crops would require a modification of this index that considers regional climate conditions and growing seasons of the main crops. In Europe, the fruit and vegetable sectors are particularly affected by frost. Despite an overall decrease in frost days, an earlier start to the growing season could increase the risk of frost damage during the growing season.

Past and projected changes

The number of frost days in Europe has decreased since the 1980s, but with considerable year-to-year variability. The fastest absolute decline has been observed in northern Europe. This trend is projected to continue throughout the 21st century, and the number of frost days is expected to decline by about half during the 21st century under the high-emissions scenario (representative concentration pathway (RCP)8.5).

Note that the bias adjustment of temperature projections around freezing point is challenging because of the abrupt albedo difference between snow- or ice-covered surfaces and bare surfaces.

Further information (European Climate Data Explorer): Frost Days, 2011-2099

Annual frost days for the European land area and sub-regions

Source:  ERA5 and bias-adjusted CMIP5 data.