Freeze

Unique identifier / Notation

MH0042

Synonyms

Frost,

Freeze event,

Killing frost.

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A freeze is an air temperature equal to or less than the freezing point of water (D °C) (adapted from WMO, 1992).

Primary reference(s)

WMO, 1992. International Meteorological Vocabulary. World Meteorological Organization (WMO). Accessed 18 November 2019.

Additional scientific description

Technically, the word ‘frost’ refers to the formation of ice crystals on surfaces, either by freezing of dew or a phase change from vapour to ice; however, the word is widely used by the public to describe a meteorological event when crops and other plants experience freezing injury (FAO, 2005).

Growers often use the terms ‘frost’ and ‘freeze’ interchangeably, with the vague definition being ‘an air temperature less than or equal to 0°C’. A ‘frost’ is the occurrence of an air temperature of 0°C or lower, measured at a height of between 1.25 and 2.0 m above soil level, inside an appropriate weather shelter. Water within plants may or may not freeze during a frost event, depending on several avoidance factors (e.g., supercooling and concentration of ice nucleating bacteria). A ‘freeze’ occurs when extracellular water within the plant freezes (i.e., changes from liquid to ice) (FAO, 2005).

Metrics and numeric limits

Not applicable.

Key relevant UN convention / multilateral treaty

Not identified.

Examples of drivers, outcomes and risk management

The economic losses due to a widespread freeze event can be enormous. Direct crop losses can exceed a billion dollars. According to the United States National Climatic Data Center, five of the billion-dollar weather disasters over the past 30 years have been caused by agricultural freezes (Brotak, 2014).

A number of different methods are available for preventing freeze damage to crops. It is important for growers to be aware of these so that they can evaluate which procedures are feasible and economical for combating freeze damage. The methods are described in terms of active and passive techniques. Active methods are those which are used when the danger of a freeze event is present and include such techniques as adding heat and covering crops. Passive methods are those used well in advance of the freeze event and include scheduling of planting and harvesting within the safe freeze-free period, and appropriate crop and field selection, among others (Canadian Ministry of Agriculture, Food and Rural Affairs, 1985).