Frost Protection - UTL Repository

F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S
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FREEZE AND FROST DEFINITIONS
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 (Blanc et al., 1963;
Bettencourt, 1980; Mota, 1981; Cunha, 1982); however, the word is widely used
by the public to describe a meteorological event when crops and other plants
experience freezing injury. Growers often use the terms “frost” and “freeze”
interchangeably, with the vague definition being “an air temperature less than or
equal to 0 °C”. Examples of frost definitions in the literature include:
the occurrence of a temperature less than or equal to 0 °C measured in
a “Stevenson-screen” shelter at a height between 1.25 and 2.0 m
(Hogg, 1950, 1971; Lawrence, 1952);
the occurrence of an air temperature less than 0 °C, without defining the
shelter type and height (Raposo, 1967; Hewett, 1971);
when the surface temperature drops below 0 °C (Cunha, 1952); and the
existence of a low air temperature that causes damage or death to the plants,
without reference to ice formation (Ventskevich, 1958; Vitkevich, 1960).
Snyder, Paw U and Thompson (1987) and Kalma et al. (1992) have defined frost
as falling into two categories: “advective” and “radiative”. Advective frosts are
associated with large-scale incursions of cold air with a well-mixed, windy
atmosphere and a temperature that is often subzero, even during the daytime
(Table 1.1). Radiative frosts are associated with cooling due to energy loss through
radiant exchange during clear, calm nights, and with temperature inversions (i.e.
temperature increases with height). In some cases, a combination of both advective
and radiative conditions will occur. For example, it is not uncommon to have
advective conditions bring a cold air mass into a region, resulting in an advection
frost. This may be followed by several days of clear, calm conditions that are
conducive to radiation frosts. In addition, the authors have observed conditions
that are considered as “micro-scale-advection frosts”. These occur when the region
is exposed to radiation-type frost conditions, but local cold air drainage leads to
rapid drops in temperature on a small scale within the radiation frost area.
TABLE 1.1
Frost event terminology and typical characteristics
FROST TYPE
Radiation
Advection
CHARACTERISTICS
Clear; calm; inversion; temperature greater than 0 °C during day
Windy; no inversion; temperature can be less than 0 °C during day