Synoptic analysis of extensive and persistent frosts in Iran

In the point of meteorological view، frost occurs when minimum temperature is below 0° centigrade degree. In total، frosts classified in two groups: a) Radiative Frosts and b) Advective frosts. Radiative frost is the most common form of the phenomenon. Nocturnal long wave (heat) radiation loss from the surface leads to the progressive reduction of surface temperature (and that of the adjacent air) between sunset and sunrise (Oke، 1987; Linacre and Geerts، 1997). Advective frost occurs under meteorological conditions that typically include the passage across a region of a large mass of very cold، dry air followed by a well-mixed، windy atmosphere and a temperature that is often subzero، even during the daytime (Scholefield، 1994; Davidson، 1997). The combination of factors such as those related to atmospheric circulation، radiative balance، soil humidity، topographic features (Muller et al.، 2005) and local factors (for example، latitude and close to the big water resources) are an example of the multiple processes taking part to produce a marked drop in temperature. The frosts have obvious effects on the energy، transportation and agriculture that among the three above-mentioned issues، frost temperature have the most effect on agriculture (Vega et al.، 1994). The aim of this study is the synoptic analysis of the extensive and persistent frosts in Iran.