Portugal - Atlas do Ambiente I.5 - Geada

Summary

Frostis a hydrometeor developed by deposition of ice on objects; it has a crystalline appearance, and grows by the sublimation of water vapour from the surrounding air. The above mantioned hydrometeor is usually called frost, the most correct name being hoarfrost, an ice deposit shouwing the form of scales, needles, feathers or fans. Frost may also occur as dry frost or black frost developed under special conditions. In such a case the plant leaves seem burnt by cold, but no ice crystals are formed on them. The general conditions for the frost formation are the advection of masses of cold, dry and clean air, clear sky, and light wind or even calm, allowing heat loss by radiation. Frosts are frequently classified according to their causes of formation, namely advection and radiation. A useful study on frost occurrences must be based on the analysis of them for each month of the agricultural year. Only a few regions of Continental Portugal are affected by frost in September, but frosts may occur in October from the North to the South of the country although less frequently in Algarve and close to the eastern border, witha few exceptions. The Chart — Duration of frost period in the agricultural year — shows that the region of Continental Portugal where frost persists for a longer number of months is Montalegre, lasting nine or more months per year. Comparing this Chart with the hypsometric one (Chart L.15 of this Atlas), one is able to see the influence of mountainous regions, d