adverse climatic event which can be assimilated to a natural disastermeans unfavourable weather conditions such as frost, storms and hail, ice, heavy or persistent rain or severe drought which destroy, in the case of agriculture, more than 30 % of the average production calculated on the basis of the preceding three-year or four-year period or of the average production calculated on the basis of the preceding five-year or eight-year period, excluding the highest and the lowest entry;