SEMIANNUAL FLUCTUATION AND EFFICIENCY FACTORS IN SUN-WEATHER RELATIONS

The changing nature of the Sun-weather relations over a 119-year period has been studied by using the geomagnetic aa index. The analysis yields consistent results on several time scales. The most important fluctuation found is a semiannual variation which can be related to the well-known variation of the geomagnetic activity. A threshold value of about aa is-approximately-equal-to 19-20 nT is necessary to detect any solar influence on the weather. The role of the aa index (i.e., corpuscular influences) seems to be determinant in certain cases.