A Review of the Variations of Optical Remote ...

The major restricting factor for space-born optical remote sensing at moderate and high latitudes is cloudiness. Significant intraseasonal and interannual differences in cloud amount and dominant cloud types are met. Cloud-free episodes are infrequent. The study of cloud restriction over Estonia was performed using hourly cloud detection data from Tartu-Tõravere meteorological station (58°16’N, 26°28’E, 70 m a.s.l.) in 1958-2011. Major features of sky coverage by almost nontransparent low and medium level clouds are considered. Monthly mean amounts of these clouds in October to February are above 7 tenths. Maximum mean coverage, close to 8.5 tenths, is reached in November. In spring and summer the mean coverage is usually between 5 and 6 tenths. The spaces of sky free from two lower level clouds often are partly covered by semitransparent cirrus clouds. Monthly relative coverages by cirrus clouds were studied statistically for March to September. Relative monthly mean cirrus cloud coverage of free spaces above one tenth was the largest, 0.40, in April and the smallest, about 0.30, in July. Overall mean coverage for March to September was 0.35. The days suitable for optical remote sensing were defined as those with cloudless conditions within ± 2 hours from noon when remote sensing activities usually are performed. The monthly averages and extremes of those days are presented as well as their time evolution in four seasons during 1958-2011. The probability of cloudless conditions is the highest in March when nearly 4 suitable days are met on average. The probability decreases from March to November and since then the conditions start to improve. In November the remote sensing conditions are available in less than half of the studied years and in March in about 90 percent out of all considered years. Monthly maxima of suitable days in all months were above 5.