Ground-based observations of clouds through both an automatic imager and human observation

摘要

The routine to determine the local cloud cover (LCC) drawn from sky images obtained using a ground-based automatic imager is validated. Almost 3 years of daily data of LCC and cloud cover measurements by ground-based human observation (CCHO) were compared for two Brazilian stations set 6km apart. The linear fitting between daily values showed a co-efficient of determination (r(2)) and a percent scattering of points (SD%) of approximate to 0.77 and approximate to 19.4% respectively. These results are evidence for the significant similarity between the two sets of data. On the other hand, the level of difference found can be explained by the fact that (1) the data refer to independent micro-scale observations of the sky from stations set a few kilometres apart and (2) the data were drawn from interpretations of cloud cover by an automatic imager and a human observer. The monthly average values of LCC and CCHO showed better linear fitting parameters r(2)=0.95 and SD%=6.3%. Average differences of approximate to-10% from LCC to CCHO were found. These average differences may reflect a systematic bias between the two processes of measuring cloud cover.