Fig. 3: Seasonal and diurnal evolution of vegetation-cloud cover relationships over two subregions.

April-September 12:00–15:45 UTC cloud cover frequency (CCF) in subregion I (a) and subregion II (b), based on ~58,560 individual HRV images. The subregions differ in the spatial scale of the green areas. The seasonal (e, f) and diurnal (g, h) evolution in CCF (boxes) and Normalized Difference Vegetation Index (NDVI) (lines) between areas with a high NDVI (green line, red boxes) and low NDVI (grey line and boxes) are shown for subregion I (c) and a subregion II (d). The high NDVI regions are defined as having a yearly mean NDVI higher than 0.38. The dashed line shows the mean relative difference in CCF (ΔCCF). Seasonal CCF variations are calculated for 12:00–15:45 UTC, the diurnal variations for April–September. Boxes show the median (line), interquartile range (box) and 1.5 times the interquartile range (whiskers) of the data. Note that one map of CCF is calculated for each month (based on ~12,200 images) (e, f) or hour (based on ~14,640 images) (g, h) first and that the boxes only represent the spatial variation. Stars (*) on the x-axis in (e–h) indicate a significant difference in mean CCF between the high NDVI and low NDVI areas (p < 0.05, using the Mann–Whitney U test). All hours and seasons show a significant difference.