How cloud screening changes a seasonal greenness comparison

Seasonal vegetation composites summarise retained observations, not an equally observed season. This methods note synthesises MODIS compositing guidance, Landsat quality-band documentation, and NOAA tundra-greenness methods to examine the interaction between cloud screening and ecological comparison. A fictional pixel history shows how an unseen peak can lower both observed maximum and available-observation mean without any underlying vegetation change. The analysis distinguishes seasonal statistics, selected observation dates, quality thresholds, reconstruction, changing spatial footprints, and cross-sensor support. It proposes coverage diagnostics and sensitivity comparisons that remain tied to the chosen question. No imagery is reprocessed and no published Arctic trend is re-estimated. The conclusion is that greenness results become more interpretable when observation timing, masks, and fitted values accompany the statistic rather than disappearing inside a seamless map.