Viking‐era diurnal water‐ice clouds

Water‐ice clouds during the Viking era are mapped as a function of season, latitude, longitude, and time of day. In the springtime hemisphere, where data are available, clouds are observed to decrease from morning to midday. They are observed to increase in extent from midday to afternoon, possibly due to increased atmospheric temperatures causing uplifting of dust which then acts as cloud condensation nuclei. During the summer hemisphere clouds are observed to decrease in extent throughout the day, indicating sufficient warming to prevent water reaching the condensation level. The Hadley cell upwelling branch is seen throughout the year via cloud belts in both the northern (aphelion cloud belt) and southern hemispheres in their respective springs and summers. Clouds extending equatorward of 60°N in late northern summer and early northern autumn are seen primarily over the Acidalia Planitia longitudinal sector and may indicate the southernmost extent of the north polar hood or clouds associated with storms. Water‐ice clouds associated with topographic features are seen, including Olympus Mons, the Tharsis volcanoes, and the northern rim of Hellas basin. There is little evidence for interannual variability in the prominent cloud features (e.g., the aphelion cloud belt) since they are present in both Martian years examined, though the detailed structure does change.