Science feed
500 t
A fair-weather cumulus cloud can contain more than a million pounds of liquid water, but this is an illustrative estimate, not a standard weight for every cloud. The familiar calculation concerns suspended droplets, not the much larger mass of all the air in the cloudy volume.
0.01°C
Water can coexist as ice, liquid and vapor near its triple point, about 0.01 °C and 611 pascals. In a vacuum demonstration, evaporative cooling can make liquid water freeze while it is still boiling.
Low contrast
The Great Wall is generally not discernible with the unaided eye from low Earth orbit, and it is not visible from the Moon. Saying it can never be seen from any altitude called space is too absolute: viewing conditions, equipment and identification all matter.