top of the cloud, these crystals have a tendency to clump together. These clouds do not last long, and they tend to change into cirrus because as the water vapor continues to deposit on the ice crystals, they eventually begin to fall, destroying the upward convection. The cloud then dissipates into cirrus. Cirrocumulus clouds come in four species: stratiformis, lenticularis, castellanus, and floccus. They are iridescent when the constituent supercooled water droplets are all about the same size. Cirrostratus Cirrostratus clouds can appear as a milky sheen in the sky or as a striated sheet. They are sometimes similar to altostratus and are distinguishable from the latter because the Sun or Moon is always clearly visible through transparent cirrostratus, in contrast to altostratus which tends to be opaque or translucent. Cirrostratus come in two species, fibratus and nebulosus. The ice crystals in these clouds vary depending upon the height in the cloud. Towards the bottom, at temperatures of around , the crystals tend to be long, solid, hexagonal columns. Towards the top of the cloud, at temperatures of around , the predominant crystal types are thick, hexagonal plates and short, solid, hexagonal columns. These clouds commonly produce halos, and sometimes the halo is the only indication that such clouds are present. They are formed by warm, moist air being lifted slowly to a very high altitude. When a warm front approaches, cirrostratus clouds become thicker and descend forming altostratus clouds, and rain usually begins 12 to 24 hours later. Other planets Cirrus clouds have been observed on several other planets. In 2008, the Martian Lander Phoenix took a time-lapse photograph of a group of cirrus clouds moving across the Martian sky using lidar. Near the end of its mission, the Phoenix Lander detected more thin clouds close to the north pole of Mars. Over the course of several days, they thickened, lowered, and eventually began snowing. The total precipitation was only a few thousandths of a millimeter. James Whiteway from York University concluded that "precipitation is a component of the [Martian] hydrologic cycle". These clouds formed during the Martian night in two layers, one around above ground and the other at surface level. They lasted through early morning before being burned away by the Sun. The crystals in these clouds were formed at a temperature of , and they were shaped roughly like ellipsoids 0.127 millimeters long and 0.042 millimeters