Diurnal and seasonal behavior of discrete white clouds on Mars

Two distinct types of discrete white "clouds" on Mars are found to show periodic activity that is highly correlated with the Martian seasons. In addition to the seasonal cycle, Type I clouds exhibit a repetitive diurnal variation, forming in the late morning or early afternoon and continuing to brighten for several hours therafter. They are still bright as they pass over the evening terminator, but are not visible at that time when they should be reappearing over the morning terminator. Type I clouds seem to be associated with warm regions on Mars and are believed to be true clouds of frozen water vapor. Type II "clouds" show no diurnal variation, are associated with cold regions, and may be CO ₂ frost rather than clouds.