The Problem of the Martian Yellow Clouds

The first extensive meteorological investigations of other planets were undertaken starting just about 15 years ago a t Lowell Observatory. only those comparatively few meteorologists who participated in these early studies can fully appreciate the keen foresight and remarkable scientific intuition that motivated Wexler's active encouragement of this project in days when research funds even for the conventional tasks were so difficult to come by. The following remarks on the problem of the Martian yellow clouds are offered in part as a reminder of the continuing validity of' one of Wexler's favorite theses, that meteorologists can with profit study the atmospheres of other planets in order to improve their understanding of the behavior of the earth's atmosphere. Observers of the planet Mars classify visual and photographic obscurations of that planet's surface features, somewhat arbitrarily, as " yellow clouds, " " white clouds, " and " blue haze. " These names refer in part to actual visual appearance, in that the yellow clouds usually have an ochre tinge; but the allusion is also to the photographic appearance of the phenomena. If Mars is photographed in the light transmitted through a blue filter, surface markings are obscured and an overall hazy appearance results. This " blue haze " is certainly an atmospheric phenomenon, probably occurring a t a considerable elevation ; but it is not yet entirely certain what its exact nature is. The " yellow clouds, " on the other hand, are usually isolated and more or less clearly delimited obscurations of comparatively small areas of the Martian disk and can be detected visually or in photographs made with yellow or red filters. The " white clouds " are also isolated features, appearing strongest in blue or violet light, and are rarely detected in the red or infrared. Because of the very small quantity of water vapor that is present on Mars, the white clouds, which are probably aqueous, are not widespread. They tend to be confined to the sunrise and sunset edges of the planet, particularly during the fall and winter seasons, and to certain presumably elevated regions, such as Hellas. These clouds, or frost deposits, normally exhibit little or no motion and have been compared to low-lying fogs on the earth. Rare as are the white clouds, the yellow clouds are even more uncommon; but, since they often persist for several days and occasionally display movement. they are of …