Viking Orbiter images obtained during the 1978 northern summer on Mars show unusual spiral cloud patterns at high northern latitudes. Well-developed spirals occurred only during early summer at a time when the sublimation of the annual CO2 frost cap in the north had cased. The systems ranged in size from 200 to 500 km and all spirals were wound in a counterclockwise sense. An explanation is proposed, based on the radiatively driven instability described by Gierasch, Ingersoll, and Williams. The model predicts disturbances with the correct scale and lifetime. However, the successful development of the spirals depends on the existence of weak mean winds, suggesting that these instabilities can only develop at special null points in the general circulation. We argue that such special circumstances are most likely to occur during early northern summer and are less likely to obtain at a comparable season in the southern hemisphere.