The Morning Glory: An Atmospheric Hydraulic Jump

Abstract

?Morning glory? is a local name given to a frequently occurring, near-dawn squall, accompanied by long, low, narrow cloud bands, on the south coast of the Gulf of Carpentaria. A series of simple computation experiments lends strong support to the suggestion that the phenomenon arises through katabatic drainage from the highlands to the east; a shallow layer of cold air tends to accumulate at the foot of the hills and to form a hydraulic jump, which propagates westward. The computed phase of the jump in regard to time of day and geographical location agrees well with that of the morning glory, while other features show reasonable correspondence. It is concluded on the basis of these experiments that the formation of such hydraulic jumps should be favored by slack pressure gradients, cloudless skies, a shallow inversion, steeper slopes, low latitudes, a low drag coefficient, and topographic funnelling. Since such conditions should not be uncommon in low latitudes, jump-like phenomena associated with orography and nocturnal cooling are to be expected here.