Sable Island:  Morning Glory Cloud

(June 2003)

On June 13, 2003, in late afternoon, a very long and narrow white cloud - a Morning Glory - appeared in the otherwise blue sky over the ocean east of Sable Island. At about 4:00 pm, the cloud, oriented roughly northwest-southeast, ominous and rapidly advancing, was located near the east tip of the island (44° 03' N; 59° 38' W).  The cloud traveled along the island at a speed of roughly 30 km/hr.

The approximate position and orientation of the Morning Glory is shown on the map.  The numbers correspond to the photos below. 

The three composite photos show the Morning Glory cloud as it traveled southwestward over Sable Island. The photos were taken, looking toward the east tip, from a high dune on the eastern end of the island (see blue arrow). The greater depth of the middle section of the cloud, and the illusion of curvature, are artifacts of multiple photos taken from a single position.

1

2

3

This type of cloud has been seen over Sable Island on at least two other occasions. In the summer of 1976 station staff observed a series of seven Morning Glories, rolling in from the north, and oriented west-east (i.e. lines of cloud roughly parallel to the long axis of the island). The first cloud passed over the station at about 6:00 pm, and the last passed just after 8:15 pm. Also, a single Morning Glory was seen by station staff in the early evening of July 8, 2000.

The Morning Glory cloud - considered one of the world's most exotic meteorological phenomena - is best known from the Gulf of Carpentaria, northern Australia, where it is observed most often during spring, usually near dawn.  The name "Morning Glory" reflects the often spectacular appearance of the long horizontal clouds at sunrise. (The Morning Glories of Sable Island might more accurately be called "Early Evening Glories".)

A Morning Glory is a cloud formed by a solitary wave in the lower atmosphere.  It is thought that solitary waves develop in the presence of a temperature inversion and are generated by disturbances such as thunderstorm activity or the collision of opposing ocean breeze fronts.  A temperature inversion is a situation in which there is a cold layer of air close to the ground, with a warmer layer on top, creating a "lid" over the cooler surface air.  This layer of surface air is highly stable and enables the solitary wave to propagate without being damped, and to travel long distances.  Thus the layer acts as a "wave guide".

Solitary waves occur over both water and land - for example they commonly form in the arid interior of the Australian continent.  Waves in dry areas normally occur without a cloud - they are low-level clear-air disturbances (a horizontal vortex) accompanied by a sudden wind squall at the surface and a temporary increase in surface pressure.  However, when sufficient moisture is available at the surface - as it would be over the ocean - a solitary wave creates a long, continuously forming roll cloud.  The cloud develops in the up-draft along the leading edge of the wave as moist air from near the surface is lifted to the condensation level.  Then, as the air descends in the downdraft along the trailing edge of the wave, the cloud evaporates.  The visible upward motion of the cloud elements at the front and the downward motion of cloud at the back create an impression of a cloud rolling backwards as it advances.

Morning Glories appear as one or more roll cloud formations extending from horizon to horizon, sometimes more than 1000 km in length.  The base of roll clouds can be 150 to 300 m above the ground or sea surface, and the cloud itself may have a depth of 600 to 1300 m.  Morning Glory clouds seldom produce measurable precipitation, but are almost always accompanied by short-lived, often intense, surface wind squall.  They are reported to travel at speeds of about 40 km/hr, occasionally over 60 km/hr.

Click on thumbnails for more information

See next page for more photos ...

<     >