Turning towards the sky, one can’t help but become mesmerized by beauty and movement of the different clouds that flow across the atmosphere. One of the most peculiar looking clouds, arcus clouds, are horizontal elongated tube-like clouds that can occur all over the world. A subgenre of arcus clouds known as roll clouds, are even more atypical as they are detached from any other cloud features.
While roll clouds can occur in many places, such as Germany, Canada, South Africa, Brazil, Uruguay, and even Florida, they are regionally known as “Morning Glory” clouds along the North Australian Coast, more specifically over the Cape York Peninsula and Gulf of Carpentaria. The clouds are so named as a result of their early morning appearance, and frequently occur during late September and through early October in this region. These phenomenal clouds may be on the order of 400-600 miles in length, ½ – 1 mile high, and may move as fast as 40 miles per hour.
As with any cloud, moisture must be present in order for water vapor to condense into water droplets. Morning Glories tend to occur when humidity values are elevated and a clash of different air masses. Once moisture levels are adequate, these clouds may form as a result of drastic temperature changes in air masses ahead of a thunderstorm, frontal boundary, or sea breeze.
To understand the physical nature of a cloud, let’s first take a look at the relationship between air density and temperature. Cold air is heavier than warm air as a result of more molecules per volume.
To better understand this, imagine a 10’ x 10’ unheated room in the middle of a Siberian winter. For a person to keep warm, they would want to fill this room with as many other people as possible, capitalizing on generated body heat. Now imagine that same 10’ x 10’ room located in the middle of hot Texas summer day with no available air conditioning. In this scenario, a person may want to remove a majority of the heat generating bodies. So if we exchange molecules for people in the above example, cold air has more molecules than warm air in the same amount of space, therefore making cold air denser (heavier) than warm air. This is what makes cold air sink downwards and warmer air upwards by nature.
A sudden influx of cold air can also force warm surface air to rapidly rise, which is often the case of what happens when cold air rushes out ahead of a thunderstorm or when sea breezes occur from differential daytime heating. A gust front is the downward and outward rush of the colder/heavier air from within a thunderstorm, usually followed by strong winds, heavy rain, and possible hail within minutes. An extremely strong gust front rush out faster, detaching from the parent storm, and creating a roll cloud. Sea breeze circulations occur as the sun heats land and sea surfaces differently, creating an onshore flow during the day and offshore flow during the night hours. When an extremely strong sea breeze occurs in the evening, the elevated chances of a Morning Glory cloud occurs the following morning.
While there’s no shortage of atmospheric phenomena to excite the average observer, it is without doubt that encountering a roll cloud is an incredible sight and definitely on any weather lover’s bucket list.