Sea Science: by Jordanna Sheermohamed
Turning toward the sky, one can’t help but become mesmerized by the 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.
A roll cloud seen on Las Olas Beach in Punta del Este, Uruguay, in January 2009. Photo by Daniela Mirner Eberl.
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 because of their early morning appearance. They frequently occur during late September through early October in this region. These phenomenal clouds may be on the order of 400 to 600 miles in length and up to 1 mile high, and may move as fast as 40 mph.
As with any cloud, moisture must be present for water vapor to condense into water droplets. Morning Glories tend to occur when humidity values are elevated and air masses of different temperatures clash. 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 because it has more molecules per volume. To better understand this, imagine a 10-square-foot 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-foot by 10-foot room in the middle of a hot Texas summer day with no available air conditioning. In this scenario, a person would want fewer heat-generating bodies in the room.
A morning glory cloud formation near Burketown in Queensland, Australia, taken from a plane in August 2009. Photo by Mick Petroff.
If we exchange molecules for people in the above example, we can see that cold air has more molecules than warm air in the same amount of space, therefore making cold air denser and heavier than warm air. This is what makes cold air sink downward and warmer air upward by nature.
A sudden influx of cold air can also force warm surface air to rapidly rise, which is often 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 rushes 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. When an extremely strong sea breeze occurs in the evening, there is a higher chance of a Morning Glory cloud the following morning.
While there’s no shortage of atmospheric phenomena to excite the average observer, it is without doubt that a roll cloud is an incredible sight and definitely on any weather lover’s bucket list.
Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a private weather-forecasting company (www.WeatherForecastSolutions.com). Comments are welcome below.