BIRMINGHAM, Ala. (WBRC) - Did you see a very dark and ominous cloud cross your neighborhood Tuesday morning? The cloud formation in this photo was spotted by many in the WBRC FOX6 coverage area. This generated lots of attention on social media, with many wondering what type of cloud this could be, and would this bring severe weather?
This cloud formation is called a “roll cloud” and while they may look like signals of active weather, they are completely harmless. There were many roll clouds spotted around the area Tuesday morning. In fact, the view from space revealed a series of roll clouds moving north to south across central Alabama.
So what caused these unique clouds to develop? These clouds were caused by an atmospheric wave known as an “undular bore.” There are many types of atmospheric waves. In fact, you’ve probably heard the First Alert Weather Team mention the term gravity wave in the past. These atmospheric waves are caused by density differences, resulting from temperature variations. The wave that caused the roll clouds Tuesday morning likely resulted from rain-cooled air from storms that dissipated earlier in the morning.
Just imagine a boat cruising across a lake, and the water being displaced upward in front of the boat. After the boat passes, water is pulled down again by gravity or think about a rock hitting the water surface, and the ripple effect it causes. The same principle can be applied to what is happening in our atmosphere when these wave processes occur.
When cooler air moves along the earth’s surface and displaces warm air in a stable environment, it forces the warm air to rise, the warm air eventually cools, becomes more dense/heavier, and sinks towards the surface. This starts the oscillating effect, which results in the type of atmospheric wave that caused the clouds Tuesday morning.
Roll clouds are an arcus type cloud, similar in look to the shelf cloud, except the shelf cloud is more commonly associated with the leading edge of storms. Shelf clouds can also be associated with severe thunderstorms. We have lots of photos of shelf clouds and roll clouds that were posted on the WBRC First Alert Weather App. You can share photos on the app and also view photos on the WBRC Weather Page. We also like to share your photos on the news!