On the morning of 17 January 2016, Pamela Heinselman captured a beautiful sight over Norman, Oklahoma: A long, extensive roll cloud associated with an undular bore.
What exactly creates these clouds? An undular bore is a type of gravity wave (meaning gravity is the restoring force) in the atmosphere (particularly the lower levels near the ground) that occurs when a low-level boundary such as outflow from a thunderstorm or a cold front reaches a layer of cold, stable air. This “disturbance” of the air is similar to when you disturb water by throwing a rock in the pond, creating ripples on the water’s surface. The disturbance of the low-level air by the front leads to ripples in the air. If the air is moist, clouds will form where the air is rising along this wave-like disturbance.
Bores have been described in the scientific literature since as far back as 1950. With advancements in our observational capabilities through ground-based and satellite-borne instruments, the formation, properties, and impact of these bores have been studied in great detail since then. The drawing above is adapted from one of these studies, in which a group of scientists described an undular bore associated with a cold front that moved through Oklahoma in 2006, much like the one photographed by Pamela in this blog. In that study, they showed that there was a strong temperature inversion near the surface in the morning, meaning that temperature increased with height indicating stable air ahead of the cold front.
Shown below is data from instruments released on a balloon that tells us how temperature, moisture, winds, and pressure change as you go up in the atmosphere. On the morning of 17 January 2016, not long before Pamela took her photos of the roll cloud, the ballon data showed a strong inversion near the surface with the temperature at the ground below freezing, indicating that the air over Norman, Oklahoma was cold and stable ahead of the cold front.
This cool, relatively stable air trapped near the surface was lifted ahead of the advancing cold front. Due to the inversion, that lifting air was trapped, leading to the oscillating, wave-like pattern that resulted in the undular bore seen in the photos over Norman. Not only did this bore show up as a roll cloud in photos by Pamela and others, the roll pattern of clouds associated with the bore showed up in visible satellite imagery, looking like ripples in a pond. Notice how far these clouds extend across Oklahoma!
When these bores pass over an area, not only can you visibly see them as beautiful, long rows of clouds, they are also associated with shifts in surface observations, particularly gusty shifting winds and rising pressure. Data from a station at Norman shows a sharp increase in pressure along with a quick increase in wind speed and change in direction around the time these photos were taken! The cold front passed through later in the day, leading to another shift in the winds to northerly and decreases in temperature and moisture.
While many studies of bores have occurred in Oklahoma due to many instruments available, they are not unique to this area. Examples from Iowa have shown bores on radar and webcams, as well as examples off the coast of Texas. Perhaps the most well-known example of undular bores throughout the world is a roll-cloud formation in Australia called the Morning Glory.
Triggered by sea breezes near the Gulf of Carpentaria, the length, persistence, and smoothness of this cloud band attracts scientists and gliders (as seen in the photo above) who like to “surf” these atmospheric waves. The Morning Glory Cloud is such a common feature in the Spring in this area that there’s even a festival named after this incredible phenomenon.