Storm School Intro: Supercell Structure.
In this first installment of Storm School, I wanted to introduce the basic structure of a supercell thunderstorm. Most of the storms we chase are supercells, so creating a solid understanding for their characteristics and processes will be important moving forward. This storm took place on May 26th, 2013 in central Nebraska. This view is looking to the southeast towards the storm, which was moving generally to the northeast. It’s important to know the viewing angles and storm motions in photographs because there is no movement and no time to elapse which would otherwise be used to visually infer this information.
The most easily identifiable characteristic of a supercell is the updraft, the rising column of air that forms the main structure of the storm. I chose to begin with this photo because the updraft structure is clearly visible. It resembles a volcanic eruption or an atomic bomb blast. Think of the heat being generated by those examples and you can begin to understand the heat energy being released by a supercell thunderstorm. In this example, the helical structure (like DNA) exhibited by the updraft is evidence of rotation within the rising column of air.
Air is being drawn into the storm at several levels, seen as tail-shaped structures along the lower portions of the updraft column. These inflow tails will usually be found lower in the storm, most often connected to the updraft at or near the base. This is because the warmest, most moisture-rich air that storms “feed” off of is normally found near the surface . In this photo, the actual base of the storm is difficult to see due to low contrast from this viewing angle.
At the top of the updraft is the storm’s anvil, named for its flat top. This is caused by an inversion (when the air above is warmer than the air rising). At this level, the air that has risen since the surface can go no higher, so it spreads out. Think of placing a glass roof over a campfire. The smoke would rise in a column until it hit the glass, when it would be forced to spread out sideways.
This second photograph from June 18, 2011 near Yuma, CO looks directly east at a storm moving east-northeast (ENE). Many of the same features from the first photo can be seen on this storm as well. The updraft rises from the base and moves away from me until it disappears into the anvil, which was much larger on this storm. The biggest difference in this photo is the big hazy area on the left side of the storm – precipitation. Precipitation typically falls from the storm in two areas – the forward flank downdraft (FFD) and the rear flank downdraft (RFD). On this photo, rain and hail up to 1.5″ are falling in the FFD region of the storm. If there was rain or hail falling in the RFD region on this storm, my view of the base would be obscured by the precipitation.
The other main difference between the first photo and this is that the base (bottom) of the storm is visible. The base is the lowermost portion of the updraft, and is the point at which the cloud matter begins. This change visually denotes the point when water vapor (clear air) condenses into tiny water droplets (cloud material). On most supercells, the base of the storm is free from precipitation, which falls further ahead of the storm along the FFD. You’ll hear the term rain-free base used instead of precipitation-free base because saying those extra four syllables just isn’t efficient.
We have now addressed the main features of a supercell. Each of these features has smaller, more intricate sub-features that we’ll address as we delve deeper into the inner workings of thunderstorms. As we add more and more entries to our Storm School page, the puzzle pieces should start to come together and form a clearer picture. If you have any questions about this lesson, or have recommendations for topics to address in the future, please comment below (or email us via our contact page if you’re shy).