WRAL WeatherCenter Blog

Why those 'anvil tops' are flat (mostly) ...

Posted June 20, 2011 8:41 a.m. EDT
Updated June 20, 2011 9:01 a.m. EDT

NC NearSpace balloon image taken from the side of two distant thunderstorms with clasically flattened "anvil" tops late in the day on Saturday, June 18, 2011.

A local group called NC NearSpace sent up a balloon with an attached camera and GPS unit this past Saturday and in the process captured a really neat set of photos that you can see at the flickr link I've included here. The balloon traveled over 60 miles downwind and topped out above 60,000 feet, and they captured some really nice photos of fair weather cumulus clouds, some lengthy contrails left behind passing airliners, and two or three shots that include some of the thunderstorms that developed late in the day.

I've included two of those pictures here (our thanks to the folks at NC NearSpace for permission to use them), the first of which led one of our Facebook page visitors to ask why the tops of thunderstorm "anvils" are so flat, something that many of you have probably observed from time to time. The first image is a nice shot from the side of a couple of anvil tops off to the north of the balloon.

Those flat tops are characteristic of thunderstorms that reach what is called the "equilibrium level." Below that altitude, the air parcels in the storm are warmer than surrounding air making them rise due to buoyancy. The equilibrium level (EL) is the height at which the air outside the storm becomes as warm as or warmer than the air in the storm, causing buoyancy to be lost and acting as an invisible "ceiling" of sorts that flattens the storm top because a sizable fraction of the cloud particles spread sideways some at that point instead of continuing upward. This level often, though not always, coincides with the tropopause, the point at which the troposphere transitions to the stratosphere and temperatures begin to either increase or remain steady with increasing height.

On Saturday evening, the EL over eastern NC was coincident with the tropopause according to soundings taken by the Morehead City NWS. On the sounding shown in the second image, obtained from NOAA's Storm Prediction Center site, you can see where the ambient temperature (red line) begins to increase sharply at around 14 km altitude, marking the top of the troposphere. The approximate temperature inside a parcel rising through a storm is indicated by the brown dashed line, and the anvil would form at about the height where those cross, in this case around 47,000 feet above the surface.

It's also worth noting that strong updrafts in some thunderstorms can sometimes "overshoot" the equilibrium level by a short distance, before becoming negatively buoyant and sinking back into the storm. Another one of the photos from the balloon launch Saturday shows this clearly, with a couple of "overshooting tops" extending a little above the anvil cloud in the lower center of the photo that is the third image in the series here. You can occasionally see these overshooting tops from the ground, and they are visible fairly frequently in high-resolution visible satellite imagery as well.