Unless you're an expert, you're not likely to recognize a supercell as anything that's particularly dangerous just by looking at it in the distance as you fly toward it. But, this relatively innocent-looking cloud could be hiding extreme turbulence, severe icing, large hail, downbursts, and maybe even a tornado or two.
Spring, when cold and warm air are still battling for control of North America, brings the strongest supercells, but they can occur throughout the summer and into fall. The biggest and strongest supercells form on the Great Plains from the Rockies east across the Mississippi Valley to the Appalachians, with Texas, Oklahoma, and Kansas seeing the most. Supercells also rip across the Southeast, sometimes in late winter with the first signs of spring warming. Supercells sometimes form east of the Appalachians from the Carolinas northward, although these are usually smaller than those on the Plains. They're rare in the Rocky Mountain and West Coast states.
Since the 1970s, researchers have rewritten the book on thunderstorms using Doppler radar and data gathered by chasing storms with the potential to produce tornadoes. Their most important findings began with the discovery that the thunderstorms that produce really strong tornadoes are fundamentally different from other thunderstorms because of a highly organized structure that gives them long lives.
When researchers first began to realize that a few thunderstorms lived much longer than most storms, they called them "steady state" storms, which is the term you still see in FAA exam questions. But researchers and forecasters no longer use this term. In fact, many will tell you that they've never heard it.
By definition, a supercell is a thunderstorm that contains a rotating mesocyclone, perhaps 10 miles across, rising into the storm. While researchers are still trying to understand the details of what causes the rotation in tornadoes, a supercell's mesocyclone almost surely plays a key role. If you see a Doppler radar display on the World Wide Web with the word "meso" - for mesocyclone-near a storm, you know that the storm is a supercell.
The existence of the mesocyclone in supercells is one of the reasons why Doppler radar is helping to improve tornado warnings. Doppler radar detects wind motions inside a cloud and can detect winds moving in opposite directions as they do on the two sides of a tornado or mesocyclone. While tornadoes are too small to be detected by Doppler radar - unless the radar is very close to the thunderstorm producing the tornado - mesocyclones are large enough to show up on Doppler radar as much as 100 miles away. When a mesocyclone shows up in the radar display of a storm, it grabs the attention of weather forecasters. They know the storm is one that has to be watched for tornado development. A rotating mesocyclone doesn't always produce tornadoes. But, it does help to pump more humid air high into the atmosphere to worsen the hazards that can be found in any thunderstorm: hail, extreme turbulence, and rapid icing.
Supercells need the same basic ingredients as any other kind of thunderstorm:
- Unstable air that will continue rising once it's given an upward shove;
- Plenty of humidity to condense into clouds, rain, and hail, releasing heat that keeps the air rising; and
- Something to give the air an initial upward shove.
The added ingredient needed to turn more ordinary thunderstorms into supercells is the correct pattern of upper air winds. To be unstable, the atmosphere needs to have relatively cold air atop much warmer air. Often, pools of cold air aloft - upper air disturbances - can move over a region, supplying the needed temperature contrast to make the air unstable.
Most pilots know that an inversion - relatively warm air atop colder air - is stable. It's normally the most stable condition. But, an inversion can actually help to create strong thunderstorms. A day that begins with an inversion but also has the other needed thunderstorm ingredients present is somewhat like holding the lid down on a pot of boiling water. As the ground is heated, thermals of warm air begin rising but are blocked by the inversion. Eventually, however, the air breaks through the inversion, and the warm, humid air that has been bottled up streams through the opening to quickly create a big thunderstorm. Without an inversion to hold the lid on, the day might have produced lots of cumulus clouds or several weak thunderstorms, but not a monster.
Once things get started, a thunderstorm cell will go through a three-part life cycle:
- The first is the cumulus stage, which is characterized by updrafts of rising air. At this stage, the cloud is growing taller.
- The second is the mature stage, which begins when precipitation begins falling and dragging air down with it. The storm now has both updrafts and downdrafts and is at its most turbulent.
- Third is the dissipating stage, which begins as updrafts end, leaving only downdrafts. The rain begins easing up.
In most thunderstorms, the downdrafts that come down with falling rain create a gust front that flows along the ground and cuts under the updrafts rising into the storm. The downdrafts eventually strangle the storm by shutting off the supply of humid air the storm needs to continue growing.
Often, the downdrafts will shove warm, humid air upward away from the storm that caused the downdraft. The air that's been shoved upward can now trigger new storms. Depending on the flow of upper air winds and other factors, the new storms can create a multicell cluster, which from a distance looks like one thunderstorm but is really a cluster of thunderstorms in different parts of their life cycles. Sometimes the new storms can form a line, called a squall line, often several miles ahead of a cold front.
When the upper air winds form the right pattern, however, they tilt the updraft. As air rises in the updraft, it cools and its humidity condenses into the water droplets and ice crystals that form the cloud. As these come together to form rain or hail, the precipitation falls away from the updraft. In a well-organized supercell, the gust front that's created when the cold air hits the ground and spreads out ends up pushing more humid air up into the storm's main updraft instead of pushing up air to create new cells elsewhere. The supercell draws on the energy that would have gone into creating new cells. A supercell's organization allows it to continue for hours as it moves across the countryside, spinning out tornadoes and creating microbursts that blast the ground - including airports - with quickly shifting strong winds and maybe large hail and heavy rain.
Air rushing upward in the updraft often overshoots the top of the thunderstorm, creating a bulge of cloud above the generally flat top of the storm. Such an overshooting top is one sign that a storm might be a supercell. The bottom of a supercell will appear mostly flat, with precipitation coming down on one side while the other side is generally rain free. Often, you'll see a wall cloud hanging down from the otherwise flat bottom of the storm. The wall cloud is usually the bottom of the mesocyclone, and you can sometimes see it rotating. If the supercell is going to produce a tornado, it's likely to come from the wall cloud.
At the top of the storm, upper air winds are blowing some of the cloud downwind, forming an anvil, which is a characteristic of large thunderstorms but doesn't necessarily mean a storm is a supercell. Often the bottom of the anvil will have mammatus, or pouches, hanging down. These are caused by cool, heavy air in the anvil sinking into warmer air below. A mammatus cloud, looks dangerous, but that's not necessarily the case. In fact, sometimes pretty harmless clouds will produce mammatus. While the mammatus itself might not be much of a hazard, pilots should consider such clouds a sign that the parent storm could have a strong updraft and should be avoided.
On the Great Plains, where the air is clearer than in the Southeast or East, you might see most of the features of a supercell in a far-away storm. But, haze or clouds or even smaller thunderstorms can often hide the main features of a supercell.
The good news for pilots and people on the ground is that supercells are the rarest of thunderstorms. The other good news is that once forecasters identify a storm as a supercell, they can do a pretty good job of predicting when and where it's likely to produce tornadoes, large hail, or downbursts.
While an extremely dangerous supercell might look as innocent as the one in Photo 1 as you fly toward it, a pilot who has obtained a good preflight briefing and keeps up with the latest weather information during a flight is likely to stay far away from the dangers of supercell storms and their smaller, but still dangerous cousins.
Related Articles
