Myth-busting: Tornadoes don’t form from a “clash of air masses”

Every spring, we hear the familiar, cliché expression:  “Yesterday’s tornadoes were caused by the clash of warm and cold air over the Plains.”   A new paper in the Bulletin of the American Meteorological Society argues that this is largely incorrect, and weather communicators should provide a more accurate description.  I agree.  Let’s look at how the process can be better explained.

What “clash of air masses” more accurately describes

As a science educator at a large public university, I frequently cringe at clichés, such as the one describing a black hole:  “Nothing, not even light, can escape its gravitational pull”.   I winced once when Neal Degrasse Tyson used similar words on the new Cosmos TV series.   This isn’t inaccurate, but I just tire hearing it, every time a black hole is described.

However, the media’s description of tornadogenesis is not only overused, it is outright inaccurate.   The bottom line:  Warm and cold air masses clash every day in North America, yet every day is not a tornado day!   Our mid-latitude location places us squarely in the domain of the polar front – a zone of hemispheric, deep atmosphere temperature contrast between arctic and subtropical air.

Extratropical (or mid-latitude) cyclones (or storms) thrive on this thermal contrast (the technical term is “baroclinicity”) and it serves as a source of potential energy, which is converted to the kinetic energy of a cyclone’s rotating winds.    These cyclones are the “bread and butter” weather systems of North America, creating rain, snow, high winds,  extensive cloud cover, rapidly swinging temps, and occasional tornado outbreaks.

The authors of the new paper, titled “Tornadoes in the Central United States and the ‘Clash of Air Masses’” discuss all this.  Extratropical cyclones frequently provide the large-scale setting for tornado outbreaks, particularly in the spring.  But those outbreaks – and their component supercell (or rotating) thunderstorms – are localized to a much smaller region of the storm, called the warm sector.  The locus for severe weather is often a hundred or more miles away from the nearest front.

So, the scale of the extratropical cyclone – which is the product of an “air mass clash” – is vastly different than that of the actual tornado-breeding storm cells.

Tornadoes actually prefer a weaker temperature gradient

The paper goes on to briefly discuss the small scale origins of tornadoes, citing the latest theories based on experimental and observational evidence.

The close juxtaposition of small-scale cold and warm air masses, beneath a tornado’s parent supercell thunderstorm, mitigates against tornado formation.   This describes the supercell’s “rear flank downdraft” (a cold, rain-chilled pool of air that spreads along the surface) and “warm inflow region”.    Several studies suggest that the warmer the rear flank downdraft, the greater the probability that a significant tornado will develop.

Figure 1. Diagram of a supercell’s tornado-spawning region.   Note that rear flank downdraft and its “mini-cold front”.   Courtesy of UCAR COMET.

Figure 1. Diagram of a supercell’s tornado-spawning region. Note that rear flank downdraft and its “mini-cold front”. Courtesy of UCAR COMET.

Why is this?  The cold, dense air resists lifting.  To “spin up” a nascent, vertical circulation into the intense rotating core of a tornado, a powerful updraft is needed, rising from the ground.  Rotating air is drawn upward and stretched.  The spinning column shrinks width-wise and thus spins faster;  angular momentum is conserved (eschewing another very worn-out cliché here!).  Updrafts are largely powered by warm, buoyant air.  So if the storm’s cold downdraft gets too close to the updraft zone, tornadogenesis fails.

In other words:   To get a big tornado, the thermal gradient beneath a supercell must be weak – the antithesis of a “vigorous cold air-warm air clash”.

How can we do better?

Science literacy in the U.S. is unsatisfactory, and the public deserves a better explanation about how tornadoes arise – especially since we have learned so much more about tornadogenesis in the past twenty years.

The authors of the BAMS paper suggest that the media adopt the following language:

 Yesterday’s storms occurred when warm humid air near the surface lay under drier air aloft with temperature decreasing rapidly with height [originating from higher terrain to the west or southwest], providing energy for the storms through the  production of instability. Large changes in wind with height (“wind shear”) over both shallow (lowest 1 km) and deep (lowest 6 km) layers—combined with the instability  and high humidity near the surface—created a situation favorable for tornadoes to form.

Now, that is quite a mouthful!   It is accurate, but I find it a tad too verbose and technical.   Here’s how I usually explain the process to my undergraduate non-science majors.  I preface the description with what’s needed to generate a garden-variety thunderstorm.

Ordinary Thunderstorm:  Thunderstorms, in general, require buoyant, moist air that rises to great heights.  This describes an atmosphere that is unstable – the air is warm near the surface, ascends into colder air aloft, forming the deep convective clouds we call thunderstorms.

 Tornadic Thunderstorm:  This requires two additional things.  First, the updraft must be especially vigorous.   This occurs when a hot, dry layer slides on top of the warm, humid air near the surface.  This temporarily puts a “lid” on the unstable air, letting it build to extreme levels.  Second, the updraft must rotate.  This requires wind shear – a change in wind speed and direction with altitude.  Rotating storm cells are much more vigorous, long-lived and process energy more efficiently.    Some of that rotation is focused into a tornado.

I usually go on to state that a rotating supercell, in and of itself, does not guarantee tornadogenesis, as only about 25 percent of these cells generate tornadoes.  But supercells are a pre-requisite for most strong and violent tornadoes.  I say this because many folks incorrectly think that the rotating updraft, called a mesocyclone, and the tornado are the same.  They are not.  A “meso” is 5-10 km in diameter, and strongest in the storm’s mid-levels;  a strong tornado is perhaps half a kilometer wide and rooted in the surface air.  To go from a mid-level meso, to a tornado, requires additional processes, and there is likely more than one mechanism involved.

For a sound bite, one might consider the following explanation:

Tornado outbreaks occur when instability in the air mass builds to extreme levels, then is suddenly released.  The rising air acquires spin from intersecting air streams moving at different speeds.

In conclusion, I agree with the premise of this new paper.  We are long overdue for a better explanation describing tornado outbreaks.   The “clash of air masses” needs to be properly contextualized.   The clash breeds extratropical cyclones.  Processes in the cyclone’s warm sector, leading to individual tornadic storms, are entirely different.   We need to first “cap” the air mass, building extreme instability, then convert some of the wind shear into a spinning updraft.  Even then, a tornado is not inevitable.

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