Wednesday, October 14, 2015

How a mulch fire and atmospheric inversion create a smoky morning commute

As we drove into town last evening from a few (well-deserved I might say) days at the beach with family, a huge plume of smoke emanated from the area around I-40 and Sycamore View near Bartlett. I didn't know what was on fire at the time, nor did I realize (since I had been away from detailed atmospheric analysis since late last week) what the ramifications would be this morning.

However, an early glance at Twitter this morning and then a drive through Bartlett on my way to work quickly made clear the situation. A combination of fire and smoke continuing through the overnight hours from a huge mulch fire and a surface-based "thermal (atmospheric) inversion" had created a very smoky commute through central Shelby County!

Photos taken about 8:30am Wednesday. This one was several miles northeast of the mulch fire, but smoke was still present in the air, creating a beautiful set of "crepuscular rays" through the trees.
Approaching I-40 southbound on Sycamore View, the smoke got thick and acrid with visibility below 1/2 mile (note you can't see the I-40 overpass from Summer Avenue, a distance of  0.6 mile).
On I-40 east of Sycamore View, the dense low-level smoke is trapped below an inversion and is thickest in the area of the overnight fire that was still smoldering this morning

The Atmosphere, Inverted!

A surface-based inversion is created when the lowest level of the atmosphere (typically a couple thousand feet above the ground) cools more quickly than the layer above it. This is not uncommon and, in fact, occurs more often than not when the wind is light to calm. When an inversion is in place, the temperature actually warms with height, instead of cools (which is the typical thermal profile during the daytime).

The typical setup during a surface-based inversion and after it dissipates, courtesy UCSI University.

An aircraft sounding from Memphis International shortly before 7am Wednesday shows the temperature (in red) rising from the low 50s at the ground to the mid 60s at 1000 feet before falling slowly above that height. The pronounced temperature increase is the " thermal inversion" which effectively "capped" the polluted surface-based air from escaping.
The warming air creates a sort of "lid" on the lower atmosphere, trapping it near the ground and disallowing smoke, fog, smog, etc. from dissipating upwards. It's not unlike the "cap" we talk about relative to summertime thunderstorm development, only this inversion occurs close to the ground, not in the middle layers of the atmosphere.

About this morning's inversion

The "strength" of the cap, or inversion, is determined by the difference in the temperature between the cool air below and warmer air above. Sometimes it is only a few degrees, sometimes much more. The stronger the inversion (or bigger the temperature difference), the stronger the "lid" is between the low levels and the air above. This morning, the inversion was very strong! In fact, outside the city, temperatures were in the mid 40s (lower 50s in the city) and the warm layer above the ground was in the mid to upper 60s. That's a 15-20°F inversion!

The other interesting feature about this morning's inversion was that it was based very close to the Earth's surface. That warm layer in the mid 60s was only about 1000 feet above the ground. That means if you were to go up 1000 feet, it would be 20° warmer than at the ground. It also means that all that smoke was trapped in air that was only 1000 feet deep.  If it were 2000-3000 feet thick, the smoke would have had much more air (by volume) to disperse through. Instead, it spread out at the surface rather than escaping up.

Air quality alerts

As we know, smoke is not something you want to be breathing, as it is harmful to the lungs due to particulate matter that is an ingredient in the smoke. This morning, a Shelby County Health Department air monitoring station at Shelby Farms actually went "code red" for a while due to the harmful particulates in the air. (It's a good thing there wasn't a station at the Summer 4 Drive-In!) The Health Department issued a statement indicating that those with breathing issues or other illnesses stay indoors and others not exert too much energy in the affected areas.

So how did the majority of the low-level smoke (outside of the source of it at the fire site) dissipate? The inversion disappeared as the low level temperatures warmed. As soon as the lowest 1000 feet of the atmosphere rose into the mid 60s (which occurred by mid-morning), the inversion was gone and air from the very low levels could mix with the air above, allowing the smoke to rise and disperse into the atmosphere above the morning inversion. However, the Health Department indicates that a Code Orange air quality alert remains in effect through midnight tonight, which means the air continues to be "Unhealthy for Sensitive Groups."

So, the takeaway is that while an inversion is not rare, it was simply stronger and lower than usual this morning and we had a source of pollution to gauge just how it works. You can "breathe easier" now that you got your weather 101 for the day! How bad was the smoke where you live or work?

Erik Proseus
MWN Meteorologist

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