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.|
About this morning's inversionThe "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 alertsAs 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.
An air quality monitor operated by us at Shelby Farms recorded levels of particulate matter at a concentration considered unhealthy.— Shelby County Health (@ShelbyTNHealth) October 14, 2015
At the levels being recorded at this one monitor, everyone may begin to experience health effects.— Shelby County Health (@ShelbyTNHealth) October 14, 2015
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."
We have issued a “CODE ORANGE” air quality advisory for fine particulate matter due to an expected monitored exceedance.— Shelby County Health (@ShelbyTNHealth) October 14, 2015
Sensitive groups include the very old and young and anyone with respiratory disorders or cardiovascular problems.— Shelby County Health (@ShelbyTNHealth) October 14, 2015
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?
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