If you watched the GA Tech/FSU ACC Championship Game in Charlotte, North Carolina tonight (12/6/14), you may have seen the smoke-filled stadium following the pre-game fireworks show. A heavy fog was already in place, which virtually guaranteed poor visibility of the fireworks; but to make matters worse, the smoke from the fireworks had no where to go. It couldn’t rise vertically, and surface winds were too light to allow for efficient dispersal. A quick look at the atmospheric thermodynamic profile reveals that the result should come as no surprise. The following a is a very brief discussion of the meteorological factors that contributed to the poor conditions and the persistent smoke.
Inversion = Stable Layer
We all know the old adage: “warm air rises, cold air sinks.” Well, this is generally true; but, for the “warm air” to rise, it must be warmer than the air surrounding it, and it must remain warmer than the surrounding air as it ascends. This configuration is known as buoyancy and it is the primary reason that convective clouds and thunderstorms form. However, if the parcel is colder than the air surrounding it, or becomes cooler than the surrounding air as it ascends, the parcel will sink due to negative buoyancy. Negative buoyancy exists in a stable layer (inversion) and inhibits the formation of convective clouds and thunderstorms.
Generally speaking, an inversion is a layer of air characterized by increasing temperatures with height. When temperatures increase with height, the atmosphere is considered to be stable and air parcels are unable to rise convectively unless forced to do so via some forcing mechanism (e.g., orographic lift over mountainous terrain or vertical motions induced by significant, large-scale weather systems). In order for parcels to rise convectively (in the absence of other forcing mechanisms), they need to be warmer than their surroundings, with ambient temperatures decreasing with height (unstable). Even hot smoke rising from a fire will cool as it rises and eventually becomes too cool to make it through a strong “capping” inversion; at the inversion, the smoke will disperse horizontally as a function of the prevailing wind at that level (see the animation below).
With an inversion in place, other factors such as high humidity, light/calm surface winds, and cool surface temperatures can cause fog to form efficiently. Prior to the fireworks show at the ACC Championship Game, fog was already in place (visibilities around 2 miles at 7-8 pm and decreasing). Fog will not magically disappear unless and until the causative atmospheric conditions change (for example, the inversion weakens, surface winds increase, or drier air advects in behind a departing disturbance). A cold front moved through around 10 pm EST (two hours after the fireworks show), bringing a slight increase in surface winds and some slightly drier air from the north. Subsequently, with the passage of the cold front, some of the fog lifted and visibilities increased from 1/2 mile (at 9 pm) to 7+ miles (at 11 pm).
Smoke in the Stable Layer
As mentioned above, a stable layer in the atmosphere suppresses vertical motions (i.e., the ability for the air to rise). Inversions are responsible for creating persistent smog and stagnant/hazy air during the summer months. When the fireworks began, the strong inversion prevented the resulting smoke from rising. Not only was the smoke unable to rise, but the relatively light winds prevented it from moving very quickly in any direction. So, the reduced visibility caused by the fog was only part of the problem; a dense smoke filled the stadium, reducing the visibility even more.
I created the following image using the thermodynamic profile valid at the time of the fireworks show (8 pm Saturday 12/6/14 at Charlotte, NC). We meteorologists are accustomed to using Skew-T diagrams to view/analyze the vertical thermodynamic profile; however, I created this particular graphic on a Stüve diagram so that the profile of temperature vs. height is easier for non-meteorologists to visualize. This image shows the vertical temperature profile from the surface to the tropopause. The “trapping” inversion near the surface begins at approximately 1,500 feet aloft (notice how the temperature begins to increase with height above the 1,500 ft line). This means that the air below that level is essentially trapped, particularly in the absence of any appreciable wind to move the smoke out of the area.
Animation of Smoke and its Dispersion in a Nocturnal Inversion
Photos of Fog/Smoke in the Stadium at the FSU/GA Tech Game (12/6/14)