WRAL WeatherCenter Blog

Smoky start to the week

Posted June 19, 2012

Trajectories showing the paths taken by air arriving at selected southeastern cities at 5 pm Monday, after traveling 36 hours along the indicated lines.

We received quite a few calls and lots of posts to the WeatherCenter Facebook page from folks who smelled a smoky odor Monday morning, as a combination of wind directions and the temperature structure through the lower atmosphere acted to both transport some smoke from a large fire in the Croatan National Forest inland a ways, and concentrate some of it enough to make it noticeable, at least in the early hours of the day.

The Croatan fire is in the lower section of Craven County on the south side of the Neuse river, and some of the smoke from the fire followed a curving path to the south, then the west and finally drifted north in order to arrive here, as the pressure pattern that brought northeast winds to the state for the beginning of the Father's Day weekend shifted and caused the winds to eventually turn more to the south. That process is reflected in the calculated trajectories shown in the first image here. Each of the starred city locations is the end point for air that traveled along the path shown in the preceding 36 hours, with each marker along the path denoting 6 hours of movement.

In addition to being carried into our area, the smoke was made more apparent by being concentrated vertically under a strong temperature inversion early Monday. As seen in the second image here, which is a vertical profile of temperature (red) and dew point (green). Notable here is that early in the morning the temperature is much cooler right at the surface than it is just a few hundred feet up. When the air is warmer above like this, and air that tries to rise finds itself surrounded by warmer air, so it is denser and sinks again, and the opposite is true as well. If some of the warmer air above is pushed downward, it is then warmer than air around it, becomes buoyant by comparison and rises back to where it was. The net result is that there is very little up or down motion, vertical mixing or dilution below a "trapping" inversion like that, and the smoke was able to remain relatively concentrated.

The same profiles a little later in the morning show a very different look once the surface is heated up some by the sun, as seen in the third image. Now the surface is warmer than the air a few hundred to a few thousand feet up, which frees the air in the lowest few thousand feet to mix much more easily. This "ventilation" process brings fresh air, flowing in from the southwest as seen by the direction of the wind barbs along the right side of the sounding, and lofts the smoke from the surface, resulting in a great amount of dilution. The end result (especially when combined with the passage of a narrow line of light showers yesterday morning) was that the smoke was no longer an issue for most of us , becoming undetectable by mid-morning or so.

Today, the general wind flow is more from the south to southwest, and should pin the greatest amounts of smoke closer to the coast. This is reflected in the forecast map (fourth image) from the NC DENR Division of Air Quality, showing Code Red levels of particulate pollution near the fire, and a larger Code Orange area covering surrounding areas. I've included a link to the DAQ forecast page where you can find links to the special fire/smoke forecasts, in case you're traveling that way this week and would like to check for updates.

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