Andy_Johnson's Blog

Usually when you think of life threatening weather, tornadoes, severe thunderstorms and hurricanes come to mind. There are times when stable weather occurs, however, that can also cause problems. I'm referring to heat waves, cold waves and even fog. Ironically I had written my blog 3 days ago regarding fog in the Bay area in the winter months. I had mentioned that of the 19 days a year where dense fog occurs in Tampa 5 of those days are in January. January has the highest number of fog days followed by December and February.

The tragic incident that unfolded in Polk County this morning with the 70 car pileup occurred in an area known for fog development. There are basically 2 kinds of fog, advection and radiational. Advection involves cold air moving over a warmer surface but radiational fog can develop in place. Since the winds were nearly calm in the area this morning, the fog was likely of the radiational variety.

Fog is essentially a cloud at the ground. It occurs when the air temperature falls to the dewpoint temperature and the relative humidity reaches 100%. In addition, cold air is "heavier" and more dense than warm air. The area around the accident site was in a bowl shaped depression that allowed the cold air to drain into. The temperatures were much colder in Polk County than closer to the coast. For instance, Bartow had a low of 48 but Tampa had a low temperature of 58 with a dewpoint of 56. The temperatures in Polk County fell to the dewpoint at which time the fog formed and then kept falling another 8 degrees or so along with the dewpoint.

Part of the reason, I believe, is that the recent cold snap last week cooled off the Green Swamp and nearby lake temperatures to very low values. This was followed by a rapid warm up where not only the temperature rose but also the dewpoint. The air temperature rises more quickly than water temperature so there was a lag in the nearby water temperatures. The dewpoint temperatures were higher than the water temperatures in some areas yesterday. It was almost as if the air were "super-saturated". From the aerial video that I saw from Randy Powers' flyover there was an area of dense fog that had pooled into a limited area. On either side of this "pool" the visibility was low but not all that bad. The fog did not appear to be very deep either. You could still see the tops of trees poking out from the fog bank. So people driving at 70 MPH or more ran into almost a curtain of  fog.

There is some question and I'm sure that it will be debated in the future as to  what additional influence smoke had regarding visibility reductions. From early aerials, though, you could see the smoke rising from the vehicles involved in the accident but not from a nearby controlled burn that was supposedly 90% contained. Also, the controlled burn was on the north side of I4.

Always use low beams when driving in fog. California's interior valleys have experienced dense fog days involving 100 car pileups. However, today's event involved more vehicles than I can remember in Central Florida.

Our thoughts and prayers go out to the many people affected by today's tragic events.