What is the Dense Fog Advisory?

By Jim Dickey
6/21/2012 10:18:38 AM

The National Weather Service issues a wide variety of watches and warnings, all of which are aimed at protecting you from harm. It really is a WIDE variety though...many of which are issued on a daily basis, but some that are rather obscure and rarely used.

As I see it, if you don't know what a warning or advisory means, then it will do you no good. And so, in my quasi weekly 'What's that Warning?' series, I will be discussing some of the more obscure warnings, hoping to give you a better idea as to what you should do when and if one is issued for your area.

This week...The Dense Fog Advisory.

I admit, this one seems pretty self explanatory. Yes, a Dense Fog Advisory means that there will be fog, and it will be dense. Putting an actual definition to it though, a Dense Fog Advisory means that fog will limit visibility to below a quarter of a mile for two or more hours.

What does this mean for you? Well, first off, in most cases, fog is most likely to form, and most likely to be at it's peak density during the early morning hours. The reason for this is that, following the normal or 'diurnal' temperatures swings that take place during a 24 hour period, temperatures are at their lowest in the hour or so before dawn. Fog is more or less a cloud layer based at the surface, forming when the lower levels of the atmosphere reach 'saturation'. A less jargon filled way to say this; when the temperature reaches the dew point. And when the temperature reaches the dewpoint, under the right conditions, fog will form.

It is often the case then that fog will be at its most dense during the morning rush hour. Remember, if a Dense Fog Advisory is in effect, this means visibility is expected to be down below a quarter of a mile. That means, if you're cruising along the highway, you will not be able to see very far in front of you. This can be very dangerous, as this lowered visibility will hinder you from seeing any dangers on the roadway up ahead.

In this way, a fender bender between two cars on the highway can quickly become a deadly multi car pile-up. In fact, fog has been blamed for some of the largest, most deadly pile-ups world wide. Take, for example, the deadly pileup on I-75 in Tennessee back in 1990, killing 15 people, still considered by many to be the worst accident in Tennessee's history.

So, if you see that a Dense Fog Advisory is issued for your area, take your time on the roadways. Much like during a snowstorm, you may want to even plan ahead, leave early, and give yourself some extra time to get to where you need to be.