Questions from the Cockpit: Burning off the fog

Denise, working on her private ticket in California, writes: Help me out! I’m getting ready to take my written, and I keep mixing up my fog types. Is there any clever way to sort them out?

Clever, no. Somewhat sorta kinda helpful…maybe.

But first a fog primer: Fog is nothing more than a cloud that lost its medical and has been grounded.

Oversimplifying it a little bit, clouds and fog — which are generally liquid water, not water vapor as some folks suppose — form when the air gets so saturated that it can’t hold another molecule and the vapor condenses. This happens at what’s called the dew point temperature. If this temperature is reached somewhere up in the sky, you get a cloud. If it’s reached somewhere at or near the ground, you get fog.

So if fog is nothing more than grounded clouds, why on earth are there a bunch of different names for the same thing? Well, the weather folks like to classify these non-aerial clouds according to how they lost their medicals — which, just like pilots, can actually happen in a number of different ways.

How many ways? Well, in the Pilot’s Handbook of Aeronautical Knowledge (PHAK) the FAA recognizes five fog types, BoldMethod lists six, the National Weather Service and the Royal Meteorological Society each list eight, Wikipedia recognizes 10, and the YouTube channel NauticaLive says that there are 23 “main types” of fog.

Thank God the FAA doesn’t subscribe to that channel. Just sayin…

On the other end of the spectrum, SkyBrary only recognizes four types. Meanwhile, Encyclopedia Britannica says the whole notion of classifying fog by cause is a bit silly in the first place (only it says it in a more stiff-upper lip manner), as in most cases fog is caused by more than one overlapping process, which might be what led Pierre Duthon et al, writing in the journal “Atmosphere,” to propose scrapping the whole damn thing and replacing it with a system that classifies fog by droplet size instead.

While that might be scientifically elegant, from a pilot perspective, a cause-based classification is actually better, because if we understand the conditions that can cause fog, we can be alert to the risk of fog when we see those conditions.

And that’s why the FAA wants you to understand the common mechanisms that cause clouds to lose their medicals.

But for your purposes, we’ll stick to the five from the PHAK, because those are the ones that will show up on your written, which is what you are concerned about.

The five PHAK fogs are (in order of appearance in the book) radiation, advection, upslope, steam, and ice.

And I feel your pain in sorting them out, it really can cause brain fog.

Recently, when traveling with General Aviation News photographer Lisa F. Bentson, we overflew a valley and saw fog hugging the river far below.

“Say, Mister Master Ground Instructor,” she asked, “which kind of fog is that?”

Immediately, my mind fogged over. So with a straight face, I simply said, “Oh, that’s graymystafog. Classic case. Best example I’ve seen in years. You remember studying that in ground school, right?”

You could almost hear the memory starter in Lisa’s mind grinding to no effect, like a weak battery trying to spin the prop on a cold, misty morning. “Gra-mista-wha?”

“Graymystafog. As in gray…misty…fog.”

I thought it was funny, but my shoulder still aches from where she punched me.

OK, but enough fogging around, here we go. Now, it might be helpful to you in committing the fog types to memory to know that one of the five is really a sub-category of one of the other four, so really, you could argue that there are only four kinds to learn. Further, all of them fit into two broad categories: Wind-required and no-wind required.

Radiation and Ice Fogs

The no-wind required fogs are radiation fog and its offspring, ice fog. They both form when the ground cools underneath wet air to the point that the ground temperature and dew point temps are right on top of each other — the recipe for cloud or fog formation.

Picture a damp, sunny day. Now picture the sun setting. What’s going to happen to the nice, warm, sun-soaked ground? Yep it’s going to radiate all its heat away overnight in the cool moonlight. And when the temperature of air near the ground cools to close to the dew temperature — within 2° Celsius or so — POOF! The water vapor exceeds the carrying capacity of the air and turns into a fine liquid. Fog.

Ice fog is formed by the exact same process, but rather than going from a vapor to a liquid, the vapor — due to extremely low temperatures — goes straight into a solid, bypassing the liquid state, forming little floating ice crystals. It needs to be -31°C for this to happen (recalling that the colder air gets, the less water vapor it can hold), so as you can guess, we don’t often see ice fog in the mainland USA.

Radiation fog tends to hang out in depressions, low-lying areas, mountain valleys and the like. It’s transient, getting sucked back into the air as the day warms or easily blown away when the day’s wind starts.

If radiation fog grounds you, just grab a coffee and a breakfast burrito, it — and you — won’t be ground-bound for long.

As to remembering the “radiation” name, the way I remember radiation fog is that the ground “radiates” off all its heat during the night, like one of those old-fashioned hot water radiators. Only, you know, the boiler is broken, so the water in the radiator cools overnight. (The sun is the boiler in this admittedly weak analogy).

Hey, I think I warned you that “somewhat sorta kinda helpful…maybe” was the best I’d be able to do.

As to “ice fog,” that’s easy to remember, thanks to its name, and as to remembering it as a subcategory of radiation, well, we’re back to that broken radiator, but now it’s getting really cold in grandma’s house.

And the next fog we’ll tackle, out of PHAK order, is also easy to sort out, thanks to its name.

Upslope Fog

Now we are into windy fogs, where wind will move air over something else or to somewhere else to create a fog-ready set of conditions.

Ever climb a mountain? Yeah, me neither.

OK, let me try again…have you ever noticed that on hot days flying, it’s cooler at altitude? Yep, that’s lapse rate in action, as you get higher into the sky, it gets cooler. I hope you can see where this is going.

If you then take some wind, and use it to blow warm wet air up a mountain slope — which is progressively cooler as the elevation increases — when the wet air arrives at the point where the ground is at the dew point temperature, POOF!

The one thing the FAA wants you to know about upslope fog is that your coffee and burrito ain’t gonna cut it this time. Upslope fog can last for days as long as the wind that creates it continues to blow. You can remember that by remembering that walking up a mountain can take days, too.

Advection Fog

This one is easy to understand, but hard to connect the name to. Advection fog happens when the wind blows wet air across a cold surface, no slope required. This is the kind of fog we get in coastal areas at dawn when sea breezes blow wet air from the ocean over the cold morning ground, as opposed to the ground getting cold under the air, like with radiation fog. Winds of up to 15 knots actually can make the fog worse, in terms of intensity, but above 15 knots it restores the cloud’s medical and you get a low ceiling instead.

As to the word advection, it’s a scientific term for the transfer of heat (or matter) by the flow of a fluid, especially horizontally. Recall that air is a fluid. How to help remember the connection between the label advection and the process in lay persons terms…. well… “advection” more or less rhymes with “affection.” And when you have affection for someone, you’ll travel to see them, just like the moist air travels to the cool surface.

Steam Fog

Also known as sea smoke, steam fog looks like steam from a hot spring, and it basically is. This time, instead of warm moist air moving over cold ground, cold air arrives over warm water for that magic mix of temperature and humidity. It’s pretty much the opposite of advection fog.

Remember it by thinking of an old black and white detective flick set in the steamy underbelly of the city. Hopefully that will remind you that with steam fog, the heat is on the bottom, not the top. Get it? Steamy underbelly?

Well, I hope that clears up the fog on fog for you.

So which did Lisa and I over fly? Well, it shared the characteristics of both radiation and steam, so maybe Encyclopedia Britannica was onto something. But regardless of how it formed, or what combination of factors went into its creation, it was still graymystafog.

As all fogs are.