Deep dark weather secrets about fog are really no mystery

It’s not accurate to say that Mother Nature keeps secrets. However, it is spot on to say that Mother Nature harbors all manner of surprises for pilots who fly on without making an effort to develop some personal weather wisdom. One key is in understanding that what you see and feel is what you get, regardless of what is forecast.

There is no question that weather information has become many times more available over the years. Forecasting has gotten a lot better, too, but a pilot who knows a bit about what is going on is still at an advantage. It’s fun, too. I really enjoyed making weather discoveries over the years and the things I learned helped me deal with some of Mother Nature’s surprises.

Here, I’ll tell you about educational adventures in foggy weather.

valley fog
Early morning fog is usually fairly thin and it tends to burn off quickly.

It took me a while to learn that the surface temperature often drops by a degree or three when the sun comes up. Because the dewpoint remains the same, if it is just below the temperature to begin and the two merge at sunrise, fog is likely to form. If it drops below freezing, pesky frost might become a factor, too.

When I was flying a lot, over 600 hours for a couple of years, there were numerous fog encounters every year. Most were different, at least in some ways.

That fog that forms at daybreak is the easiest to deal with. For one thing, not many of us end flights at daybreak but a lot of us start out at that time. Fog at the beginning of a flight is not like fog at the end of a flight.

Early morning fog is not usually thick and if the sky was clear before it formed, which is often the case, the sun will get to work and the fog will burn off in a couple of hours.

A low-visibility takeoff before the fog burns off is a matter of preference and should be done only if a pilot brings excellent flying ability and a good understanding of the risk. There have been frequent loss-of-control accidents after low-visibility takeoffs and when there is such an accident it likely just means that the pilot was not up to the task.

How low the visibility can be and still allow a takeoff is an individual decision. The military used to teach zero-zero takeoffs. Hoods used to be literally that – hoods that closed over the pilot and instrument panel, allowing no outside view at all. The directional gyros were those big old ones with a horizontal readout of heading, calibrated to a degree. That was used for directional control on takeoff and, unless the pilot misread the direction of any change in heading, this worked pretty well.

I always used the centerline striping as a guide for takeoff minimums. If I could see down the centerline for a few hundred feet, I was good to go. FYI, centerline stripes are 120 feet long with 80 feet between each stripe so I was looking for a view of a whole stripe plus at least the start of another one.

The worst accident in aviation history happened when a 747 crew started a foggy takeoff run before another 747 cleared the runway at Tenerife. The airplane taking off hit the one that was taxiing and the tragedy killed 583 people and injured 61.

That defines a clear risk on a low-visibility takeoff. I always thought about that and felt like the worst case I might encounter would be an animal, like a deer, on the runway. If that happened, I had experience hitting both a horse and a deer in a car and knew that the way to do it is to drive straight ahead through the hit and come to a stop. Don’t ask me about the horse. It’s a long story.

Really bad luck might find a piece of equipment on the runway and I remember one case where a snow plow got in the way of a jet on a low visibility takeoff. Remember, when you can’t see down the runway the tower can’t see the runway at all so you are definitely on your own whether at a controlled or uncontrolled airport.

The top of fog that forms in the early morning is usually low and it’s common to pop out after climbing just a few hundred feet. The sun shining on the wings doesn’t mean the risk that was assumed is all gone. The residual risk relates to what happens next if there is some problem with the airplane or the pilot or one of the passengers.

Farrenberg airport
Is that a good takeoff alternate?

Was a takeoff alternate considered? That would be a nearby airport with the weather at or above landing minimums. I would add that if a pilot is serious about a takeoff alternate, he’ll look at the temperature/dewpoint spread and if it is four or less consider that the airport is only potentially viable for a takeoff alternate. Fog not only forms right after sunrise, it also thickens and both the ceiling and visibility can drop at that time. I always thought of the best takeoff alternate as being one where it is still clear and with a spread of four or more.

I was in Kansas City one morning, prepping for a foggy pre-dawn takeoff. The fog had been there all night so I knew that the top of the fog might be relatively high. I did understand that the fog was widespread because it was related to an upslope condition where cool air at the surface is moved over ever higher ground and when there is an inversion, or warmer air, aloft. Because the elevation increases westward over the Great Plains, this can result in a lot of fog over a wide area when the conditions are right. On this morning, I don’t think there was a reporting station within 400 miles of Kansas City that did not have fog.

So, while there was no good news, there would be no surprises.

I was flying a Mooney 252 because my P210 was in for an engine overhaul. I had flown Mooneys quite a bit and felt comfortable making a low-visibility takeoff in the airplane.

The terminal forecasts were calling for steady improvement through the morning but when I popped out on top of the fog just as dawn was breaking I knew this was not going to happen. There was a high overcast so the sun wouldn’t be working on that fog.

That left me with no option other than the original plan because none of the airports that I overflew had anywhere near minimum weather for an approach. So I would motor speedily on (over 200 knots groundspeed) toward my destination, Mercer County Airport in New Jersey, and hope that nothing interrupted that motor. I did know that the upslope condition would not be present farther east and I flew into CAVU conditions about half way into the 914 nm trip. The area around Kansas City stayed fogged in all day.

Dealing with fog on a departure is pretty simple. If you are good at low visibility takeoffs, and if nothing goes awry with the airplane or the people, or on the runway, everything works well. Arriving in foggy conditions is far more complex.

The standard IFR weather minimum for a precision approach is 200 & ½. The 200 would be considered the decision height: nothing pertinent in view there, it’s a missed approach. The visibility is a requirement in for-hire flying. Major airports have runway visual range measuring equipment and minimums are expressed in feet of required RVR.

Under Part 91, not for hire, it is perfectly legal to attempt an approach if the reported weather is below minimums and it’s legal to land out of that approach if the proper things can be seen because our requirement is for flight visibility and if you can see the light(s) at the appropriate time, you apparently have the flight visibility. And if you tell a fib and then flub, nobody will know about it unless and/or until the NTSB tells them.

RVR sign
When you start measuring visibility in RVR, things are getting foggy.

If the visibility is 5/8ths of a mile or less, the obstruction to visibility is referred to as fog (FG). Above that, it is mist (BR). If it is foggy, vertical visibility (VV) replaces ceiling height in the report. If the VV is given as 400 feet you might look straight down and see ground or lights from that altitude but what you can see when looking straight down doesn’t really count for anything on an approach.

The problem with reported weather at most of the airports we use is that it is but a snapshot at one point on the airport. With automatic equipment, a vertical sensor measures cloud height and a horizontal sensor takes care of visibility. The information is massaged for best accuracy. That massaging is complicated and if you want to read about it, query ASOS on the NWS web site.

Humans still make a few weather observations, and they can add things to an automated report, but however you slice it, there is no report on the conditions on approach when a half a mile from the end of the runway.

Fog is the most difficult condition to deal with on arrivals because when it is present, the forecasts of improvement are not often accurate unless something like a front is coming through to get rid of the fog.

I learned an enduring fog lesson years ago. I have told of it before so will be brief.

To accommodate the number of passengers, we used two brand-new Piper Apaches for the trip from Little Rock to Monroe, Louisiana, and back. The weather was fine going but for the late evening return, fog was starting to develop at Adams Field in LIT.

An old friend was the only person in the tower and when we were in range he told us there was no ceiling or visibility. The old-hand ace of the base flying the other Apache was ahead of me, the new kid pilot of the base. He said he’d try it and in a few minutes reported down and clear of the runway.

Do you want to try it, Richard? I thought about Ansel’s question for a moment and then said Yes. The other pilot made it and if I flew my folks to the nearest good weather they would be going to bed 110 nm from home while their friends in the other airplane would be at home.

Glideslopes had not yet come to this class of airplane, but I had an idea of how to fly the approach. I did, and I landed N3222P without seeing much other than a runway light or two glowing in the dark.

The passengers said nice job! but before I found my way to the ramp in the dense fog I had decided that was the stupidest thing I had done in an airplane and vowed never to do it again. When totting up the things that could have gone wrong, the list was pretty long. That was about seven years into my flying career. In the 50 years that followed, I managed to avoid anything exceeding that burst of stupidity.

I did learn lessons that stuck with me over the following years.

A lot of pilots feel like the lower you go in inclement weather, the more you will see. That is certainly not true in dense fog. The temperature/dewpoint measurements are taken on the ground and if they are together that means the clouds are on the ground. The night I made that dumb foggy landing, the visibility sure didn’t improve right at the last. If anything, it got worse.

Fog around approach lights
With fog, conditions don’t get better as you descend.

If fog is present as night falls, improvement will come only with a basic change in the weather, regardless of what might be forecast.

Another lesson was and is that airports in close proximity to rivers have more frequent fog problems than others. Is that why the airport founding fathers built so many airports next to rivers? It probably had more to with the availability and value of land than with fog.

Adams Field was right on the Arkansas River. Mercer County Airport (now Trenton Mercer), TTN, in New Jersey, where I based for 22 of my most active flying years, was right on the Delaware River. I learned a lot of fog lessons there.

I didn’t make a lot of low-visibility takeoffs at TTN but there were a lot of foggy evening arrivals. The ILS to Rwy 6 came over the river and the approach was over a swale off the river that would fill with fog. The result of this was a lot of missed approaches, most with the reported weather at or above minimums. I learned early to not even try if the reported weather was below minimums.

I had two close-by alternate airports, North Philadelphia (now Northeast Philadelphia), PNE, and Philly International, PHL, which was hard by the Delaware River. PNE was the farthest from the river and occasionally I did a successful divert to PNE. I think I was able to use PHL once. The tried and true real alternate was Allentown (now Lehigh Valley International Airport), ABE, and that was used a number of times. By road, it was probably 55 miles from ABE to my home in New Jersey, over hilly, winding two-lane roads, and more than once I wondered, after reaching the dense fog, if driving in it wasn’t at least almost as stupid as landing in it.

I have seen what might be called patchy fog a few times though I always thought of it as scud, defined as small, ragged low cloud fragments.

I was returning to my Hagerstown, Maryland, base one day with forecasts of pretty good weather. Actual conditions were pretty bad, below minimums, and smoke was coming out of my ears as I was considering alternate airports. The closest good weather was out around Pittsburgh with Dulles, IAD, the only nearby airport reporting 200 & ½. My suspicious nature had led me to arrive in the area with enough fuel to try an approach at Dulles and, if not successful, fly to good weather with a generous reserve.

Fog on runway
Those centerline lights at big airports do help.

I guess the airlines were not launching airplanes to IAD because I seemed to have the whole place to myself and vectors to final for Rwy 1R started as soon as I requested the approach. Dulles has the excellent approach and runway lighting found at all major airports and the runways are wide and long and welcoming. I guess I am hinting that you would have to be a real kiwi to miss an approach at Dulles in a light airplane.

As I was nearing the decision height, I had some doubts because in my peripheral vision I was aware that there were what appeared to be cloud tops below me. At the DH I looked ahead, saw nothing but white, and started into a missed approach. Then I saw runway lights, enough to descend and land visually. Scud or patchy fog was sure present that day,

I was a little amused when the tower asked a vehicle driver on a taxiway if he could see whether or not I had landed. He said that I had, but that I had landed a bit farther down the runway than normal. That was true enough but on an 11,500 x 150 foot runway, I could afford to give a little back.

When flying a low visibility approach, we are dealing with slant-range visibility, which is not addressed in any report. Even if low clouds are scattered, or scud is a factor, it doesn’t take much cloud to obscure the view of the runway from the DH. If it comes into view during the missed approach, do consider how much runway might remain for use if you decide to land.

Weather wisdom: What you see is what you get. When it comes to fog, what you see is. . . not much.

4 Comments

  • Thanks RC for the “lack” of insight in fog!!
    A perfect application for a drone would be at FAF ? DH? with the capability to read the slant range…… “lack” of visibilty.

  • Great article, Dick, as are most of your articles.

    For more than thirty-five years, my home base was Anchorage, Alaska. Surrounded as it on three sides by water, fog was often a visitor. I remember one night when I had made a return flight from Talkeetna, about 100 miles north of Anchorage, only to find a thick fog layer over the whole town. The tower at Merrill Field was reporting an observed ceiling of only 60 feet.

    My Super Cub was instrumented rated, after a full rebuild, but Merrill had no instrument facilities, so I was a bit stuck. Kenai, to the south, was wide open, and only about 45 minutes away. With a gentle north wind, I knew that fog would move along toward the south in a short time. But, how short?

    I throttled back to best glide speed plus 10%, flew closer to the Chugach Mountains to the east to hold above the fog, carefully calculated remaining the flight time I had left, and decided that I had enough fuel to lollygag around for about 45-minutes before I’d have to light out to my alternate field at Kenai. I listened to C&W music coming through my big Davie Clark headset for the holding time.

    About 30 minutes later, the fog had moved south enough that I could make a nighttime VFR landing at Merrill. Knowing, by past experience, that the fog would be moving out was a blessing. Without that experience, I would have voted to go to my alternate right away, blowing the costs of two meals and a night’s lodging at a local Kenai motel.

  • Great article. When you mentioned Trenton Mercer County it brings a story to mind. My friend and I were dropping off a Baron 58 at Trenton Mercer County for maintenance and we departed Monmouth county executive in the early morning with light fog. We were on top at two hundred feet with a short smooth ride to KTTN. Listening to ATIS we knew we would have to fly the ILS 6. We flew the approached and with nothing in sight, executed the missed due to heavy fog from the Delaware river. Upon climb out and breaking out again at about two hundred feet on our way to the hold we noticed that only half the field was covered in fog including the tower. We contacted Philly approached and asked for the visual for runway 24. We landed on runway 24 in VMC conditions.

Leave a Reply

Your email address will not be published. Required fields are marked *