Chapter 7 - Stratus Clouds and Fog
It is hard to remember when I first became fascinated with clouds. I know it was when I was very young. As a teenager, I made two types of cloud chambers, trying to capture them and see what they looked like close up. Even then I couldn't see the stuff that made up a cloud. As a Boy Scout and later an Explorer, I learned the names of the more common clouds and fogs. It seemed to me that each name meant, at least in principle, that each type of cloud could be made of different stuff. I think it first came relatively clear in high school chemistry class that radiation fog, cirrus clouds and arctic sea smoke weren't made of different substances. For the record, natural clouds are all made of the same stuff, water.
The water in clouds will generally be in two phases, vapor and liquid. But, occasionally, the solid phase of water, ice, will come into play. I know some meteorologists and acid rain specialist will disagree with that statement because the water will have some contaminations in solution, but even then, clouds are still mostly the same stuff that comes out of the taps in your kitchen sink; the acid component really goes along for the ride, doing its chemical reaction thing, until it lands on cars and vegetation and then the reactions can be more visible. Also, volcanologists do have a point. The dust cloud coming out of an active volcano contains very little water. Should you decide to penetrate one, your engine may complain and falter. That sharp, mostly silicon dioxide dust can clog filters, grind down pistons, and enlarge cylinder walls. It doesn't do much for the paint job either. If you have a question, ask Flight Watch. The system seems to be pretty good in identifying, locating and following volcanic dust clouds.
Water in its three phases is the very same material which makes the clouds which resemble billowy castles in the sky one day and flying dragons the next. It is also the same stuff we call "steam" which comes out of the teakettle or when you can "see your breath" when you go outdoors on a cold day. The term "steam" when applied to the plume emanating from a teakettle is a misnomer. Technically it is cloud; steam is reserved for water vapor under pressure, usually in steam boilers and pistons which drive steam locomotives or in steam heating systems.
The cloud which you see coming out of a teakettle is really made up of little droplets which are condensed when saturated air at high temperature mixes with humid air at low temperature. Mixing is straight-forward. One cubic inch of stuff at boiling temperature mixing with one cubic inch of room air gives two cubic inches of mix at the average temperature. Some of the water vapor which came out of the spout precipitates out on any surface it can find, usually dust.
Fog and stratus clouds are really mixing clouds, formed in much the same way. Probably the best way to think about it is to look at how simple stratus clouds are formed. Simple stratus are produced when there are two layers of distinctly different but moist air, one over the other as in Figure 7-1. The smallest stratus I've ever seen were only a few inches thick and hung over the frozen foods in the chests at the local supermarket on a very humid summer day when a supermarket air conditioner wasn't working right and there were too many people going in and out the doors. The thickest stratus can extend from pattern altitude upward into elevations where turbocharging and oxygen are required. And, the only real difference between simple stratus and fog is that the lower layer is the thin skin of air just over the ground or water which is modified by the underlying surface.
I almost got caught in some very thin stratus when I was just learning to fly. I had been signed off for solo and was happy and sassy on a clear blue day. There were some slightly reddish patches high in an azure blue sky which I didn't remember ever seeing before. Once I had completed my planned air work for the day, I realized I had an hour left. So I decided to go exploring. Moving carefully away from the TRSA, now class B airspace, I started up, head on a swivel, just to see what the world was like at 7,000 feet.
As I climbed, the sky darkened into a gorgeous deep blue, perhaps the first time I had really seen the sky that shade. I remember thinking maybe this was the way it looked from the space shuttle. Must be that the double windows on the 727s cut out part of that hue. Off in the distance another big jet sparkled as the sun glint caught my eye. I redoubled my scan for aircraft nearby.
I was concentrating on the scan near the horizon and marveling at the way the ground looked from my front row seat. I never noticed the patch of reddish haze above me. Suddenly the mountains off on the horizon started to disappear. The flight visibility was deteriorating rapidly. The airplane suddenly seemed to be awfully warm. Remembering what my instructor told me, I realized that the airplane was well trimmed so I reached over, set the mixture to rich, turned the carb heat on and moved the throttle to idle. I put my hands in my lap and feet flat on the floor. Slowly the altimeter unwound slowly and the visibility started to improve. My shirt was still wet when I tied the airplane down.
Thinking back on the experience, I was legal in the strictest sense all the time. But, the proximity of the big three, BWI, IAD, and DCA, increased the probability that a large aluminum chariot would be descending to some intersection in preparation for landing its hundred or so passengers. Since it would be doing three miles a minute and I was doing two, the time for action could have been relatively short. And I was not being watched by ATC.
The stratus which gave me a very damp cause to reflect were
probably simple stratus with a bit of mountain wave thrown in. But the mountain
wave just made the mixing of the two layers a little more localized; the mixing
between the two layers formed the clouds. The same principles applied as they
do to all meteorological clouds; it was caused by the cooling of moist air and
the moisture condensing out on dust.
The condensation of water isn't a simple thing. The equation which describes condensation on dust is quite complex and involves the saltiness of the dust itself. The equation which gives the maximum allowable water vapor content at a given temperature is simpler and is shown graphed out in the curved line in Figure 7-2.
To see how this graph works, suppose you had 2.9 pounds of water vapor at 30 degrees. If you cooled it to 20 degrees you would have, according to the curved line, 1.6 pounds of water vapor and the rest of it, 1.3 pounds, would have condensed to become liquid water. It really shows how much water can be in vapor form at any given temperature; it is approximately equal to the number of kilograms of water vapor per 770 cubic meters. Incidentally, the same graph works in space where there is no air. The result is most noticeable in the formation of a comet's tail.
When a simple stratus cloud forms (the type shown in Figure 7-1) the air between the layers mixes so the dew point temperatures in between the layers equation follows the straight dotted line in Figure 7-3. The layer below the cloud is moist and cool with a temperature just above the dew point. The layer above the cloud is also moist and just warm enough to be stable over top the lower one. Both layers are clear. When they mix at the interface, the temperature of the mixed layer becomes the average with the warm air being cooled and the cool air being warmed.
Suppose the dew point temperature in the lower layer is 5 degrees C and the one overlying the cloud 25 degrees C. This situation can happen in the atmosphere in the winter and almost always causes instrument meteorological conditions (IMC) to develop. When the warm air mixes and cools, the air is saturated. As it cools further, the moisture content is unstable and the dew point of the mixed air will be at the air temperature. The water must condense. Since the average is 15 degrees C, the difference in the moisture content between the straight and curved lines condenses out on dust, sea salt particles or whatever else is there to collect dew. In real clouds, the differences aren't as pronounced, but the idea is the same. If the mixing is just right, a nice layer of stratus cloud results. If the mixing is too violent, the stratus will be torn apart as soon as they are formed.
A jet engine produces hot exhaust which contains a lot of water vapor, a product of combustion. The air coming out of the exhaust is at a pretty high temperature and nearly saturated with water vapor. As it mixes with the very cold air around the jet exhaust, the air mixes, and if the environmental dew point is high enough, contrails will form. As you are watching a jet cross the sky leaving a contrail, the little gap between the plane and the contrail will give you an idea of how fast this mixing can occur. The length of the contrail tells you about the humidity at that flight level. Sometimes the environmental air is dry enough that the contrails evaporate rapidly or don't form at all. When the air at flight level is moist enough, the contrails persist, gradually spreading out across the sky. In most cases, the liquid water freezes and slowly move downward; however, very tiny droplets can exist at temperatures of -40 degrees for long periods of time.
If you live in an area in which you rarely see contrails, look near the spout of a teakettle when it is boiling to see the gap, or check out your breath on a cold humid winter's day. They are formed by the same processes, when the excess moisture condenses out on condensation nuclei forming stratus clouds.
I have seen thin stratus a number of times since my first encounter during my solo days, but rarely in a clear blue sky. Usually higher stratus clouds are fairly thick. The thin stratus layers seem to me to be found downwind of the mountains. Stratus formation is enhanced by upward motions caused by the undulations of the mountain waves. It's easy to see how a VFR pilot can be trapped between layers of stratus. They aren't always easy to see when they are forming. When viewed from the top or bottom a thin stratus layer is almost invisible. They are most visible nearly edge on.
One Saturday, the wife and I were flying just to get her acclimated to the
Piper. We decided to fly to
The nice thing about stratus is that they are rarely a problem for the VFR pilot and are generally an advantage for a nice flight. The smooth air below stratus clouds makes flying much more comfortable as the bounces and jiggles caused by rapid surface heating are missing. Folk flying IFR generally don't like them as well because they are caused by mixing which means some turbulence. And, being in the soup, an IFR pilot runs the risk that there is a developing cumulonimbus just ahead. If there is a chance of imbedded thunderstorms, the best advice I have for an IFR pilot is to request an altitude which takes you below or above the deck, depending on the most advantageous winds. Then you can see and avoid the big thunder bumpers and the buildups. The simple truth is that a buildup can provide substantial interest in the fifteen minutes or so before it produces the first rain or snow or the first lightning bolt. Even then, a really active buildup can toss aircraft around before they start producing particles big enough to show up on radar or lightning detector.
The other place where simple stratus causes problems is the warm front. A warm front is a place where cold air near the surface is generally retreating from the front. Just north of the warm front, moist air is streaming in overtop the cold, the same air which was, a little while ago, moving north in the warm sector of the storm. Mixing occurs readily in a situation like that. In fact, the meteorologist looking for a warm front will look first for the tell-tale reports of fog east of the low. Fog, coupled with rapid decreases in pressure as the cold air departs, are the major clues to warm frontal location. Warm fronts are often difficult to locate and keep track of; they sometimes appear to jump. If the cold air is thin enough in the vertical and the mixing is strong enough, the mixing may occur over a relatively large area, perhaps the size of the state of Massachusetts. Relatively violent mixing (and this vertical mixing is probably not noticeable from the wind reports which only measure the horizontal components of the winds) can cause the fog in an area, just used to locate the warm front, to dissipate. There will be new fog formation where the warm sector meets the retreating cold air and the front will reform there. Occasionally the phenomenon will be called a "jumping warm front" but it really doesn't jump.
Stratus clouds have some advantages for people who study clouds and what they are made of. One of the best places to study them is off the west coast, where the upwelling oceans produce fine stratus which hangs in there for days. Specially instrumented aircraft can probe them to find out more about the details of the material we see as clouds. The thought was, the more we know about clouds, the better decisions we can make about seeing through them. And, maybe there is a device which can provide pilots a better way they can bring home the aluminum and all it contains safely.
A number of years back, a research associate in our research project came up with a good way of measuring the size of the droplets in clouds. Before that, if a person wanted to find out what the drop sizes in a cloud were, he or she took some oiled microscope slides up and held them out a window of an airplane. Not only did they have to look at the slides with a microscope quickly as the water would evaporate, the prop wash probably influenced the measurements. Paper tapes impregnated with water soluble dye were an improvement but paper wasn't uniform and the size measurements couldn't be trusted.
The instrument we used was built from the sprockets and some extra parts from an old 16 mm projector. Slung in a pod below a wing, the motor driven sprockets pulled a film coated with gelatin past a carefully constructed opening where the droplets came in. When the droplets hit the gelatin, they dissolved the smooth gelatin surface leaving pits when the water evaporated. Back in the laboratory we would look at the film with microscopes and measure the diameters of the pits. Of course, every once in a while the sampler would run into a big drop and the water would smear out a few inches of tape. But these were actually relatively rare. Even though it sometimes looks as if you're flying into a solid wall of water, there are actually only a few really big drops per cubic yard.
The droplet collector worked well for measuring drop sizes in clouds and was used to provide statistically significant results on the droplets in various types of clouds. It, as well as other similar samplers, has been used to sample stratus clouds and fogs. If you had just the measurements, you couldn't tell the difference between the fogs and stratus clouds. Figure 7-4 shows a typical distribution of droplets from my research with the horizontal axis showing the radius of the droplet. While these were sampled from stratus clouds, the droplet distribution in a fog looks much the same.
The vertical axis represents the number of cloud droplets which occur in the interval on the horizontal axis. For example, there should be around 32 droplets which have a radius between 5 and 7 micrometers. I hasten to add that this histogram is a schematic example of all types of clouds. Different types of clouds may have different shapes; however, there are very few droplets which have a radius less than 2 micrometers.
I had the opportunity to examine a few hundred feet of the film using a powerful microscope. All I had to do was identify any and all items smaller than 2 micrometers as well as determine the droplet distribution for each half meter of coated film. Using every trick in the book including light interferometry with oil immersion techniques, I found dust, dirt, wiggles in the gelatin, but no droplets. Either the tiny ones don't exist, or they do exist but don't last long enough for the collectors to measure them. The equipment was able to pick up particles of that size in the lab. But, there were apparently none in the natural clouds we sampled. I found out only after my searches that I hadn't discovered anything new. Other people hadn't seen them either.
The physics says that larger droplets attract water vapor more than small droplets. It may be that the smallest droplets form and then evaporate as soon a larger droplet comes near. The evaporation or condensation can be rapid. In some cases, when the dew point is low enough, the evaporation of water vapor from fairly large drops, even an eighth of an inch in diameter, is enough to support fairly large drops. Probably you have seen these droplets dancing across the car hood sometime when you are washing the car.
Cloud, fog and haze droplets are formed when there are excess water molecules in the air. The excess molecules condense out on sea salt, dust, dirt and other tiny things in the air called nuclei. There are three types, condensation nuclei, freezing and sublimation nuclei. Condensation nuclei are soluble in water and usually form lots of ions. Sodium chloride, ordinary table salt, is a quite good one, but sea salt is better; to paraphrase that old advertisement, when it rains it doesn't pour.
The oceans are a remarkable source of material for the atmosphere. Each breaking wave produces bubbles, which in turn shoot tiny droplets into the air when the bubbles burst. These tiny droplets in turn dry out in the air leaving bits of sea salt, called condensation nuclei. Many of the droplets rise in the air and dry out leaving tiny crystals of sea salt. Sea salt is a mixture of salts, NaCl, MgCl2, MgSO4, and others. The magnesium salts are the impurity in inexpensive table salt which absorbs water in the shaker causing the stuff to stick. It does the same thing in the atmosphere and the sticky feeling of hazy air owes something to the sea.
Not all of the dusty garbage we throw into the air from daily living and commuting works as condensation nuclei. One year, the cane fields on the Yucatan Peninsula were unusually dry and burned. Looking down from the satellite perspective, we were able to follow the smoke plumes across the Gulf of Mexico and as they crossed Florida. There were no clouds in the smoke plumes. None. It is as if the sugar-laden smoke soaked up any moisture and then dropped out sight. Central Florida had a drought that summer, only breaking when the burning cane fields went out.
Once up in the free atmosphere, the water vapor and the condensation nuclei move along with the air. If the air goes up, so does the water vapor and nuclei. If there is enough upward motion in the air, at some point, the temperature and the dew point will converge and water will condense out on the condensation nuclei, forming clouds. Once formed, droplets take some time to dry out. Researchers have taken some of the droplets from the sea spray and then dried them out in a chamber, then increased and decreased the humidity around the droplets while measuring the radius with a microscope. Figure 7-5 is typical of the type of distributions which were found. This one, taken from Fletcher's book The Physics of Rainclouds, is typical of the various types of salts that a number of studies have shown.
Cloud nuclei need to be bathed in air almost at 65% humidity before they start to grow.
And, then they grow to haze size as long as the humidity doesn't change. Once the particles have grown to a certain point, if the relative humidity increases, the particles will grow larger to drops. If, however, the relative humidity drops but stays above 40%, the particle's radius drops down the curve and but will still be seen as haze even with humidities around 40%. A haze particle will dry out at low relative humidities. If the humidity starts to rise again, the haze particle will start up the left-hand path again. People who have studied some physics and engineering will recognize a Hysteresis Loop.
When the temperature reaches the dew point, at cloud base, the humidity is very nearly 100% and the droplets are at the upper right of Figure 7-5. Once the humidity reaches 100%, dew starts to form on these particles and the cloud is born. Droplets grow rapidly from haze to cloud drop size, but they are still very small. Most of them have a diameter less than 10 micrometers.
A number of years ago, one airline actually hired cloud seeders to clear a path for unloaded air carriers to take off through the shallow fogs at San Francisco. Expensive hardware sitting on the ground did not help the bottom line. When a fog occurred, the drill was to line up a "Follow Me" truck with the seeder plane right behind and then the departing aircraft all lined up on the runway. When all was ready, the truck revved up and started down the centerline of the runway with the seeder plane right behind. When the parade reached takeoff speed, the seeder plane rotated and the truck got out of the way. The seeder plane then made a number of passes over the runway, dropping finely ground ammonium nitrate (fertilizer) dissipating the fog. The fertilizer soaked up the moisture and settled to the ground. When the visibility was good enough, the air carriers launched, one after another.
The experiment sort of worked, but the project was canceled because the winds would often carry the fog upwind of the runway too fast to allow more than two planes to launch. At other times, vortices of the first airliner mixed the fog into the cleared area too fast for more than two big planes to get off per seeding run. When the bottom line was discussed, it turned out to be just too expensive. Then, too, the operation was more than a little risky. And, I sure wouldn't want to have been the driver of the "Follow Me" truck with those two spinning blades snuggled up behind me and having to concentrate on following the line at 80 mph in fog.
The fog problem for almost all pilots usually occurs during the last few hundred feet of an approach. If you can't see the runway there is a chance of missing it and coming to a very abrupt halt elsewhere. While the ILS beams run right down the center of the runway, unless you can hold the needles right on the centers, and there's no other recourse (as in the situation faced by Trever in More - I Learned About Flying From That, Dover Press), knowing where your alternates are and which one will be most likely to be usable is important. To judge that, you need to know something of the nature of the fog and what's happening to keep it foggy.
The other piece of the puzzle is the nature of light itself. If you combine the properties of light with the distribution of droplet sizes, it's pretty easy to see why you can't see a fog. The explanation begins to work when you make sense of the scales. The diameter of the average cloud droplet is just about ten times one wavelength of the light you look at them with. Think of light as a bit of space containing a train of electromagnetic energy containing at least 30 wavelengths; there may be many more waves in this train. Nobody is sure of the width of one of these bits of light called photons but it is probably pretty large.
Somewhere within that space, think of a tiny little mass which varies with the color or wavelength of the light. The bluer the light, the more concentrated the mass, and the more massive. If the light hits something large and opaque like you on the beach on a sunny day, the mass converts to heat energy. That mass is much smaller than that of an electron and the size of the mass is around one thousandth of a wavelength. The photon has no mass after it hits something and the energy is absorbed. If it did, we would be picking up weight just walking outdoors on a sunny day.
When a bit of light hits a molecule of air, if the wavelength is compatible with the molecule's energy levels, it is absorbed, energizing the molecule. But being a lot lighter, it doesn't do much to move the molecule. Sort of like a motorcycle hitting a fully loaded semi. Most molecules can't stay energized and release the bit of light immediately but the light's direction is changed. This is called molecular scattering as the light can be re-emitted in any direction. Some molecules can hang on to the light for some time but air molecules release it almost immediately. I have a couple of key chain fobs which glow in the dark made up of a plastic containing some of the molecules which hang on to their photons and release them grudgingly.
The color of the clear blue sky is the result of molecular scattering. The blue part of sunlight scatters much more than the red part. Lord Rayleigh showed it was related to the fourth power of the wavelength. The blue part of sunlight traveling overhead scatters quite well and some of that comes to your eye. The red part of sunlight doesn't scatter well and just goes on through. If you go out some clear evening a couple of hours before sunset, somewhere to the west, the sun is setting. People out looking at the sunset there will see the red which came through your area. You will see the blue light which is being scattered out. As the evening wears onto sunset, the light coming through the air to the west will have the blue scattered out. The remaining light coming straight through will be red. Sunset on Earth, as viewed from the Moon, is almost totally red.
Haze particles are larger than air molecules. These scatter red light as well. So haze often has a reddish cast. The colloidal chemists, especially those who work in photography, know that particles of a certain radius are bathed in light, they appear to be black, that is absorbing all light. There have been some reports of "black stratus"; however, there has been so much argument about these, I feel it is best (read politically correct) to remain undecided as to whether they exist. No reports of these clouds have ever been connected to unflyable weather. If they do exist, the particles should have a diameter around the average wavelength of sunlight, according to the colloidal chemists.
When light hits an object which has a diameter about ten times the wavelength or so, it sort of oozes around the object and scoots off in another direction. To compound the problem, the light doesn't scatter just in any direction, but it has preferred directions which are related, not simply, to the size of the droplet. For real small drops about the size of fog droplets, light prefers to scatter either close to the original direction, called forward scattering, or back in the direction from whence it came, called backscatter. As a gross oversimplification, the amount of forward scatter and backscatter is about the same.
You can think of forward scattering light as bullets which have just grazed a pool ball. The light is just deflected a little bit from its original course. In a fog, there are enough particles so the light scatters a number of times each time at slight angles from its original path. And since there is some backscatter involved, the light may bounce back and forth enough that the world looks just white. Since the light coming from a landing light bounces off fog particles many times, you can't see the direction it came from originally. The same process occurs from illuminated objects such as the centerline of the runway or worse yet, the landing lights.
Backscatter is similar except that the light bounces back towards the source. But the sort of reflection is not as nice as in a mirror where the mirror particles are much smaller. Tractor trailers have an edge on four-wheelers when it comes to driving in fog. The angle between the headlights, the fog particles, and the driver's eyes is larger so drivers in the office of their semis are not as affected by backscattering light. For smaller cars, the angle between the light coming out of the headlights and the driver's eyes is smaller, so we see much more backscattered light.
Occasionally a pilot will get into the pattern at an airport reporting one quarter of a mile visibility and wonder what the problem is. From the air, the ground is clearly visible. What is happening is the bright sunlight is coming in being scattered at an angle nearly the same as the angle of the sun above the horizon. This usually happens just after dawn.
One person who got into a situation like that described the predicament well. He and his wife were up before dawn and, on a whim, decided to fly from their home airport to a neighboring airport for breakfast. He could see one plane at the pumps, another headed that way, and a number of pilots preflighting their planes as he came around the pattern. Yet, the Unicom operator reported a tenth of a mile visibility with the sky obscured. He described the visibility from his point of view and said that he was going to try it.
He reported on final, but being somewhat suspicious, he made it a somewhat longer final and spent a few extra seconds trimming. As he descended below 500 feet, all of a sudden the fog was apparent and what had been visible a few feet higher disappeared. Fortunately, he was well trimmed on final and just went around. When he gained a few feet the visibility shot back up to 14 miles. Faced with a dilemma, all he did was to go out over some nearby farmland, throttled back, leaned for maximum range on nearly full tanks, and made some lazy circles. He told the Unicom operator what he was doing. When Unicom reported enough visibility to land he and his wife came in for breakfast. Of course, he could have gone to an alternate or gone home for breakfast, but he elected to stay until the sun "burned off" the morning fog. Evidently the airport restaurant put on a pretty good breakfast.
These few hundred of feet of fog and then severe clear above the fog has frustrated many a pilot standing on the ground. Sometimes you can see contrails if you look straight up through the fog. If you are up to launching IFR into fog, and rumor has it that some commercial operators are, there really is no problem as long as the hardware doesn't break and your technique will get you up and away. Otherwise, the problem becomes the same as the folks going to breakfast except that your choice of alternates is greatly diminished.
The next time the weather is zero-zero, walk out into it and take a look. Try to identify an individual fog particle. It's tough even if you have perfect eyes. In fact, except for an occasional small mist droplet, the foggy part is really invisible without magnification. Yet the total effect of tens of thousands of these fog droplets is certainly visible.
There are at least six relatively common categories of fog: radiation fog, advection fog, Arctic sea smoke, up-slope fog, precipitation-induced fog, and ice fog. There are more. The difference in names (except for ice fog) really come from how the vapor got into the air and cooled enough for the fog to form in the first place, the processes which go on to form the cloud-on-the-ground. So, naming of the fog tells something of how it was formed. All the fogs are made up of water droplets or ice crystals which are just small enough that they aren't visible to the naked eye.
Radiation fog is a common type in the late summer. It occurs in the relative calm of a late summer or fall evening (until an hour or so after dawn) when the ground cools because there is no sunshine to balance the outgoing infrared radiation. When winds are calm, the temperature of the air next to the ground is at a minimum an hour after dawn. Since the earth radiates all the time, it takes about an hour for the incoming sunlight to become stronger than the earth's outgoing infrared energy headed to space.
As the land and vegetation cool in the evening, the air in contact cools as well. The gradual mixing of the air cools the air above it. At some point, the air at some level cools to the dew point, and the water vapor in the air starts to condense out on something. It may start to condense on the leaves, trees, and cars as dew, or it may condense on condensation nuclei in the air as fog. Most probably it will condense on both, resulting in a fog over a dewy surface. An example is the "cat's paws" type fogs which are common in late summer and autumn.
This type of fog can cause all kinds of problems. I was in a conversation with a friend, an instrument instructor at the local FBO when she asked, "Why do you guys lie?"
Somewhat taken aback, I inquired as to what she meant.
"Well, yesterday I was instructing a student on filing an IFR flight plan and we were all set to go when we couldn't get clearance because of fog," she replied, somewhat indignantly. "The sky was clear, almost severe clear."
Thinking back to yesterday, I replied. "Yes, I had to go over to the forecast office for a meeting and the weather was nice. But when I was about five miles from the office, I ran into a wall of fog. At the forecast office the fog was really thick. And all of the data we received showed fog, the best was a half mile visibility. Where was your certified observer?"
All she said was "Humph." But her tone implied I didn't want to know.
Back at the office the next day I looked up the satellite picture. Sure enough, there in the center of a hole in the fog, in beautiful clear skies, was Montgomery Airpark. I made a copy of the satellite image and gave it to her the next day.
"That didn't help any," she said. "But now I see why. Can I keep this image? I want to show it to some people"
About three months later, I heard that Montgomery Airpark was going to get
an AWOS unit. Evidently radiation fog wasn't about to spoil her day again.
The general rule for fog formation is that if the temperature difference between the air and the surface of the earth or water temperature is about 20 degrees or more, look for fog. Or, if the temperature and dew point are almost the same, again consider fog in your planning. The idea behind the 20 degree difference is that moisture from the ground or water will evaporate retaining the temperature of the surface for a short while. It will either cool the air causing condensation of water vapor already in the air, or warm the air by condensing itself. Either way you get fog.
Advection fog occurs when warm moist air moves over a cool surface land or water. The well-known fogs of the Grand Banks and those which roll in over San Francisco and Erie, Pennsylvania, are examples. The cold water cools the air just above it to the dew point and droplets form, or as some would say, fog sets in. The rolling flow over the cool water surfaces mixes the cooler foggy air upward and can reduce the average temperature through a considerable depth of air. In the case of the legendary fogs over the Grand Banks, the cold water is the Labrador Current which flows from the Arctic. In the San Francisco case, the cold water comes up from the ocean bottom off the coast. Frequently the size and shape of the ocean upwelling current can be estimated by measuring the size and shape of the fog off the coast of California.
This type of fog can also be induced by any falling water drops. A fellow I know built a short grass strip which was adequate for the Aeronca he had. Unfortunately, the only place on his property which was level enough for the strip had a waterfall near the end. On warm muggy days you could see the fog coming off the cool water droplets coming over the falls and going right over the end of the runway. The cool water droplets set off the chain of events and it sort of limited the use of the strip.
Steam fog, or Arctic sea smoke is a close cousin to advection fog except that the temperatures are switched. In this case, cool air moves over warm ground or water. The faster moving molecules in the liquid evaporate only to be surrounded by cold air. They then condense on nuclei and, if there are enough of them, become fog. You don't have to be in the Arctic to see this kind of fog. I've seen a steam fog form over the Potomac River late in the fall. The wind may be blowing along at a brisk clip while the fog was forming, so a 10-knot wind at the airport doesn't mean that the fog can't be bad news for visibility. It can. Fortunately, steam fogs from small water bodies are rarely thick enough to cause a problem, but along the seaboard, if the winds are off the ocean, it can build to totally obscuring dimensions.
Steam fog also frequents airports along the Great Lakes. At least one
airport is built on a fill out in the lake. Here, cold air flowing from almost
any direction over the warm water in late fall can sock the airport in. If the
air is cold enough and the flow is from the northwest, the fog then proceeds
inward and helps produce the dandiest snowstorms, called lake effect storms by those
who love or hate them.
Up-slope fog is the same as clouds obscuring the mountains, although the
mountains may actually be a gradual slope upward. The cooling due to expansion
as the air gradually moves upward brings the temperature down to the dew point,
and fog forms. The term RDGS OBSC is one clue; however, the temperature-dew
point spread at nearby airports may give you another clue to the likelihood of
cumulogranite.
Precipitation-induced fog occurs when warm precipitation from the warm sector aloft over a warm front is cooled by cool air below. The warm molecules evaporate from the rain drops falling through cold air and, finding themselves in cool air at the dew point, condense on nearby nuclei. You probably have noticed a precipitation-induced fog in your bathroom after taking a hot shower. It is usually spotty, but in rare instances can affect a large area. Hopefully you won't run into it on approach. The difference between the fog which caused problems with the fellow with the Aeronca mentioned earlier and a precipitation-induced fog is only that the cold drops which caused his fogs came from a stream, not precipitation from aloft.
Ice fogs are simply other types of fog which contain ice crystals instead of water droplets. Most often, the ice crystals are ice needles formed at very cold temperatures. Alaskan pilots are most conversant with these fogs as the conditions which cause them frequent the North slopes. However, they can occur in the lower 48, usually on extremely cold days. If you are going to fly on a bitter winter's day, while you are doing the preheat, you might look around for halos or sun dogs which are a clue that ice crystals are close by. However, absence of halos does not guarantee the lack of ice crystals.
The pilot's problem of seeing the runway and of visibility in general has been studied extensively by both civilian research personnel and the military and the results are not simple. Even the FARs talk about three types of visibility: flight visibility, runway visual range (RVR), and slant range visibility. Only one of these (RVR) is measured and then only at a few airports. AWOS and ASOS measure the amount of light which gets through a meter of air somewhere on or near the airport which may not be the same as the visibility on the runway.
With all of the problems with visibility measurements, why bother? All the different types of visibility are aimed at trying to tell us when things are dicey. When it comes right down to it, the ability of the pilot to see to perform the operation is the critical thing. Horror stories are told about commercial operations under the pressure of get-home-itis. The attempt to quantify the problem to give the reasonable pilot some way of knowing when a problem might arise and some basis for making the decision which will keep the aluminum from substantial deformation.
So treat these visibility measurements with care. Where measurements by people are made, treat them with respect even if you know that the observer can't really tell what is visible from the cockpit. The rules were made by pilots for pilots, especially those who care about their own and their passenger's skin. Compensate, but don't disregard.
On to Chapter 8
- Cumulus Clouds and Thunderstorms or
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