Chapter 10 - Atmospheric Mirages

Mirages have fascinated people for centuries. Seeing things which aren't there is always a bit scary. The Isle of Glass seen when the weather is right, an island which seems to float on air or a ship which disappears before it should, probably raised the hair on the back of a number of necks in the past. An atmospheric visual mirage is caused by light bending from its source to your eye but you are so used to thinking that light travels in straight lines that your brain gets confused. The most common mirage is the wet pavement on a clear hot sunny day. Light from the blue sky on the far horizon is bent upward by the air next to the pavement. You are looking at the pavement but the bent light is out of place. The brain searches for similar occurrences in the past and comes up with wet pavement. After you've seen it a number of times, your brain switches gear fast enough, and if you are close to touchdown, you have no problem landing, but if you are fast, expect to float for a while.

Fortunately, there are no visual mirages which cause serious problems with flight and these are only minor problems. I have never heard of the NTSB citing a visual mirage as a cause of an aircraft accident. In fact, there are only two mirages which can be seen aloft; the others are visible only when you are on the ground, usually very close to water. In fact, they are best seen when your eyes are a few inches away from the water's surface looking over a cold lake in the north land.

There are other types of mirages, sound and mirages in radar waves which can cause problems with flight. Sound mirages aren't a safety factor, but they can cause problems with neighbors to airports. They are very real and usually cause the most problems at night. Mirages on radar images can be confusing and might cause real problems in a few rare instances.

I found out the reason. Seems water waves travel slower in shallow water than they do in deep. At Daytona, the beach has a very gradual slope to the water, and the bottom slope is very gradual out to perhaps an 80-foot depth halfway to the Bahamas. So each wave that comes down the coast has one end in the shallow water along the beach. The shallow end slows down.  Since the wave doesn't have a gap in it, the shallow end turns into the shore. Figure 10-1 shows a top view of some crests of waves moving from left to right as they curl into the beach. In a sense, this is a mirage in water waves. Since wind causes water waves, looking at the surf isn't a good way to estimate where a storm is. Even if the storm is to the north, the waves at the beach make you think the storm is almost straight out. You can get a general idea of the winds, but an accurate determination is almost impossible.

The same idea seems to work for other mirages. Unlike water waves, the speed of sound through air is dependent on the temperature of the air, actually the square root of the temperature. When the nocturnal inversion sets in, sound travels differently from a hot sunny afternoon. The inversion is an increase of temperature with height so sound travels faster aloft than it does near the ground.

Figure 10-2 gives an illustration of the sound waves, the maxima of pressure, coming from a source such as a loudspeaker on the ground. A graph of temperature with altitude is shown on the right. The path of a point on a particular sound wave, what one might call a "ray of sound", moves out from the source. The sound waves and their energy are concentrated near the surface.

I learned at an early age that fishing in a boat at night on a cool lake is not the place to discuss things that you don't want everybody on shore to know. If you sit on the dock, you can hear every word that is said in a boat halfway out in the lake. I've even listened to parties taking place a mile across the lake.

Pilots should be aware of this property of sound. A small plane or helicopter makes a great sound source. When the inversion sets in most of the sound your plane produces goes downward. In fact the greatest intensity of sound produced is directly underneath. If you are flying a police helicopter in an inversion, forget trying to be stealthy. The noise will carry for miles in an inversion situation. If you want to be a good neighbor to the people who live around airports be aware that the noise is a maximum just underneath while you are flying in the inversion. When taking off at night, try to fly over uninhabited land as much as possible.

The flip side of the coin is that sound goes upward during a sunny day. When the sun is baking the earth and the winds are gusty, the wavefronts of sound move fastest along the ground, turning the "rays of sound" upward. The Greeks constructed their amphitheaters with this in mind. The seats aren't all the same height, the farthest back are somewhat higher so the sound from the stage reaches all the audience. Taking off during a hot day like this is a bit bumpy, but at least you don't have to worry about the noise. Most of the sound energy is being bent upward away from the ground all because sound moves fastest in warm air. Figure 10-3 shows the path of the sound energy along the rays of sound during a hot summer day. Balloonists often find they hear conversations from the ground but cannot make themselves heard by people on the ground during a hot summer day. But, after the inversion sets in and they descend to get in it the reverse is true. They can be heard quite well by people on the ground, but they cannot hear what the people on the ground are saying until they are really close. The moral is on a hot sunny day, don't worry about sound getting to the ground from above pattern altitude, but on late afternoon and night time flights obey sound restrictions as much as possible.

Light is different from sound. Light and radar waves are electromagnetic waves which move faster through less dense air than they do in more dense air. But the process is much the same. Years ago I was introduced to a prism, a triangular piece of glass which separates light into colors. I noticed that the blue light was bent more than the red light. Later, when I learned that red light had a longer wavelength than blue, I guessed that the wavelength might have something to do with it. Radar waves are electromagnetic waves, the same as light or radio waves.  The only difference is that the radio waves used have a longer wavelength than red light. It turns out that in air the amount of bending isn't simply related to wavelength. Radar waves are bent more than visible light. And, bending happens frequently when inversions are around and can happen when there is strong convective activity near the Earth's surface.

Perhaps the best way to see this is to look at one of the most familiar mirages around, the "wet" pavement on a clear hot sunny day, a scorcher. This mirage can occur even if there are some breezes around. The sunlight beating down on the black asphalt heats it to a high temperature. TV news commentators may be seen cooking eggs on the pavement on one of these days. The air just above the pavement is also heated to a high temperature and its density decreases according to the old perfect gas law. Of course, it becomes less dense than the air above it, but it takes a little time before it pops up like an underwater cork. All over the pavement, little parcels of less dense air are popping up and cooler air is replacing them only to be quickly heated by the black asphalt. While this is a very active area, there is so much of it going on the layer appears to be stable. It is very much like the waviness in the air close to the top of your car on a hot sunny day.

Figure 10-4 shows the situation. The large circled area in the diagram is an expansion of the small circle on the runway. The lines perpendicular to the light ray are the sum total of all of the wave fronts traveling to the right. As the electromagnetic waves move faster through the less dense air below, the total wave front is turned upward.

Since the light travels faster in less dense air, the light waves very near the asphalt move slightly ahead of the light waves in the cooler air. Light from the blue sky just above the horizon off the end of the runway which would have hit the pavement is bent by this density change upward and enters your eye. The pavement looks blue with just a hint of shimmer so it appears to be wet. You need to be very near the pavement to see this mirage; you won't see it on final unless you are very low on final. But if you should see it on takeoff be prepared for a high density altitude and a long run. Once you are a few feet off the runway, things should be back to normal and your climb related to the normal density altitude of the airport. Low-wing aircraft are more affected than high-wing aircraft; the few feet makes a difference in this highly dynamic situation.

The only other visual mirage which might affect aviation is looming. This is a mirage most frequently seen aloft in high mountain regions. You need to be flying in a valley to see this one, so it is not a factor for most flights in eastern North America. In fact, I've only seen it a few times.  And then only towards evening when I was on a boat on very cold Otsego Lake and the winds were warm and light. In mountainous territory, the effect can cause mistakes and can be a factor in safe flight. Figure 10-5 illustrates the problem.

Say you're flying high in a valley with very cold dense air in it. Above the valley is less dense warm air. The dot-dash line in the figure is the boundary, it happens to be an inversion as well. The solid line from the mountain top is the path light takes to your eye. The curvature of this line is caused by the same thing as in the "water on pavement" mirage.  The light waves travel fastest in the less dense air than they do in the cold air below the inversion. The dotted line is the apparent path, and the dotted peak is what you see. Simply gaining some altitude and getting above the inversion is enough to get out of the mirage. Of course, if you're at the top of your service ceiling, the mirage should tell you to go someplace else. If you can't get above the inversion, the chances are better than average you won't be able to get out of the valley.

Mirages affect radar. Bending of radar beams gives radar a longer useful range than is possible if light were used. My father used to point out that standing on the beach, we could see the tops of ships only if they were within around 15 miles offshore because of the curvature of the Earth. A straight line between the tops of the masts of tall ships fifteen miles out and our eyes would just brush the surface of the ocean. If the ship were farther out, the straight line would go through the water and we wouldn't see it. The range of a radar beam is around 300 miles simply because the beam is curved by the atmosphere. The normal density distribution of the atmosphere with the least dense air aloft is enough to provide a normal curvature to radar waves. This gives the "extended" range of the ground based radars and makes it possible to have national coverage with only a few radar installations.

The down side of radar is that because the curvature is exaggerated, you really don't know exactly what you're seeing unless you use extra clues. Any mirages you have seen are also there in radar, only more pronounced because the curvature is more for the longer wavelengths.

Looming is slightly more pronounced with radar than with visible light and is a reason for what might seem at first glance as excessive IFR separation in some metropolitan areas. The FAA has taken it into consideration in the rules. In some areas of the country, ATC controllers use more than one radar unit for routing. Where one radar is on a mountain peak and the other in a valley, and the old familiar inversion exists, the possibility for error exists. Figure 10 - 6 shows the problem. As long as the plane is above the radar, the radar on the mountainside, in the less dense air, gives the best position. The radar in the valley shows the plane at a different altitude and position. As soon as the plane descends into the inversion and the more dense air, the radar on the mountain starts to be less accurate and the radar in the valley becomes more accurate. My recommendation is that if you are coming into an approach area to land in a valley and you are anywhere near an inversion, that you be extra careful about altitude excursions from the one assigned by ATC and you have accepted. Although gimbaled radar altimeters don't have the same problem since they are looking straight down, pay particular attention to the pressure altimeter.

Aircraft descending into inversions often have four different altitudes, the real altitude, the altitude measured by a radar on a mountain top, the altitude measured by a radar at the airport in the valley and the altitude an on-board radar altimeter shows. Fortunately, these altitudes are usually within 300 feet of each other, even so, separation is a must.

One of the worst problems for aviation is the phenomenon known as ducting simply because it occurs when you are already into weather which could turn severe. The most likely location for ducting is when you are flying just below a warm inversion. If your radar is pointed directly ahead of you, the radar beam will bend downward just as in looming. Figure 10-6 shows a plane using radar in an inversion situation. The warm air above the inversion speeds up the radar electromagnetic energy causing the beam to curve downward.  Then, if there is very strong heating at the surface, density change of the atmosphere may curve it upward as though it is being forced down a heating duct. In a sense it is. The return follows essentially the same path back to the aircraft. It is entirely possible that the radar in a ducting situation may not be able to see a storm which you can see very nicely from the cockpit because the beam is being refracted downward. You may have to tilt the radar beam up a significant amount in order to have the storm show up on the radar.

The inversion shown is quite typical on days when thunder is around, so you should be using the tilt feature if you have one. If you aren't sure, time to hit the manual. In fact, when you are in IFR you should be using your tilt as well. Flying in the stratus just below a warm or summer stationary front is a good way to get ducting. You could miss the storm development above any inversion if you leave your radar tilt set on horizontal. I suspect that this was the case for one flight of an air carrier in the southeast where the plane was suddenly battered by hail. Hail falls at around 30 miles per hour in calm air. In a downdraft it comes down faster. If a thunderstorm was producing hail at 30,000 feet as the updraft stopped and a plane was headed towards the hail shaft at 20,000 feet but was five minutes away, the hail and the plane would arrive at the same point in space at the same time. Remember, if you aim a radar beam by eye, the radar return probably is not from what you're looking at. Probably the radar is looking below where you are looking but possibly above.

There are problems with radars but for big thunderstorms, on-board radar units are worth their weight in gold. But they must be used with care. As with any instrument, unless it has had sloppy maintenance or has some mechanical problem, the data you see are correct, the only problem is the interpretation. The radar is displaying what it sees. It is up to the pilot to interpret the information.

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