Mirages do not exist only in the domain of the parched desert traveler; they can actually influence your flying, to a greater extent perhaps, than you might expect. I suppose that doesn’t explain the title… sorry, keep reading.
SEEING IS BELIEVING
We’ve all seen them. You’re driving on a long stretch of blacktop on a clear hot day. The sun heats up the asphalt, and the air immediately overhead warms, becoming less dense. Light from the horizon hits that air, moves ever-so-slightly faster than surrounding light, and gets bent upward. The bend is just enough to allow your eyes to see that light, with a hint of shimmer, instead of the pavement. Your brain, searching for a match, comes up with: wet pavement.
HEARING IS BELIEVING
It happens with sound, too. Sound energy is different than electromagnetic energy, of course. First of all, it travels about a million times slower, but it’s also more likely than your landing light to annoy the ground-pounders living near your local airport. Sound is also very different in how it propagates. Sound is a ‘longitudinal‘ wave — a series of compressions and rarefactions of air, traveling outward. Light and other radiation, in general (ocean waves too, for that matter) are ‘transverse‘ waves — their oscillation is in planes perpendicular to their travel. But both can be refracted, or bent, by temperature (or other) gradients.
Unconventional Mirage Example: The waves coming in from a storm further down the coast will slow down in the shallower surf as they begin to ‘feel‘ the bottom, and will bend in parallel to the shore. As the wind kicks up, and you see the waves coming in, you think a storm is approaching from the sea, straight out. That’s a mirage.
WHAT THE HECK DOES THIS HAVE TO DO WITH AIRPLANES?
Vision Scenario 1: Getting back to our wet runway — if you’re ever taking off on a hot dry day, especially in a low-wing aircraft, and you see water at the end of the runway, you’d better plan on a longer takeoff run than usual. The same conditions that cause the mirage are indicative of a very hot layer of air just above the pavement, which will rob your wings of some of their lifting potential. Consider the mirage your early warning for poor takeoff performance.
Vision Scenario 2: In high mountain valleys with cold dense air, light from surrounding peaks takes a curved path downward into the denser air, but if you’re flying in that valley, the apparent path of light from the mountain peaks will make them appear higher than they actually are. There’s even a name for this: it’s called ‘looming‘. I suppose the good news is that it doesn’t make them appear lower than they actually are…
Sound Scenario 1: Things to consider for noise-abatement… At night, especially with a nocturnal inversion, a small plane or helicopter will sound louder than on a hot sunny day. The noise can carry for miles. And the loudest sound will be directly underneath. On a hot sunny day, sound will instead bend upwards, as it moves fastest in warm air. So the bottom line here is that being a good airborne neighbor is even more critical at night.
Sound Scenario 2: This refraction of sound hasn’t escaped the notice of balloonists. Depending on the atmosphere’s temperature profile, conversations between those in the gondola and passers-by on the ground are often only one-way. Voices from the ground can either clearly be heard from the balloon, or they can be clearly heard from the ground — but the other party on the ground or in the balloon, respectively, can’t hear a thing.
FLYING WITH RADAR… AND MIRAGES
Radar waves are electromagnetic waves and, just like light, they refract when moving faster through less dense air. Actually, radar wavelengths are bent much more than visible light. (And this happens more strongly when there are inversions, as well as during convective activity.) Radar in fact gets most of its useful range from this. A radar beam (even one originating from 200 feet up, where a visual horizon might be perhaps 17 miles away) can travel two hundred miles. The down side of this, however, is responsible for the excessive IFR separation that we sometimes perceive. This is especially true when ATC uses ‘mosaic-ing‘ of multiple radars, and where one radar unit is at a sufficiently different altitude (or temperature domain) than another. But it gets worse…
Warning 1: If you are landing in a valley and are descending into an inversion, separation and altitude excursions become more critical.
Warning 2: For those folks who have their own on-board radar… If you are flying below a warm inversion and your radar is pointed straight ahead, its path will be bent downward, just like with looming. Here though, it actually is called ‘ducting‘. Your radar might not see something right ahead of you as a result.
Warning 3: Flying in stratus clouds below a warm front, if you have (but don’t use) a ‘tilt‘ feature on your onboard radar, you could miss storm development above the inversion layer entirely.
BOTTOM LINE: If you aim a radar by eye, what you see may not be what your eyes are looking at. If you fly without considering the consequences of refracted sights and sounds, you may not understand what’s really going on.