Chapter 9 - Thunderstorm Avoidance

Of the things which can ruin your whole day, thunderstorms come close to being the scariest. I was out with the wife doing her shopping on a beautiful spring day. She was picking out flowers for the garden at a garden center which just happens to be under the final approach to our neighborhood airport. As she pondered and chose, I stood by driving a cart load of her selections when I would have been much happier in the left seat above. She took her time. Just as she started into the peonies, the sky towards the northwest started to get darker. As she pondered her selection, it got darker and darker. Wandering out of the greenhouse, I looked up and saw the nicest line of thunderstorms which stretched from horizon to horizon. Urging her on, she made her selections and we wheeled the cart towards the main building. Unfortunately, the main building is filled with all those nice-to-have geegaws which must be perused. While she was investigating these, I wandered out to the front of the building to watch the storm come in. The feeling started to grow that being in the left seat up there was not the place to be and that I was much more comfortable on the ground.

As the rain shafts came over our five mile visibility marker to the northwest, a low wing plane came in full throttle from the east. He entered downwind for runway 14 just as a big gust burrowed its way through the main building sending papers flying. Perhaps the sight of the clouds made him decide to do a short downwind; which was now an upwind, he cut the corners into base and final. He was still 200 feet above the runway when he crossed the numbers. Making a hasty go-around he entered base the other way and curved around to final in one continuous movement. The headwind from the thunderstorm enabled him to make a short-field landing with probably half power. He got it on the ground and turned off just as the rain hit. Aside from taxiing in almost zero visibility and gusts probably hitting 20 knots, the thought of getting out in that deluge to tie it down left me cold. I wondered why he didn't stop at College Park and have a cup of coffee or whatever when he first noticed the sky getting really dark to the northwest.

It didn't really hit me until we were driving home with the trunk full of plants. That guy obviously didn't know the winds at GAI. He hadn't listened to the AWOS or he would have chosen to land into the wind on the first pass. Probably he took off sometime earlier using runway 14. By the time the storm hit, 32 was the only possible choice as GAI only has one piece of tarmac. He was fortunate or the storm was cooperative. The outflow from the storm was blowing gustily right along runway 32 at around 30 to 40 knots. Had the storm been to the west of the runway, he would have had a mighty strong crosswind.

A few weeks later, a friend made a comment about a meeting he had just attended concerning a particular incident involving a thunderstorm during landing. The copilot who made the landing was discussing information in the cockpit with some of the systems people. The systems people were promoting more information in the cockpit while the copilot pointed out that there was already too much information flowing in as it was. On some reflection, both were right. What the pilot needed were the golden gems of information for action - at that time and at that place. And what that golden gem is often depends on the pilot's training and education in meteorology. Most training emphasizes avoidance; thunderstorms are bad - go someplace else. I've always found that a reasonable amount of knowledge about a phenomena goes a long way to help my thought processes and quell the panic which ignorance causes.

I used to think the problem is not the isolated thunderstorm. Those you can see and avoid by the requisite 20 miles. Nature set the distance, not the FAA. The big problem with thunderstorms is the ones imbedded in other clouds when you are on IFR. Those you can't see until it's too late. But, lately I'm not so sure, especially in the Eastern US where busy airports conspire to confine us to the lower levels under Class Bravo airspace. Summertime haze can do an effective job of hiding a thunderstorm. And, crowded beltways around cities are extra good sources of water vapor from burned gasoline, reducing the visibility in these corridors. Of course, you can always work with ATC and go up to clear air if ATC isn't too busy. But, assuming it's clear somewhere up there under the aircraft's service ceiling, there is other VFR traffic to contend with during the climb to clear skies. Yet, if conditions are right, flying during a day when thunderstorms are around is perfectly acceptable, as long as you're aware of them and avoid them. When they are around, I generally confine my activities to air work which can be done in or very near the pattern, such as making sure I have the required three takeoffs and landings so if something untoward happens, the insurance company cannot point a pious finger at me.

I've never been flying in a thunderstorm; the pleasure is one I've been avoiding. I've flown near them, but never in one. Everything I've ever read or heard says that if you're caught, slow to or below maneuvering speed so the turbulence will not bend the airframe and fly as straight and level as possible to keep the hail from entering your lap through a broken window. Whatever you do, fly the airplane. There's nobody else there to do it. Try to keep it in a reasonable attitude and don't worry about altitude excursions until you are in the clear and well out of the turbulence which often extends beyond the edge of the cloud.

When the subject of thunderstorms is mentioned during "hangar flying," the first thing which comes to most pilots' minds is either "I'm thankful I've got a lightning detector and/or radar." or "I wish I had a lightning detector and/or radar." But there should be three items in the list:

• thunderstorm sensitivity,
• lightning detection and
• radar.

Without the first, the devices are a lot less useful.

Thunderstorm sensitivity is a whole lot less expensive and weighs less than the hardware. And it can help pick up the beginnings of those nasty thunderbumpers before radar has anything to look at and well before lightning begins its violent transfer of electricity between cells or the ground. Thunderstorm sensitivity means you have been generally aware of the location and intensity of thunder for the past few days. It means that in your preflight you paid attention to those places where thunder was occurring. It also means you are aware of those places where thunder is forecast during your likely involvement with Mother Nature. It means that you include sigmets, airmets, and convective sigmets in your briefing and read them carefully. It surely means you are aware of the last location of any summer stationary fronts where you will be IFR, in cloud, and are most likely to run across strong unseen convection which may or may not grow into Thor's anvils.

In the cockpit, thunderstorm sensitivity means you are listening to the radio with heavy weather at least in the back of your mind. It means that as you fly in IFR you are scanning the clouds as well as your instruments. When it starts to get unusually dark you need to make the mental connection between possible turbulence and heavy rain/hail ahead. During approach, you need to be aware of any thunderstorms in the area and the likelihood of gustiness or serious wind shifts on final. And, you need a plan if you have to go elsewhere.

Most pilots would also like to avoid even light turbulence. A few passengers have a tendency to get green around the gills even in light turbulence. The problem is, turbulence phenomena don't reflect radar waves well, or any other type of wave for that matter. Researchers and engineers alike have attempted to develop a device to warn of turbulence, but, so far, nobody has been able to measure turbulence out ahead of a moving aircraft with sufficient lead time for the pilot to do something about it. All the attempts have led to dead ends in terms of inexpensive equipment which is light enough for general aviation; however, the engineers are still at work. Until a good detector is devised to work in clear weather as well as IMC conditions, you will have to estimate the likelihood of these meteorological phenomena from a knowledge of the stability and wind shear, and the images which do appear on the instruments we have available.

Radar has been a boon to weather forecasting and aviation. It has provide meteorologists their only look at the things in clouds which return a signal, mostly precipitation. In the forty years of operational use, the weather radars have provided the meteorologists with the only information on what is going on in the clouds. For pilots, radar is a primary source of preflight briefings. It helps to know where the echos were a few minutes ago, in judging what the intensities of the returns were and where they are headed. The average cell lasts 30 minutes, so that time can be easily chewed up heading from the briefing room to the runup area. The cells you saw on the radar are history and new ones, usually nearby the old ones, have taken over the jobs of water distribution and stabilizing the atmosphere.

The new NWS weather radar gives a much more detailed picture of the particles in the storm than the old radars did. In addition, the WSR-88D measures the velocity of the particles away from or towards the radar. From this information meteorologists can infer the velocity of the winds towards or away from the radar. This means that meteorologists can spot the violent vortices in thunderheads which can breed tornadoes before the tornado touches down. Since the adoption of the WSR-88D, tornado warnings have doubled in accuracy and the false alarm rate has been halved. It's very handy for the forecasters to be able to see what they are talking about.

The NWS radars, with some help from the Department of Defense WSR-88D radars, pretty much cover the 50 states and the larger possessions. There are only a few gaps and these are really only in valleys where the radar beam is blocked by the mountains or where there are buildings in the way of the beam. The people who were responsible for siting the radars did their utmost on some very messy situations. While some people have complained there are coverage gaps in some areas of the country, there really aren't many gaps. If you watch the composite radar images available on the Web, it is soon apparent that the radar coverage is excellent.

The FAA has installed versions of these new radars, called Terminal Doppler Weather Radars, at major airports so the ATC managers and Center Weather Service Unit meteorologists can have at least a chance of observing the precursors to microbursts and gust fronts. The information from these does not go to the national network as the range of these radars is limited to the neighborhood of the pattern, and the threats to aircraft they should detect are limited to the landing phase of the flights. The radar information is not available on the Web or used in images you see on the TV but is available to the folk in the tower who need it. In the near future it may be available in the cockpit.

Ten years ago I didn't see how the weather radar image could be seen by the general public - and there it is on cable TV. And, there it is in a graphic form from the DUATS suppliers - radar images - in living color, and elsewhere on the Internet. If you are on the World Wide Web part of the Internet, there are plenty of sources of radar images. I regularly use http://www.intellicast.com, and http://www.weather.com/. I also use the NWS's Aviation Weather Center and DUATS.

Interpreting one of these is relatively easy. Figure 9-1 is a typical example of some meteorological excitement as shown by a composite radar image taken off the Internet. The image shows some echo over the Montgomery County, Maryland, where I watched this storm from the ground. It was well forecast and my contemporaries at Montgomery Airpark (GAI) also felt the ground was the place to be. It was a slow moving storm, with almost continuous lightning, and the thunder rolled as though Rip van Winkle's miniature keglers were all at a national bowling convention which used all 96 lanes they must have built just to the north of my house.

Of course, the precipitation isn't actually colored this way. The intensity is color coded for your ease in decision making. The choice of colors, blue and green for light precipitation, yellow for medium and orange and red for intense is a logical one to carry over from stop lights. Each radar image on the web is a composite of a number of layers of radar return. In some radars, each pixel is colored to represent the most intense measurement of all of the returns at all elevations over that location. In others the pixels represent the base reflectivity (lowest radar scan) of the precipitation

The colors are selected to represent the approximate level of precipitation with a few caveats. For rain, the relationship is relatively easy; the amount of return is proportional to the sixth power of the mean raindrop diameter, although there is still some discussion of this in the literature. Hail is similar to rain unless it is wet hail. However, dry snow doesn't give hardly any radar return, especially if it is in small crystals. Should the crystals grow, stick together and move into warm environments and have a tiny bit of melting on the surface, the return is greatly enhanced. In a vertical cross section, you can easily spot snow melting by its bright horizontal band in the precipitation. It also appears on the horizontal displays but it isn’t as obvious.

So what the developers of radar have compromised on is to slice the return signal into various categories of return. The first level is the background noise, no return. Then next level is the first perceptible level above the background noise when they can be sure it is return from something. The next level is the level where they can be sure there is a difference between that and the level below. The next level is chosen where they can be sure there is a difference between that level and the level below, and so forth. The difference work out to be a logarithmic scale very similar to the dB scale for hi-fi, and is called dBZ. The Z stands for radar return. Of course, some engineering adjustments are made during development of the electronics which do this slicing automatically, but the numbers on the scales refer to these dBZ values for the highest peaks in the volume the radar beam surveys. Obviously, they are different for radars which have different wavelengths.

It's dangerously easy to assume everything on an image is precipitation. Birds and bugs also provide return but these are easy to spot if you are looking for them. Birds tend to leave their nighttime roosts early in the morning and the flocks return at just before sundown. Migrating birds also cause radar returns in season. Most migrating flocks begin their flights just after sundown and fly until just before sunrise. At dawn they land for refueling, rest and to hide from predators until they have to make the go/no-go decision for the next night's flight. While there are automatic bird migration removal programs, they sometimes miss a flock or might remove a few light showers if they happen to be moving at the same speed and direction as a flock might.

The other new bit of information which you can get from the new radars is the Doppler storm relative velocity. Figure 9-2, after an image from Intellicast.com, shows the Doppler velocity of the same storm. Interpreting Doppler images is pretty easy if you remember you are looking at the velocity moving toward or away from the ground location of the radar. The radar location in this image is the blue spot in the lower right. Shades of red mean the winds there are blowing away from the radar. Green shades represent winds moving toward the radar. So, the winds to the lower left of the diagram are moving away from the radar and winds to the right are moving toward the radar. The light line in between the darker red and darker green is the zero line where the winds are not moving toward or away from the radar. There are winds there, to be sure, but they are moving perpendicular to the line between the location of the return and the radar.

Imagine you are standing on the median of an interstate highway looking at the cars coming at you from only one direction. The cars which are just beside you have zero velocity relative to you (You know they are actually moving pretty quickly.), and the cars which have gone past are also still moving. If you had a Doppler radar gun, you could detect the cars coming at you, the ones going away, but not the ones at your side unless they are really moving at you. In that case, duck! In the case of the Doppler radar image in Figure 9-2 the winds are reasonably uniformly out of 120 degrees although there is some wind coming at the radar from the bottoms of the thunderstorms in the right center of the image. The winds to the north of the thunderstorms are red, moving away from the radar.

There is one more thing which complicates the Internet Doppler radar velocity images labeled "Storm Relative Velocity." The images are made by subtracting out the motion of the storm. If the storm isn't moving much, there isn't much effect. But if the storm is moving fast, the winds are adjusted considerably. To see how this works, go back to the interstate example. Imagine it's rush hour. Instead of just standing on the median, you are now moving with the general flow of the cars in both directions. There isn't much interest in going into the city, so cars headed in are clipping right along. The cars heading out to the suburbs are traveling slowly, bumper to bumper. The movement of all of the cars is slowly outbound, so imagine you are traveling with the overall speed. The speed of cars moving to the city is enhanced while the speed of the cars moving to the suburb is decreased. In the same way, the storm relative velocity enhances speeds of winds moving away from the storm while slowing those moving in the direction of the storm's movement.

Most of the velocity images have blank or black spots in them. Don't be fooled into thinking that the wind is calm there. Probably it isn't; it's just that the radar only sees the velocity of reasonably big drops, snowflakes, birds, or insects. If there aren't any particles there, there's no return, so the system can't make any measurements there.

The difficulty with the present network of Doppler radar is that it doesn't detect the actual wind speed and direction, it only measures wind speed to and away from the radar itself. You have to estimate the total wind. Where we have two radar units which overlap, there are research computer programs to calculate the actual horizontal winds, but this is not done on a regular basis right now. It may be in the future.

A few very different Doppler radars called profilers actually look almost straight upward with three separate radar beams. These units can make measurements of the horizontal and vertical components of the wind, all three components of the vector wind. The down side is that they are expensive, their antennas are huge, and they only cover the area right above the antenna. The NWS has perhaps a dozen of these around the country to evaluate what, if any, additional information they can contribute. If they provide enough good information and the equipment is reliable enough to be used all the time, more of these may be installed across the country.

These images are most useful in the pre-flight phase, the planning. They can tell you where the turbulence and nasty updrafts are, or were in the last half hour or so. Since the timing on my bladder is shorter than the fuel capacity of the plane, intermediate stops are a must. A look at the radar images on Internet has a spot on my intermediate stop planning if I am heading into any questionable weather situations. And the ATC folk do have access to the information through the NWS staff at the center.

Someday, I hope to have an on-board weather radar display. The big reason one plunks down the bucks for a radar unit is to help you avoid getting your wings bent or worse in or near a big thunderstorm. I did say near. A university professor friend of mine, also a pilot, studies thunderstorms as a profession. Bill rarely flies the specially armored and overpowered plane he sends into thunderstorms with instruments on it. Specially trained pilots equipped with extra intestinal fortitude do that kind of flying. And, Bill's weight can better be used in packing more instrumentation on the plane. After analyzing data from many flights in and around thunderheads in Colorado, he reported that the data show the most severe turbulence is sometimes in the clear air surrounding the visible cloud. The rule I learned in ground school, to stay at least 20 miles away from the visible edge of the thunderhead, is well founded!

Most of us have been brought up in the age of radar, yet few of us who drive light singles get to use it in the cockpit. I'm sure you, like I, have read articles about airborne radar that swear that sliced bread is a trivial accomplishment compared to this potentially life-saving device. But, down there in fine print you find out that there is almost no way to put one into a light single. True, I've seen a couple of Cessna 210s that came with a pod hanging under the wing. And there have been some discussions at the club about buying one to complement our lightning detector in the Piper Dakota. So far, nobody can figure out where to put it.

If you regularly fly IFR and use your plane for business, you should consider adding both a radar and a lightning detector to your panel. One does not duplicate the other. A radar detects the position and intensity of particles such as dust, rain, snow, and hail in the clouds. It also detects birds, insects and mountains. The lightning detector gives you the position of lightning going on around you and, occasionally, the start of a motor on the ground. If you regularly fly VFR and stay legal, you probably don't need them, although checking the lightning detection during your scan is nice since it lets you know if something has popped up. There are occasional false strikes; but, sometimes, they aren't false. So, I include it in my scan and occasionally reset it even on clear days, just to make sure those clouds on the horizon have not grown into really big and nasty ones.

Turbulence avoidance is the main reason for using weather avoidance radar. Of course, icing, hail, and lightning are also to be avoided. And, having on-board radar will help avoid these dragons. But the main point of using radar is to avoid the towering thunderstorms which contribute to these sometimes fatal encounters. The concept behind weather avoidance radar is that the correctly functioning radar unit sees something out there. Like any good instrument, it displays the data to you - the interpreter. The data are correct. It's only the interpretation that can go astray. So the first job is to understand what the radar data mean.

Radar operates on the idea that a beam of radio waves travels at the speed of light. A pulse of radio waves sent out will echo off of anything out there and some part of the echo will return to the receiver. The time between the outgoing pulse and the incoming return signal is used to calculate the distance to the object. For those who like equations, this one is the old familiar distance is equal to rate times time. The rate is reasonably well known, the speed of light (3 x 10⁸ meters/sec) in a vacuum and not far from that in air. The time used to calculate the distance for display is half the time between the outgoing and returning pulse, after all, the radar waves travel out to the target and then back to the receiver. The height of and bearing of the target are calculated from the position of the antenna.

While the internal circuits have changed drastically over the years, the basic concept hasn't changed from the days before World War II. During the war, echoes from weather were a nuisance, something to be worked around. After the war was over and people had time to think, it was realized that the nuisance returns, would be helpful for warning people about impending dangerous weather. In fact, the designers of the radars used by air traffic controllers thought of weather as a nuisance. Only later, at pilots prodding, was any attempt made to display any of the little weather information in the data.

In the late 1950's, the National Weather Service (then the Weather Bureau) installed a chain of weather radars across the U.S. These radars allowed the meteorologist to see the precipitation in all the clouds in their area for the first time. The resolution wasn't particularly good but it was better than anything meteorologists had before. The big difference between the weather radars and earlier radars was the wavelength they operated on. It was changed so the radar had a decent chance to "see" the precipitation.

Any radar sees the objects in a volume of space. If there are a few drops or flakes in that volume the echo returned is weak. However, intense rain or snow multiplies the returning echo so even the long 22 cm. FAA enroute radars will show some return. But each component of that return is the result of an interaction of the wave with a single drop. When the enroute radars were designed, weather returns were to be suppressed. The engineers tried very hard to eliminate weather errors from the radars. They succeeded quite well by choosing a wavelength which does not respond to these ranges of objects and by filtering the returns; however, the only reasonable tools they had to work with were electronic time smoothers which average a number of radar pulses. Airplanes provide a single strong return, but weather is distributed over a larger volume with variations over time. As the averaging techniques work on these returns, they set them to zero. If, however, they run into a relatively rapid increase as in a thunderstorm, they will give some time-averaged return.

The selection of radar frequencies, like most of life involves tradeoffs. First, you want to see the rain, snow, melting snow, and hail. Second, you don't want to see small objects like dust, condensation nuclei, and cloud material. Third, you don't want to just see the outside edge of the rain and etc., you want to find out what's going on in the storm, and ideally, if there's another storm behind the first. Lastly, the longer the wavelength, the bigger the antenna must be.

Long radar wavelengths, between 15 and 30 cm long, don't get much return from rain or snow; the waves go right around the drops with little radar return to detect precipitation. The return signal reflected from aircraft are easily detected by the longest wavelength radars while the weather information is missed because the radio waves don't reflect much from the drops and flakes. The FAA 22 cm widely publicizes that their radar units do not provide much reliable weather information. The returning signal is close to the noise levels. Only if large numbers of rain or wet snow are detected in an area will the radar report any appreciable return. But, even in the most intense rain, the waves interact with individual drops or flakes.

Studies of detecting rain and snow show that the shorter of the radar frequencies, which are about the same wavelength as the drops, illuminate the droplets the best. A 1 cm radar would be the best for seeing the outline of the 1 mm raindrops; however, these radars are not be able to penetrate into the rain shaft very far; the engineers call it attenuation. So a longer wavelength is usually used. Typical on-board radar uses a 3 cm wavelength (X band), so attenuation is a real problem. It is not uncommon to use your radar to avoid a cell, but just as you get around the corner, you find another one lurking.

All radars have attenuation, but the shorter the wavelength the more severe the problem. All radars operate on a part of the electromagnetic spectrum, the same spectrum as light, radio, and television. Early in the development of radar, a number of bands of the electromagnetic spectrum were set aside for radar transmitters. The letters have remained with us and are often confusing to the neophyte. The frequencies and wavelengths for these bands are rarely stated if the letter(s) is used to denote the radar frequency. Table 1 lists the names, the wavelengths in centimeters and meters, and frequencies in giga Hertz (10⁹ or billions of cycles per second for us old timers) for the radar bands. The detection column provides information on the smallest things it detects well. Obviously all radars will detect aircraft if they are close enough; however, the manufacturers' engineers may adjust the display parameters to remove "extraneous material."

Table 1 - Frequencies and Wavelengths for Radar Bands

The best type of weather radar is one which "sees" raindrops, snowflakes, and hail. It doesn't see cloud droplets. If you wanted to see cloud droplets you would simply look, or use TV cameras with big telescopes if you wanted to see what's going on over another airport and also stay out of the weather yourself. In order to "see" the raindrops, snowflakes, and hail without being stopped by the intervening cloud, the radar should look at the optimum wavelength for these things.

Using the same physics that explains our inability to see through fog, there is a portion of the electromagnetic spectrum which is best for seeing rain, snow and hail. If the wavelength is too large, the objects won't interact with that wavelength and no backscatter will occur. If the wavelength is too short, cloud droplets, drizzle and little drops will obscure the hail

Generally most meteorologists get excited when pea-sized hail or larger appear on the ground. This tends to excite most everyone else so meteorologists look for these in the radar return. The peas in my garden are about half a centimeter in diameter. Thunderstorm research studies show that most of the big drops which hit the ground during a thundershower were snowflakes or hail around one centimeter in diameter not long ago. The one-centimeter snowflakes were single crystals when they were in the tops of the cloud. As they start to melt when they hit the warmer air below the freezing level, they start to clump together, reaching a size of three centimeters or so. As they continue their fall in the warm air, they melt before they hit the ground. This is why most summer thunderstorm drops are really big, they were clumps of snowflakes just minutes before they hit the ground. So a radar with a wavelength of 10 cm is probably the best to observe these things. Now a 10 cm radar requires a really big antenna. The NWS 10 cm radars have an antenna which is about the cross sectional area of a 727. This would be a tad big to carry on a small single.

Most airborne radars are in the X band, around 3 to 5 cm. While the manufacturers set the colors on the screen to the appropriate return, the attenuation is more severe and the range through rain is less. This is very understandable as weight and size are at a premium in any aircraft.

The modern radar receivers use a color arrangement to display the returns. Most in-cockpit displays show you a horizontal slice through the air out in front of you or a vertical section of the same region. One of the newest ones allows you to look at both at the same time while looking at a GPS moving map as well. If there is no return (just static), the digitizer assigns a zero to the pixel at which the radar beam was looking. The number assigned corresponds to light green or the GPS map background on the color screen of the computer. A little return causes the digitizer to tell the computer the value at the pixel's location should be the one which corresponds to green. More return, color the pixel yellow, and so forth, on to red for the highest returns. The cutoff numbers have been chosen to correspond to the average values for rain of various intensities. Green usually corresponds to light rain or moderate snow whereas red corresponds to a "wall of water" or wet snowflakes.

Older on-board radar has only enough computer capacity to provide color displays of the sweep that the antenna just made; however, newer ones can store images, and turn them on the display if you make a turn. There are some other features which help especially if the air turns bumpy as it often does near thunderstorms.

Figure 9-3 shows a diagram of what a particular set of thunderstorms might look like from an on board radar unit. The storms appear to be somewhat widely spaced. The different shadings are the reflectivity levels which use a single intensity on the screen for all returns which are in a certain range of return from the radar.

The range rings which are supposed to represent 25 and 50 miles ahead are represented by arcs from one side to the other. The lines from the small airplane are supposed to represent the limits of the radar "map" as seen from a point over the center of your future jaunt through the atmosphere.

As I mentioned earlier, the small X Band (3 cm wavelength) units for light twins often suffers from attenuation. Be careful. If the radar shows a pattern similar to the one shown in Figure 9-3, what's behind the storms in the foreground may be hidden from view. The little transmitter sends out its energy which must go through the foreground storm, be reflected from the drops in

the background storm and then survive coming back through the foreground storm again. Not much energy makes it. If we magically remove the foreground storms, the background storms might look like that in Figure 9-4. What appeared to be a hole on the right to go through turned out to be perhaps the most dangerous route through the storm. If you picked the hole on the left, you may have won. But keep in mind that a cell might be starting there but not have enough wet snow or big drops to give a high reflectivity as yet. By the time you get there, the cell might have grown to become an unruly mature storm.

Attenuation may have more serious results if you are using the radar in the rain. If the rain gets more intense and hail starts to form, the center and the backside of the storm may be hidden. It looks for all the world like you’ll break out in the clear when the worst is really hidden by the hail. Going there could be the worst thing to do. If you are aware of attenuation and sensitive to the structure of the big cumuli, you might try going towards what you can see. What you can't see might be worse.

The tilt feature on radar is also important. As you scan the storm ahead, tilt the radar beam down until it is seeing the ground. If the ground is visible around the storm but not under it, the rain or hail is attenuating the beam. Some people call this "shadowing." Since rain shafts can harbor hail and strong wind shear, they are places to avoid. If you can see it visually, avoid it. If you play it safe and avoid flying under the overhanging edge of a thunderstorm, you will generally avoid the rain or hail shafts.

Figure 9-5 shows a reconstruction of a case of attenuation. It is a side by side comparison of the type of information one might get. In the right hand side, the blue line gives the path of the aircraft which is about to turn to avoid the worst part of the storm. The depth of hail off the left wing attenuated by the first hail the radar beam ran into. The hail was probably four times as thick as the 3 cm radar indicated. Fortunately, the pilot recognized the attenuation by the sharp gradient on the upper left part of the storm displayed on the on-board radar and turned right instead of into the thinner line of return displayed to the left of his course. He survived with only a few dents to the airframe.

Most of us even in the early part of the twenty-first century will probably not have on-board radar. You may, however, be using portable PCs or hand-helds in the cockpit to help determine weather hazards. This means you will need to know how the radars take the data and how the data are processed to interpret them correctly. Surface weather radar is quite different from airborne radar. For one thing the data are bigger in all ways. Not only is the wavelength longer, around 10 cm, but the components are bulkier requiring massive supports for the 10 meter diameter dish. Image sizes are larger requiring bigger computers to process the data. Ways of getting the 10 cm information into the cockpit in a timely manner are being tried out now.

Radar and lightning detection are fine, but use them wisely. The toughest part of avoidance is thunderstorm sensitivity. Then lightning detection and radar will be much more useful to you and make your flight through the soup both safer and more enjoyable.

That's all fine and good, but occasionally a pilot will report that the radar is "screwy," often when he or she needs it the most. Unless the program in the radar's computer has crashed and the radar needs restarting, the radar is telling you the truth as it "sees" it. The key word here is "sees." It may be "seeing" a mirage, the subject of the next chapter.

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