Take the example of a line of storms moving from the northwest toward the southeast across southern Wisconsin and northern Illinois, across Lake Michigan and into Michigan and Indiana. If you had been watching the local news when this image appeared, you would have seen the meteorologist point at the orange and red at the southern end of the line and talk about the strong thunderstorm that was expected to hit Lafayette, Indiana, in the next half-hour.
A pilot in Lafayette who had looked at the loop on AOPA Online (www. aopa.org/members/wx ) earlier in the day would have seen that making a flight toward Chicago would have been a bad idea. Even without knowing exactly what the orange and red colors mean, the pilot would have correctly surmised that they depict something that an airplane should not tangle with - no matter how well-equipped the aircraft or how experienced the pilot.
Radar works by sending out radio waves that are reflected back by various objects. Air traffic control radar is designed to detect radio waves reflected by aircraft and also to display information from aircraft transponders. Weather radar is designed to reflect radio waves reflected by raindrops, hailstones, and snowflakes. This is called a reflectivity image because it depicts the amount of radio energy that precipitation - in this case, rain - reflected back to a National Weather Service (NWS) radar antenna.
With the DTN reflectivity images used on AOPA Online, green colors generally show rain falling at a rate of less than one-tenth of an inch per hour, while orange shades indicate more than two-and-one-half inches of rainfall per hour, and red means 12 inches or more per hour. The red shades can also indicate hail, and this is normally noted on the image with an h to show that hail may be present and an H to show it is likely present. Shades of blue indicate snow, freezing rain, or sleet, and purple shows a mixture that can include rain, snow, sleet, and freezing rain.
The important point for pilots to remember is that the heavier the rain, the greater the likelihood of severe or even extreme turbulence. Hail is a definite sign that you can expect severe or stronger turbulence.
Several different companies use data from National Weather Service radars to produce the radar displays that you see on the Internet and television. These companies don't all use the same color codes, but in most cases orange and red colors show the heaviest precipitation and the likelihood of dangerous turbulence. Still, as with any chart it's a good idea to check the legend to learn exactly what you are looking at.
Another view of the same line of thunderstorms shown in a reflectivity image chart is the radar summary chart produced by the National Weather Service, and it shows the storms about one-half hour after the reflectivity image chart. You'll find the summary chart on the NWS's Aviation Weather Center Web site ( www.awc-kc.noaa.gov/awc/ fltfldr/2080.gif ). FAA written examinations include questions about this type of chart. A radar summary chart divides precipitation intensity into as many as three levels, with each area corresponding to two of the six levels that the NWS uses to classify precipitation intensity.
To see how this works we look at a radar summary chart covering northern Indiana near the letters NA. (This means that data from a radar site there is not available, but other radars in the region fill in the gap.) The lines indicating areas of rain show three contours. The innermost contour-on the Illinois-Indiana border southwest of the NA - contains Level 5 or even Level 6 precipitation. The middle contour outlines Level 3 and Level 4 precipitation, while the outer contour shows where Level 1 and Level 2 precipitation was detected. If you colored the innermost level red, the middle area yellow and the outer area green you'd have a display somewhat like a reflectivity image chart.
The radar summary chart has some useful information that's not shown on a reflectivity image chart. Underlined, three-digit numbers such as the 460 at the Illinois-Indiana-Kentucky border indicate that precipitation was detected as high as 46,000 feet in the area that the thin line extending from the number leads to. Always add two zeros to the number to obtain the altitude.
Since radar detects precipitation, not tiny cloud drops, the actual tops of clouds are higher than indicated by the figures on the chart. A good general rule is the higher the clouds, the stronger the updrafts and possibly the downdrafts inside them. Fierce thunderstorms in the Great Plains can grow higher than 60,000 feet, but thunderstorms that top out at less than 30,000 feet can also create dangerous winds and turbulence.
While some television stations have their own weather radar, almost all of the images on television and the Internet - as well as those used to create the radar summary chart - are from the new National Weather Service NEXRAD or 88D radars that were completed in December 1996. NEXRAD stands for Next Generation Radar, and 88D shows that the radars are the 1988 model and they have Doppler capability. Doppler capability means that the radars can detect wind speeds and directions.
Most television stations describe their radar images as Doppler, which is true in the sense that the radars have the capability to measure wind speeds and directions, but the images that include this data are extremely complex and are almost never shown on television.
Strong tornadoes come from thunderstorms with mesocyclones, which are areas of rotating wind maybe 10 miles across ("Supercell Thunderstorms: The worst of the worst," AOPA Flight Training, June 2000). Doppler radar is good at detecting mesocyclones, but the images are extremely complex because they show only the component of the wind moving either toward or away from the radar antenna. This means that the image isn't easy to interpret. In fact, the images produced by today's Doppler radars are so complex that computers are used to detect and highlight important storm features. Since Doppler images would make little sense to anyone other than a radar meteorologist, the radar images on television and the Web are almost always reflectivity images or radar summary charts. Nevertheless, many online radar summary charts carry a MESO notation next to radar returns that show a potential for mesocyclonic circulation.
While large aircraft, including airline jets, have onboard weather radar, most small, general aviation aircraft are not radar-equipped. This means that most general aviation pilots have access only to radar images like those in a reflectivity chart or the radar summary chart. These images can be a good preflight planning tool, but they have serious limitations that pilots need to understand. First, radar does not detect clouds, fog, haze, poor visibility, or low ceilings. Second, the images or the summary chart are likely to be at least 30 minutes old when you see them.
Since an individual thunderstorm or a line of thunderstorms can move across the countryside at 40 mph or faster, storms could have moved a long distance by the time you see the chart. Radar, of course, does not tell you where the storms will be an hour or two after the image was captured or how strong or weak they will be. In other words, radar images or the summary chart are only one of the many pieces of information a pilot needs to gather for a preflight weather briefing. But this could change.
Some time in the not-too-distant future, a data-link system for transmitting images directly to the cockpits of aircraft-including small general aviation aircraft - could make it possible for pilots to display images like a reflectivity image chart as they fly along. This image could be combined with a GPS map that shows the location of the airplane superimposed on the radar image.
In the meantime, however, pilots should use weather radar images or the summary chart to help learn what the weather has been doing, which is the first step toward figuring out what it will do in the near future. Once in the air, pilots should check regularly with flight watch on the radio frequency 122.0 MHz for aircraft flying below 17,500 feet. This puts you in contact with a specialist at a flight service station who has immediate access to a variety of up-to-the-minute data.
Radar On The Web
Weather radar images, other weather images, and text weather products are available to members on AOPA Online (www.aopa. org/members/wx ). The National Weather Service Aviation Weather Center standard briefing page where the latest radar summary chart is found is available to everyone ( www. awc-KC.NOAA. gov/awc/aviation_weather_center.html ).
For more detail about weather radar and the relation between radar images and the odds of turbulence, see two articles in Rod Machado's Safety Tip of the Month series on AOPA Online (www.aopa.org/ special/machado/). The two articles were posted in May 1998. They are "Operating within the envelope, Parts 3 and 4."
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