Page 20 - Volume 18 Number 8

By Archie TrammelRadar displays seem simple enough to understand; green is “Go”, yellow is “Caution”, red is “Stop”.Right?Wrong!A long time ago, radar manufacturers ignored science and elected to display echoes in three levels of green intensity. Then, when color radar displays became possible, it seemed like a great idea to show the three intensities in green, yellow and red. But, facts are, those colors are misleading, almost to the point of homicide.To understand just what a radar display should display, we need to turn back in history about 55 years. In the late 1950s, Jim Cook, a freelance pilot in Nebraska, was kept busy cloud seeding for an association of sugarbeet farmers. They had been losing big money due to hail damage. To reduce the losses, Cook flew his war surplus P-51 through thunderstorms, seeding them with iodine crystals, thus causing them to disperse before hail developed. At the same time, J.T. “Jean” Lee, a young scientist working for the Kansas City office of the Weather Bureau, as it was called back then, was tasked with discovering a way to prevent thunderstorms from chewing up airplanes.Jim and Jean were a natural fit. Their budgets were just enough to hire Cook to fly through thunderstorms and for Jean to acquire a two-way radio, which he set up in the basement of his home in Kansas City. When Jim came across a promising thunderstorm, he would fly into it and radio his position to Jean, explaining what level of turbulence he was experiencing. Jean would then compare that data to recorded images of the storm from the Bureau’s radar. From that crude beginning, Jean began to compare the radar reflectivity at various locations within a thunderstorm with the degree of turbulence found in each level of reflectivity.Eventually, the research resulted in a table of the turbulence to be expected per mile of flight within six levels of radar reflectivity in a thunderstorm. That 1964 report has been the basis for the design of every weather18 • TWIN & TURBINEradar since then, and no doubt will be forevermore. No one has ever found reason to dispute it.In the beginning, those levels of radar reflectivity were called “VIP Levels”, in ranges based on decibels of radar reflectivity or dBZ for short. The accompanying chart is based on that scientific data for turbulence and other risks in thunderstorm echoes.Obviously, echoes of VIP 1, 2 and even 3 present an acceptably small risk of a severe upset — which is a momentary loss of control; no problem for properly trained and skilled IFR pilots. But note that at VIP 4 the turbulence hazard level is such that only exceptional research pilots should ever venture into them.Back in the old days, as a result of that data, thunderstorms were shown on “scopes” in shades of gray with patterns in the gray revealing internal details. Those gray patterns were not easy to read, but they showed exactly where the shears, swirls and meaningful shapes were located. Where the storm was especially intense, the gray was dropped out into a black hole, called a “contour”. That black hole signified that the storm, the entire storm, was potentially very dangerous. Those old displays were great — certainly hard to read, but full of details to show the skilled how dangerous an echo was.Then, in about the mid-1970’s, digital techniques came in and radar engineers seemed to lose all sense of what information a pilot needs to make safe decisions. Because of limitations in digital techniques, rather than present weather echoes in patterns of internal detail,AUGUST 2014