Radar myths and misconceptions

Flying Safety, May, 2002 by Dave Gwinn

Sferics devices sense static energy created by the up-and-down drafts in a storm long before the storm is mature or paintable on radar. A herd of dots indicates a stampede of electrical activity. No need to guess intensity. Avoid it at any range.

Interestingly, airborne radar is calibrated to status rainfall rates, not the frog chokers inside a thunderstorm. This makes radar very sensitive to nuances in rainfall rates; it turns Radar Red at a rate of 1/2-inch per hour. The entire state can be Radar Red, or apparently hazardous. Nope, it's just wet. We have to know more about the accumulation of water in what kind of clouds and what the forecaster says before radar reveals a definite hazard. Radar is best at confirming what we knew or suspected was developing.

Geography has a lot to do with it. In Kansas, for example, water is not plentiful. Red in Kansas, the product of billions of tons of air and water transported northward from the Gulf, is guaranteed to be the severe hazard the radar says it is. ("Grab Toto, Auntie Em!") Compared to the fat rain droplets found off Florida, a midwestern storm has smaller water droplets and almost certainly more violent up and down drafts.

West of Kansas City is the area of High Plains or desert-type thunderstorm; a convective storm without the water accompaniment. What goes up must come down. High cumulus cloud bases, evaporating rainfull and little reflectivity are typical, but they are full-blown thunderstroms. It's a violent ride through them and deadly beneath them.

In one season, the strongest windshear ever recorded (97 knots differential) originated from a benigh-looking green return. Radar interpretation must be calibrated to the region in which you're flying.

Limited Penetration

Remember, radar interacts with waterdrops when drop diameters equal .1 to .2 the wavelength of the transmitted frequency. As radar energy penetrates a dense storm, the water blods may approach the size of the wavelength. When that occurs, the radar energy is totally absorbed by the water. This is called attenuation, the lessening or loss of all energy. It's solely a function of wavelength and droplet size, not power. Failing to recognize attenuation contributed to that DC-9 accident in Georgia. A highly attenuated display may look like a soft spot in a line when, in fact, just the reverse is true.

There are clues to an attenuated signal and pilots must learn to recognize them: bowed out backside, steep color changes on the backside, absence of distant returns and an inability to ground-paint beyond the target being examined. Always remember, radar is a weather avoidance device never intended for penetration. The people who designed it know that the more hazardous the threat, the less effective the radar. It can see it, but not through it.

Altitude

To acquire the weather picture, we aim or tilt the radar cone up and down, to as much as 15 degrees. The diagnostic levels to examine thunderstorm intensity are in the 18,000 to 25,000-foot altitudes. Higher than that and you'll find crystalline and frozen particles that won't return the radar's energy. Low-altitude water is there for the viewing, but it may be only non-hazardous rain showers. If it's a thunderstorm, it builds to these higher altitudes.