The mission was a short one. Fly across Lake Michigan from Milwaukee to my home base at Muskegon on the Michigan shore. The trip is less than 70 miles. But looking at the Nexrad radar picture before takeoff made that short flight look like it would be a big challenge.
For days thunderstorms have been pounding the upper midwest and Great Lakes. Many of the storms contained straight line winds powerful enough to flatten trees and knock out power. But heavy rain has been an even greater threat with slow moving storms dropping several inches of rain on the same spot in an hour or two.
Nexrad radar has done a good job of showing where the storms are, and where they are headed. You can see Nexrad radar continuously on TV, on hundreds of online sites, and in our airplanes. People who don’t know a cumulonimbus from a cirrus cloud are constantly checking the radar on their phone or iPad. And I don’t know a pilot who takes off without checking radar if there is a cloud in the sky or a chance of precip in the forecast.
Nexrad images are compelling. The color coding that ranges from a cool blue through a magenta to a dark red appear to instantly show where the trouble lies. We have been conditioned in all areas of life to take green for granted, start to pay attention when something is yellow, and go on high alert at red.
Weather radar didn’t always work this way. Before the late 1970s radars used a display storage tube (DST). Radar echoes appeared on the DST but immediately began to fade. Radar operators worked in darken rooms so they could more accurately interpret the brightness, thus the strength, of radar returns on the DST. Pilots couldn’t darken the cockpit so many radar displays had a hood over them. You pressed your forehead against the hood to block the light so you could see the dim radar image on the DST.
Bendix changed all of that when it perfected the digital weather radar display in the 1970s. What Bendix engineers did was to chop up the radar returns into three levels of strength. Each of the three levels were then shown as a distinct brightness of green on the display. Because the three levels were distinct they could be shown continuously without fading.
Whala. With the Bendix radar all of us pilots were radar interpretation experts. No need to see how bright a target is, or how quickly it fades. With digital radar we worried only a little about the dim level one, stayed away from middle brightness level two, and gave level three returns a wide berth.
RCA was first to make the next step and assign a color to each radar return. Color made it even easier to see where the heavy stuff was but it didn’t do a thing to eliminate the ability of a weather radar to mislead. And Nexrad is no different. Nexrad appears to present an unambiguous picture of the weather, but it often isn’t.
That was the case for my flight from Milwaukee to Muskegon. The Nexrad image I looked at in the FBO, and then again on the satellite system in my cockpit, showed almost the entire route covered by medium intensity yellow returns. There was no rain at Milwaukee and the clouds were starting to break, but just a few miles offshore the radar showed significant precipitation.
I took off and climbed to the assigned altitude of 5,000 feet. The air was smooth, and there was no precip. Muskegon approach controllers, who look at the same Nexrad returns I was seeing, asked for my flight conditions, I’m sure wondering why I wasn’t complaining. I told them I was in the clear with an overcast that looked to begin at about 8,000 to 10,000 feet. Smooth ride. They were surprised. I flew the visual approach and landed, all with yellow Nexrad targets over the airport.
Nexrad wasn’t lying, only misleading. I was seeing “composite” mode where the Nexrad antenna scans at multiple levels up and down. There was precip up there, somewhere, and the Nexrad composite mode could see it. In “base” mode the Nexrad antenna scans at a fixed tilt angle just above the horizon. When I looked at the Nexrad online and could select to see either composite or base mode, the precip disappeared when I looked at base because the precip was not falling low enough to get to the base antenna angle sweep.
The controllers wondered how I was doing, Stancie, who had been looking at the radar, asked how rough the ride was, but I sat there flying in good VFR.
We can’t and shouldn’t ignore Nexrad but we need to understand how composite mode works. If you can stay out of the clouds it really doesn’t matter what the composite mode shows. If you can’t stay clear of clouds and the Nexrad shows precip, then it’s time to deviate.