Nexrad Weather Radar Fakery

Our TV cable system has a channel that continuously shows the Nexrad weather radar while the audio is the drone of the NOAA weather broadcast. The radar display is a single-site from the Nexrad station at Grand Rapids.

Most Nexrad images we see are mosaics of the radar network. Over most parts of the U.S. the Nexrad coverage areas of the more than 150 radar stations overlap for both redundancy when a station is off the air, and to see greater detail by looking at storms from different angles and different ranges. The Nexrad images we see in the cockpit sent down by satellite, or now up from the ground by ADS-B, are all mosaic pictures that combine the data from all available radars.

What is revealing about the TV cable Nexrad display is that it is a fake. Well, fake may be too strong of a word, but it is misleading. The fake part of the display is an antenna sweep line that rotates once about every 52 seconds.

I’m sure the guys at NOAA put the fake antenna sweep mark on the display so people would think the radar is working, and not just static or broken. All of us expect to see a radar scanning when it is operational so the sweep line looks normal.

I have to say, I was taken in by the fake Nexrad sweep line. Or at least I didn’t really think about how disingenuous it really is. But this morning I had the radar channel on with the sound down. An area of convective activity kept brewing up new lines of extremely heavy rain over Lake Michigan. A chance of severe storms was in the forecast, and there was plenty of lightning and thunder nearby.

I was reading while watching the Nexrad with only glances when I noticed a cell increase from yellow to bright red and move at least six or seven miles as the radar sweep line passed. I thought, wow, that is sure a cell to watch if it can grow and move that fast.

Then it dawned on me what was happening. A Nexrad radar requires five to six minutes to complete a full 360 degree sweep depending on what mode the operator selects. The incredible growth and rapid movement of the cell I had noticed was the result of completion of that minutes long Nexrad sweep, not the 52-second rotation of the fake radar sweep line.

I hacked my watch and it took more than five minutes for anything to change again on the radar display. The fake sweep kept going round and round, but no cells moved, or changed shape or intensity until five minutes had passed since the last new picture.

I have written many times about the delay inherent in Nexrad, particularly the delay of receiving a Nexrad mosaic image in the cockpit. But this was the first real live example I have seen of how crucial that delay can be. Before my eyes a cell had grown from moderate to very heavy rain and moved many miles since it was last presented by the Nexrad. If you had been trying to weave around that cell using Nexrad you could have had a really big surprise.

Like I said, I understand the purpose of the fake sweep line on the TV presentation, but what a vivid demonstration of how Nexrad, at its best, is showing us weather history. When looking at a single site like I was the weather history is only five or six minutes old, but that still made a huge difference in cell development and movement. Add in all the delays of creating the mosaic and then transmitting it to the cockpit and we are really looking at old news.

Nexrad in the cockpit is the best thing to happen for pilots in a very long time. But we can only use it to miss by many miles, not to weave our way around storms that may no longer be where the radar shows.

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7 Responses to Nexrad Weather Radar Fakery

  1. Roger Halstead says:

    Although I think you are correct, NEXTRAD is a computer generated display that “updates” from a series of what we would call normal scans. There is no 360 degree sweep every 5 or 6 minutes.

    This is an over simplification, but think of a computer with a new input every 45 seconds or let’s say ever minute for simplicity. each of these sweeps is processed for many things, among which are the tine of the return, the Doppler shift o the return, the asmuth of the return. all of these sweeps are processes, combined, and I believe, processed again so the NEWEST a display can be is 5 minutes because of the processing time, but the critical part for aviation is that display can be 10 minutes old the instant before it updates. That is if everything works as it should. That is why you should always wait for an update to see what a storm is doing. A single display tells you where the storm was, 5 to 10 minutes ago and with cell vectors turned on you know which way each cell is going. Reading the display is not as simple as red means bad. I’ve flown the Debonair, IFR through areas that were mostly red and outside of getting it washed thoroughly it was one of the smoothest rides I’d had in a long time. The view was surrealistic between layers with columns joining the two layers along with the occasional solitary cloud drifting here and there . It reminded me of an animated sci fi movie from a few years back.

    Waiting for an update tells you if the storm is changing in intensity. Remember the colors are only showing the intensity of the precipitation. it takes a different screen to look at differential winds and they are in relation to the RADAR site.

    I have a subscription service called Weather Tap (TM) so I can see many aspects of a storm in detail, but I know what I view is never less than 5 minutes old. It can’t be any newer, even for aviation weather down or up linked to the cockpit.

    But as to that sweep you mentioned, it could be a real representation of the individual sweeps or just a cosmetic addition for the general population.

    BTW I understand the new RADAR just coming on line may only require half the time to generate the image which would be a real boon to aviation as images in the cockpit would only be 2.5 to 5 minutes old. Then you really could depend on it. As it is now a display could be of a storm moving 60 MPH. If the display is 10 minutes old the storm may have moved 10 miles and you do not want to be 10 miles (or less) ahead of a severe thunder storm. That’s somewhere near where the hail falls at altitude.

  2. Ed Yess says:

    I’ve been retired from the NWS for nearly 10 years, but my recollection is that we did not add a “sweep” to any NEXRAD products. These “sweeps” appear on the radar mosaics during local TV weather broadcasts but, if you’ll notice, are not on any NWS radar products available on the NOAA websites. I firmly believe these are added by the TV station. The general population has long been conditioned to associate these “sweeps” with all radars and it implies real-time data.

  3. KB says:

    If you want a fascinating and practical lesson on the workings and interpretation of NEXRAD for aviation, seek out and attend a presentation by Dr. David Strahle. He’s outstanding. Also, for a good lecture on airborne radar, I’d add Erik Eliel. Both are easily Googled ™.

    I’ll add, in addition to the ~six minute update lag inherent in NEXRAD itself that Mac mentions, is any lag in the system that delivers the presentation to you (e.g. XM) which could worst case be up to 20 minutes total. A whole lot happens in six minutes, let alone 20.

    Be careful out there…

  4. trevor Smith says:

    Mac: Good observations and the reader comments are perfect, the key being that the radar is 10 minutes old. The radar is good but only for long range flight planning, not dodging cells. The really sad part of this is that aviation could have had this radar for FREE years ago. The NWS has both the public and AWOS voice WX reporting. Turns out that they could have added a “data tone burst” to all those VHF channles. That tone burst would have contained the radar data set (much like the old modem days) and the vhf receiver with a tiny modem (very cheap) could display the radar. My boss, Kevin, emailed the NWS about this 5 times and got not response. By the way, we have a free software utility called QUICK RADAR (windows) at http://www.vfrcopilot.com/1_QuickRADARfree.htm that is much faster than the NWS web site (we preload all the backround layers for the entire US converage).

  5. Greg W says:

    Great points about the use of nexrad “radar” in flight. I simply do not understand why so many pilots “need” the latest electronics including the nexrad displays. So many seem to fly into weather they should avoid because of the “safety and situational awareness” afforded by the electronic systems. They seem to use these systems to justify pushing their abilities and that of their airframes. If all this makes us safer then why with nexrad do we still have so many VFR into IMC accidents? This holds true to me for CFIT and running out of gas, the GPS true ground speeds and terrain warnings were supposed to stop these accidents. We need to change the culture back somewhat so we are PILOTs again and not system operators. Look out the windshield, stay away from the ground, go WAY around the storms stay safe and get out and fly.

    • Roger Halstead says:

      If you know your weather well enough, know your airplane and are proficient then flying through rain, even heavy rain and no more than mild turbulence can add a great deal of utility to an airplane . Notice I said proficient, not current.

      If you only have one engine, then it’s a good idea to either have at least marginal VFR under you or close by. Know which way the upper level winds are blowing and never try to skirt AHEAD of a thunderstorm. Ever see hail with a clear sky overhead when the TS is 10 miles “over there”? I have and many times.

  6. Pingback: The Nexrad Delay | High Altitude Flying Club

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