If you paid attention to NEXRAD training, you know that the colors of pixels show the maximum activity at any altitude in that air column. (It’s a “composite” view.)

NEXRAD itself samples in three dimensions, so that integration into the composite is being done on the ground before being sent back out on SiriusXM or ADS-B.

Given ADS-B’s bandwidth constraints, it makes sense that you only get maximums—it’s the thing that most pilots will care about. The bandwidth constraints are the same reason pixel granularity falls off with distance—the stations can’t send nationwide data in the bandwidth allotted.

But could SiriusXM give you the raw 3D data in addition to the composite view? They have the bandwidth where you could dial in a given altitude and get a 2D slice rather than just maximums. And you can get other data like wind on an altitude basis, so the capability seems to be there.

I don’t know if it’s a case of cost (considering how SiriusXM charges for subscriptions, I imagine they’d be happy to offer this to pilots as an additional tier), a worry that some pilots could take on even more risk around storms by trying to do 3D rather than just 2D daisy-clipping, or something else (like a technical issue with NEXRAD—as far as I can tell, none of the weather sites offering NEXRAD give the ability to dial in altitudes).

Given how many accident studies (even into incidents where the PIC’s were instrument-rated private pilots) have shown that pilots who flew into unexpected IMC or icing could have survived had they made a different decision about what altitude to seek for safety, it feels like it could be an improvement in situational awareness and safety, and usually we seem to accept those technologies in aviation, even if they may also present greater risk of recklessness.

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    $\begingroup$ An aircraft's shipboard weather radar can provide that 3D picture (by use of tilt and vertical modes). An aircraft's NEXRAD display is delayed with several minutes of latency and is typically used for big picture strategic planning. Shipboard weather radar is better used for tactical purposes. $\endgroup$ – J Walters Aug 18 '19 at 11:06
  • $\begingroup$ One thinks that the pilots flying into unexpected IMC wouldn't be able to figure out how to use 3D data in a timely manner. The risk is not about the altitude, but the lateral confines of the IMC and precipitation. Additionally, clouds don't show up on radar, only precip does. So NEXRAD won't help pilots avoid clouds. $\endgroup$ – Mark Jones Jr. Aug 24 '19 at 22:50

Data about the height of precipitation would be useful in real-time avoidance of weather phenomena. In fact, this information is provided by NEXRAD and is known as "echo tops." Echo tops also appear in suitably sophisticated SiriusXM weather displays.

However, ground based radar is not the primary tool used for weather avoidance, and in particular, datalink radar data, like Sirius XM should not be used for weather avoidance. According to the FAA:

Unlike airborne weather avoidance radar, weather data linked from a ground weather surveillance radar system is not real-time information. The radar data displays recent rather than current weather conditions. As the current location of a thunderstorm cell may be different than the broadcast weather product, do not attempt to find a hole in a thunderstorm solely using data-linked weather. Pilots must avoid individual storms by visual sighting or by airborne weather radar.

Airborne weather radar does include three dimensional data. The image here shows that a radar beam has height, as well as range and width. enter image description here

In those cases where weather cannot be avoided completely, the pilot chooses the path with the least intensity precipitation. This can include a climb to an area of lower intensity precipitation.

Finally, icing information, including pilot reports and forecasts is included in the SiriusXM aviation products, as detailed here. Thus one does not have to use NEXRAD height data and correlate to temperatures to avoid areas of icing.

  • $\begingroup$ Your answer addressed the question as asked, so thank you for that. This question occurred to me frequently in reviewing accident reports when I noticed that icing and other Wx issues where the same flight path at different altitudes would have resulted in radically different outcomes also seemed to be excruciatingly long in duration, compared to most accident report emergencies—sometimes many hours between recognition of issue and point of no return. So there was time to consider things like condition stratification; but would the tech to do so have helped? Thanks for the answer. $\endgroup$ – Trey Aug 26 '19 at 19:29

There is not much gained by the third dimension for your use case. In GA you generally want to avoid any active weather. Why make it more complicated?

  • can't go over it (performance limited)
  • can't go under it (microbursts can kill you)

=> only way is to go around it and therefore no need for that 3D information.

  • $\begingroup$ Chris, I may have made a big mistake by mentioning “GA” in the question—the only reason I did was because there hasn’t been a weather-related airliner fatality in the US in so long that the recent incident reports are all GA. But many involved aircraft—e.g. (video) in a Socata TBM-700—aren’t so performance-limited as to make altitude information completely useless. $\endgroup$ – Trey Aug 18 '19 at 20:38
  • $\begingroup$ @Trey, you linked an icing-related accident, but radar does not provide any information about icing. It only provides information about precipitation and convective activity (storms). $\endgroup$ – Jan Hudec Aug 24 '19 at 20:04
  • $\begingroup$ @JanHudec sure—but you can get freezing level with ADS-B or SiriusXM, and the formula there is pretty straightforward: above freezing level and in precip, icing. Climbing out of precip will avoid icing, even if aircraft below you might be icing. There have been three incidents since 2000 just in those that AOPA’s Flight Safety Institute has reviewed where climbing above precip rather than trying to descend below freezing level would have been better. $\endgroup$ – Trey Aug 24 '19 at 20:59
  • $\begingroup$ Correction: I mean’t “climbing above precip”, not “climbing out of precip”. Obviously if you’re already getting ice accumulation, you have to get below freezing level to get rid of it (if you don’t have a de-icing system). But climbing to where you never get ice in the first place can do the job, too. $\endgroup$ – Trey Aug 24 '19 at 21:07
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    $\begingroup$ @Trey, no, it's not that simple. The radar sees both water droplets (where you get icing) and ice crystals (where you don't), but only if they are large enough and only if they are affected by some turbulence. Rain not associated with convective activity shows weakly or sometimes even not at all, but is more likely to cause severe icing as the water droplets are staying at their level while convective activity would bring them higher where they'd freeze. $\endgroup$ – Jan Hudec Aug 25 '19 at 10:24

The danger of VFR flight into IMC is flying into clouds when you aren't properly trained for it. Icing generally only occurs in clouds between -20°C and +5°C. NEXRAD shows the strength of the radar returns, which is usually an indicator of precipitation intensity, though, not clouds, so it doesn't really help avoid either of those things.

Regarding why NEXRAD isn't 3D, precipitation almost always falls to the surface, so there is no need for a minimum altitude. On the other hand, taller storms tend to be more intense, so you have a good idea what the maximum altitude is just by looking at the color. Generally speaking, if precipitation is too intense to fly through in a particular plane, it's probably too tall for that plane to climb over as well. So, there's no practical benefit to 3D.

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    $\begingroup$ In more severe storms, much of the precipitation does not fall to the surface initially, but is carried about with up and down drafts. This is an important consideration when looking for possible areas of hail (for the pilot this is typically done with onboard radar, though 2d radar like NEXRAD can also be informative). $\endgroup$ – J Walters Aug 18 '19 at 10:57
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    $\begingroup$ I would question your comment about the precipitation intensity related to the height of the storm. The aircraft that I fly can fly at FL450, and top many medium intensity thunderstorms. I have flown over large, severe storm systems that have brought tornados and caused deaths on the ground. Flying through many parts of these storms could have been deadly, but flying overtop the storm was smooth and uneventful. $\endgroup$ – J Walters Aug 18 '19 at 11:01
  • $\begingroup$ StephenS—again, like in my response to the other answer, I may have injected too many assumptions by mentioning “GA”. In this case, I didn’t necessarily mean non-instrument-rated pilots entering IMC or icing. Unexpectedly entering IMC with moderate-to-severe Wx can be a problem even for instrumented-rated pilots (though pretty much exclusively GA pilots, since their IMC flight hours logged will be much lower). A pilot flying e.g. @JWalters FL450-capable aircraft (including jets!) may, in fact, be GA—even though that’s not at all what we imagine when we hear “GA”. $\endgroup$ – Trey Aug 18 '19 at 20:46
  • $\begingroup$ @JWalters FL450 is exceptionally high, possible for maybe 1% of GA traffic. And by the time someone is flying such planes, they should have the experience to not fly into danger despite a lack of 3D NEXRAD. The other 99% will be flying around any purples, reds and maybe yellows rather than trying (and probably failing) to fly over them. $\endgroup$ – StephenS Aug 19 '19 at 0:04
  • $\begingroup$ Ok, we must be working from different definitions of GA. I have only ever flown GA by some definitions, even under §135, but flying a light jet under §91 would generally be included under GA. Let me assure you that there are a great many §91 jets and turboprops that can top storms that the pilots would otherwise not fly through. $\endgroup$ – J Walters Aug 19 '19 at 21:10

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