Sunday, June 29, 2008

Weather in Contrast

It's thunderstorm season. Yay.

Every afternoon it seems, big towering cumulonimbus clouds get annoyed with us mere humans and blow out sheets of rain, bolts of lighting, and gusts of wind that make life complicated for everyone. If you happen to be in the air in a thin-skinned metal airplane, that includes a bunch of fun possibilities like turbulence, wind shear, icing, and hail.

Some pilots apparently think that we controllers have some kind of God's eye view of the weather. Unfortunately, T-storms don't have transponders and don't file flight plans.

What the Pilot Sees

Before you took off, you probably looked at all the cool weather charts on, iPilot, AOPA, and other sources. You pulled up nice detailed charts with all kinds of data, like some of these examples from and Before you'd even begun preflighting the airplane, you developed a weather picture with trend information, intensities, and forecasts. On your PDA or in your flight bag, it's likely you had printouts with Area Forecasts (FAs), Terminal Area Forecasts (TAFs), and Routine Weather Reports (METARs) on-hand.

Once you're in the air, if your aircraft is equipped with a glass cockpit system like the Garmin G1000 or one of the newer handheld GPS systems like the Garmin 496 below, you'll have access to satellite-delivered weather data. You can look ahead on your route of flight and view recent METARS at different airports, examine trends, and generally get a big picture view of the weather. Everything's in lovely shades of colors, telling you what areas from which you need to remain clear.

If your airplane's a swanky model with its own built-in weather radar, you can scan the sky yourself and see trouble ahead. Of course, this is subject to attenuation, meaning that the radar pulses get weaker or are "bent" more the deeper they get into the weather. The result is that it's difficult to see what lies beyond the first layer of weather.

Of course, as a last result, you can just look out the window and see what's going on out there.


You've got access to all kinds of data - web-based, satellite-based, airborne radar-based, and "Mark 1 Eyeball"-based.

What do controllers see?

Different facilities have different weather equipment. Since the ARTS-based one is the only one I'm familiar with, I'll just stick with that one for this post. However, a terminal facility with STARS or an En Route Center will have something very different.

How does a storm look on ours? Um, well, it kinda looks like this:

Isn't it beautiful? I think it's got a very "retro video game" feel to it. Just huge blocks of pixels all over.

I mean, look at Mario! He fits right in, doesn't he? :)

But in all seriousness, that is in fact what we're looking at when we're issuing weather to the pilots on our frequency.

Well, that doesn't look like a thunderstorm...

Nope, it doesn't. Those people looking out the window may see huge towering cumulonimbus clouds extending to FL450 and a thick overcast layer blanketing the entire county, but we don't see that.
  • Our scopes show only precipitation. No cloud cover, no turbulence, nothing. It could be a solid overcast sky and our scopes will be blank. When there is actual precipitation - rain, snow, hail - we start getting a primary target return on it. In essence, we don't see the storm itself but rather its footprint, the water it's leaving behind.

  • The stronger the intensity of the precip, the more opaque the image will be on our scope. There are six levels of intensity available to us:
    • 1: Light
    • 2: Moderate
    • 3-4: Heavy
    • 5-6: Extreme
  • However... our scopes can only show two levels of precipitation at a time. That's right: just two. Unlike, say, a satellite radar map where a storm image can go steadily from blue to yellow to orange to red, our equipment is much more limited. And that's two levels of green, since our scopes are of course monochrome.
Below is a shot of the control panel touch screen. Once again, there's big differences between different facilities' equipment. In a facility with STARS, for example, each controller can set up his individual scope to display whatever weather he wants. On our ARTS system, one control panel changes the settings on an entire bank of scopes. On a weather day, you'll frequently hear people calling back and forth, "Hey guys, I'm going to show 1's and 5's...." to give the other controllers a heads up on the display change.

We'll typically show levels #2 and #4, as depicted in the image of the control panel. This provides a good base, since we can see the moderate stuff and the heavy stuff. Any empty areas inside the 4's are interpreted as extreme weather.

When I was in art school, a lot of my drawing studies dealt with the concept of negative space. In short, rather than seeing what's there you see what's NOT there.

In the graphic below, #2 and #4 are selected. There are dark spaces between the #2 and #4 bands, and the center of the #4 is completely dark. So, we interpolate the combination of visible precip and invisible spaces as follows:

That sounds like a lot of limitations. What's on the plus-side?

Well, the precip returns we're getting are actual real time returns from the primary radar antenna. This means they're instant and reasonably accurate up to the moment. The satellite-based weather displays like the Garmins are updated every five or six minutes, and when you're talking rapidly building thunderstorms that's an eternity. Your GPS may be showing a gap in that line of storms ahead, but in two minutes it could already be closed. If we see a gap, it's probably real.

The real time nature of our displays allows us to guide pilots around the precip to an extent. For instance, if we see an aircraft tracking towards a precipitation return, we'll say "N123, there appears to be an area of moderate to heavy precipitation extending from your 10 to 1 o'clock, approximately 8 miles in diameter." This lets him know how wide the precip is and how "deep" it is. You can also inform him of possible gaps in the precip, though you should leave it up to the pilot to decide whether or not they should try to squeeze through that hole before closes.

Always remember what level of weather you're looking at. Different level displays give vastly different pictures:

Levels 3 & 4 makes it looks like there's just a small cell in the southeast corner...

...but Levels 1 & 2 give a vastly different picture, showing that the full storm system is quite large (at least 30 miles).

When there's a lot of storms in the area, you can plan on pilots deviating all over. "Deviations left and right of course approved. Fly heading XXX when able." The pilot's the one up in the thick of it, experiencing that weather first-hand, so you need to be flexible to their needs. The smart pilot will deviate anyway - controller instructions be damned - but an inexperienced or scared pilot may listen blindly to the controller and fly whatever heading they're assigned, no matter what lies ahead. If you order a pilot to fly a heading that takes them directly into a storm and he gets smacked down to his death by severe turbulence and down drafts, you will be up on a courtroom stand explaining why you thought it was a good idea to restrict his heading.

So that's the overview of weather from the ATC trainee's point of view, learning this stuff as I go. It was definitely a bit of a shock to me going from a pilot's perspective to an ATC's perspective as far how different the weather display is, when I was used to pretty maps and charts. Regardless, you do the best with the tools you have, and they generally do seem to work well.

1 comment:

Anonymous said...

QR Anguilliforme Newswire/Rockland County, New York - Tuesday October 7, 2008

Sources tell Quiet Rockland that U.S. Department of Transportation (USDOT) Secretary Mary Peters is now making preparations to brief the “new Administrator” of the Federal Aviation Administration (FAA).

In a related story, FAA promotes Ruth Leverenz to “Acting Deputy Administrator” as Internet-listed second-in-command Key Official, in anticipation of Acting Administrator Robert Allan “Bobby” Sturgell’s departure from FAA office:


For the full story, please see: