Wx Watch: Icing on the Internet

Where's the ice? The Web can show the way

November 1, 2002

It's ba-a-ck! Icing season, that is. Fortunately for us, more and more progress is being made on the icing prediction front. One Web site leads the way — http://cdm.aviationweather.gov/cip/ — with its current-icing-potential (CIP) feature. It's produced by the National Weather Service's Aviation Weather Center.

The CIP reports the potential for the most commonly encountered types of icing (clear, rime, and mixed), plus supercooled large-droplet (SLD) icing. As you may recall from past "WxWatch" columns, SLD icing is especially hazardous because its effects can be sudden, and it can coat an entire airplane very quickly. That's because the droplets remain liquid well after they strike an airplane's leading edges. This supercooled liquid runback doesn't freeze until it's well aft of any ice-protection devices — where it can change the shape of a wing's cross section and ruin both a wing's pressure distribution pattern and the airplane's overall handling characteristics in the roll and pitch axes.

Large-droplet icing occurs in the most moisture-laden areas of the world. In the contiguous United States, the Pacific Northwest and Great Lakes regions take the prize. New England is a close second.

The CIP is a stone-simple site. Vertical columns on the left side of the screen have buttons corresponding to altitudes. The column labeled Total refers to garden-variety icing; the SLD column is for identifying altitudes that may contain supercooled large-droplet icing.

Click on the button labeled Previous and you'll see the maps for the past two hours' worth of icing potential. The Update button reloads the latest information. Updates occur automatically every 10 minutes.

On the right side of the display is a vertical scale with colors that represent the potential for icing conditions to exist, expressed in percentages. The CIP's word isn't gospel, just a computer model's estimate of where icing conditions may reasonably be expected.

The color-coded map is accessed via a MouseOver feature. Just move your cursor over the altitudes you plan to use and up pops the icing potential. The imagery appears the instant you pick an altitude, so it's easy to run the cursor up or down the altitude scale to see where the highest icing potentials (the reddish colors) begin and end.

Unlike the icing information contained in airmets and sigmets, which can blanket entire clusters of states, the CIP plots specific areas that are ice-prone. Its information comes from a blend of GOES-8 satellite imagery, surface observations, and RUC (Rapid Update Cycle) model algorithms. After some data-crunching, the three-dimensional extent of cloud is calculated, then the national radar mosaic is applied to identify the locations and likelihood of both conventional and SLD icing across the United States and Canada.

Right about now you're probably thinking of ignoring any Airmets Zulu (the airmets that identify freezing levels and warn of moderate icing) or sigmets (which warn of severe icing) in favor of the CIP. Well, hold your horses. Across the bottom of the CIP map is a disclaimer: "[The CIP] does not substitute for the intensity and forecast information contained in airmets and sigmets. It is authorized for operational use by meteorologists and dispatchers only."

So there. The CIP is considered supplemental information, merely an adjunct to the information you receive from flight service station briefings. Its rarified insight has been deemed worthy of an inner clique only, not the pilot rabble who may actually confront icing face-to-face.

This raises the issue of a "legal" FAA-endorsed preflight weather briefing. Are our operational decisions limited to the insights provided by flight service alone? The answer is a most definite no. The regulations even say so.

Federal Aviation Regulations (FAR) Part 91.103 does not specify where pilots may receive their preflight weather information. To paraphrase, it says that each pilot, prior to a flight under IFR or a flight not in the vicinity of an airport, must "become familiar with all available information concerning that flight [including] weather reports and forecasts." Flight service isn't singled out, and Internet sources aren't ruled out.

Does this mean we can skip an FSS briefing in favor of visiting the large number of aviation weather-related Web sites available? That thinking may reflect the letter of the law, but it's certainly not the intent. Besides, you still need to check with flight service — or DUATS — to learn of any notams, temporary flight restrictions (TFRs), and ATC delays affecting your flight.

Pilots need the structured format of an FSS briefing and the advice provided by any and all relevant National Weather Service products. Most pilots aren't savvy enough to brief themselves, or make independent weather-related operational decisions. And not all aviation weather providers on the Internet are up to snuff.

To remedy this, the FAA published a proposed advisory circular (AC) in the January 14 Federal Register. It set out the requirements for Internet weather providers to earn what amounts to an FAA seal of approval, making them — in FAA-speak — Qualified Internet Communications Providers (QICPs). The requirements insist on reliability, accessibility, and security.

Reliability means users must be able to retrieve requested data from a provider with no outage lasting more than 10 minutes, and no more than 30 minutes of total outages in any continuous three-month period.

Accessibility means that providers should be able to transmit requested data to 100 percent of their users within two minutes.

Security means that providers must have security practices in place to prevent unauthorized access to, or modification of, provider data, software, and hardware.

So far, the proposed AC has yet to become official. AOPA, of course, is in favor of it.

With or without becoming QICPs, aviation weather sites on the Internet are there for you to use. Just be sure to check for disclaimers, pay attention to any valid- and issue-times posted with the information, and verify your data with flight service as part of your flight-plan filing routine.


E-mail the author at tom.horne@aopa.org.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.