Wx Watch: Icing on the Web

Two must-see icing Web sites

November 1, 2004

How times change. A mere 15 years ago, flight service was the gospel of aviation weather. If you wanted weather information for your preflight planning back then, it was 1-800/WX-BRIEF. Then it was a matter of copying down the information rattled off by a telephone briefer. This wasn't — and still isn't — always an easy task. Some briefers are better than others. You can talk to one briefer, hang up the phone, phone another, then get a vastly different mental picture of the weather at hand.

The Internet has forever changed the briefer-only world. Now we can access a slew of aviation and nonaviation weather Web sites and have more objective, visual imagery of all kinds of weather conditions. This makes it easier for us to both understand the weather and make those go/no-go calls. A lot easier than trying to imagine what a telephone briefer is trying to describe.

An active weather research community spooled up at the same time the Internet did. The result has been a dramatic uptick in graphical aviation weather products, many of them — of course — focusing on convective activity and the icing environment.

Some products are experimental, and shouldn't be used for preflight planning purposes. Others have made it through the trial stage, are now operational, and the National Oceanic and Atmospheric Administration (NOAA) makes these products available to Internet sites upon demand.

As we enter another icing season, it's worthwhile to review two relatively new products — the current icing potential (CIP) and forecast icing potential (FIP). The CIP is operational; the FIP is currently in the experimental phase.

These can be found on the National Weather Service's Aviation Weather Center (AWC) Web site — the Aviation Digital Data Service (ADDS) — although they are also on some other private and public aviation weather sites. Access the CIP on the AWC Web site ( http://adds.aviationweather.gov/icing/) and the FIP on its Web site ( http://aviationweather.gov/exp/fip/).

Current icing potential (CIP)

The CIP is the go-to site for icing. Its home page gives images that expand with a mouse-click and show great detail about the vertical and horizontal extent of potential icing conditions.

The most complete information is at the top of the page, where six altitude levels can be selected. Click on the buttons listed to the left of the altitude schematic to call up the time frame you're interested in, then click on the altitudes you plan to use, and up comes a plot that shows the areas expected to produce airframe icing conditions. Notice that you can choose between "regular" icing (i.e., clear and rime) and SLD (supercooled large droplet) icing.

SLD icing is especially dangerous. It mainly occurs over and near New England, the Great Lakes, and the Pacific Northwest, and consists of water droplets several times larger than those that cause clear and rime icing. When these droplets strike an aircraft, they splatter and run far aft. Where more typical icing conditions create ice accretions limited to an aircraft's leading edges, the runback from SLD icing can range as far back as mid-chord of a flying surface — well aft of any deice boots or other ice-protected leading edge surfaces. Obviously, this compromises the aircraft's lift and handling characteristics. The damage can occur suddenly, too, as evidenced by the circumstances surrounding the fatal crash of an American Eagle ATR 72 over Roselawn, Indiana, in October 1994. It was meteorological analysis of this landmark accident that first identified SLD icing and the environment that produces it. Before this time, no one knew that icing could involve water droplets of SLD size.

To find the anticipated tops and bases of any icing clouds, click on the CIP's "tops" and "bases" links. Up pop color-keyed images showing the vertical limits of any icing layers.

Forecast icing potential (FIP)

The FIP, legally speaking, can't be used for preflight planning purposes by pilots. However, a disclaimer at the top of the FIP's main chart states that it's authorized for use by meteorologists and dispatchers. I guess we can't be trusted with the FIP quite yet. It's worthwhile mentioning that at this writing the FIP sometimes can't be called up. Perhaps it's because it's being updated; perhaps it's only available sporadically.

But it still is worth a look, and the "mouse over" feature really helps visualize forecast icing zones. Just move your mouse over the altitude and time you're interested in, and areas of icing appear on the basemap. Follow the color key on the left of the map to see the icing probabilities. Blue colors indicate lesser chances; yellows, oranges, and reds mean stronger possibilities. The FIP also has buttons you can click on to make the map show areas with airmets or sigmets.

Be especially careful to note the valid times posted on the CIP and FIP. Forecasts go out to 12 hours and there are projections for the next three hours every time an update is posted. Click on the Info link at the top of the mouseover box for full details on navigating the FIP page.

The computer algorithms used in the CIP and FIP are pretty simple. Basically, their models are set up to look for a combination of relative humidity greater than 50 percent (this would indicate clouds or precipitation) and temperatures below freezing. But simple though it is, professional aviation meteorologists rely on the CIP and FIP.

"ADDS and the CIP and FIP are some of the best Web sites out there," said Paul Hamilton, a senior meteorologist at NetJets' weather bureau. "We do our own forecasts in support of the dispatch requirements for the fractional ownership fleet, but we still check the CIP to get a first look to confirm what we've predicted. We also check the relative humidity on the Eta [a forecast model] and use pireps to help verify what we think is going on."

In icing season, our two biggest icing questions boil down to these: Where are the cloud bases with icing potential? Where are the cloud tops? What we want are tops low enough to safely outclimb. Cloud bases must be high enough to allow ice-free flight over high terrain, and allow any inadvertent icing accretions to be shed during the descent for landing. The CIP does a great job of answering these questions. Check it out and see what you think. Just remember to use common sense when flying around icing conditions this winter, avoid icing conditions if you can, and escape them immediately if you can't. We'll talk about flying strategies in the icing season in the next "Wx Watch."

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