Airframe icing has been a safety hazard since the first pilots flew in cold clouds, or in freezing precipitation. Can you imagine those early mail pilots trying to get across Pennsylvania in the winter with its lumpy terrain, perpetual cloud cover and freezing temperatures? With all of those leading edges, struts and flying wires to collect ice, it's a wonder any pilot made it.
In case you have forgotten somehow, icing is caused by supercooled water droplets freezing to the surfaces of an airframe. The word "super" means the droplet temperature and the air it is suspended in is below freezing, but the droplet remains liquid until disturbed by impact with the airplane.
This concept of supercooled appears to defy what we all learned in elementary school science class. That is, water freezes upon reaching a temperature of 0° C. Like most of nature's phenomenon, that is true -- but not always, and that's why icing can occur.
The potential for airframe icing is present whenever there is visible moisture and air temperature is at or below freezing. That is the definition used often by regulators, and you can find it in airplane operating manuals. And it is accurate because the "potential" for icing is present under those conditions, but actual icing occurs only a tiny percentage of the time when moisture is visible and air temperature is below freezing. That's what is so frustrating about ice. Only a tiny minority of cold clouds contain icing conditions, but there is no way to know with certainty which clouds will create ice, and which won't.
There is some potential confusion about icing with air temp above freezing, but the issue there is the so-called "ram rise" experienced by old-fashioned air temperature probes when an airplane is flying at high airspeeds. An air temperature reading that is not corrected for the effects of airspeed is called RAT (ram air temperature). At high speeds RAT can be as much as 10° C above static air temperature, so that's why some turbine airplane manuals advise of possible icing conditions with RAT above freezing.
In order for a drop of water to become supercooled and remain liquid it must be transported from air that is above freezing to cold air rather quickly. If a droplet moves gradually from warm to cold air it freezes into snow, or ice pellets, or even sleet. Snow isn't much of an issue for airplanes in flight because -- except for a thin white strip on the leading edges -- it doesn't stick. The same is true for ice pellets. Sleet, which contains a mixture of ice and liquid water, is a different matter altogether because the liquid water can freeze on the airframe, causing the already frozen ice pellets to adhere, too.
In a typical icing scenario the liquid water is moved from warm air to freezing air by some lifting force in the atmosphere. Convection -- the natural lift of warm air into colder air above -- is the most common means for getting a liquid drop of water into freezing air without actually freezing the water. The most extreme cases of convection are, of course, thunderstorms, and that's where you find the most extreme icing. But if you fly into a thunderstorm, ice is only one of your problems as you deal with severe turbulence that can break the airframe, the possibility of hail that can destroy the airplane, and the chance that rain and or hail will be so heavy that it drowns the engines. Avoiding thunderstorms is a different topic and one for which no pilot needs a reminder.
But there are mild forms of convection that are almost always present in the atmosphere, so air of various temperatures and moisture contents is always lifting or sinking. It is that almost continuous convection that can lift warm water into freezing air quickly enough to supercool the droplets and cause ice. That's why every cold cloud has at least a tiny chance of containing icing conditions.
Another atmospheric means of supercooling water is mechanical lifting. Air moving over rising terrain is lifted rapidly and water can easily become supercooled. Big mountains like the Rockies can lift droplets quickly and generate severe icing conditions in their lee and pilots of all categories of airplane are wisely alert to those conditions. But the lower mountains of the eastern U.S. are actually more consistent and threatening ice producers, because they have a great moisture supply upwind in the Great Lakes and from low-pressure systems that circulate moisture up from the Gulf of Mexico. If there is a place most likely to have icing conditions it is over the mountains downwind of the Great Lakes because there is mechanical lifting, a moisture source and long periods of cold temperatures.
Among the many frustrations when it comes to forecasting the presence of icing conditions is the very local and confined area of the special conditions it takes to make airframe ice. If the moisture travels from warm to cold too slowly, there is no ice. If air temperature changes only a couple of degrees, the unique conditions for icing are removed. And if the moisture source does not produce as predicted, icing won't occur either.
If you want to know how uncommon icing conditions are, talk to any airplane manufacturer who is conducting a flight into known icing conditions certification program. Special meteorological consultants advise test pilots on where icing is most likely to occur, but they are often wrong. Test programs can stretch out over weeks or months, as test pilots try to find icing conditions that match certification criteria and then fly in those conditions long enough to collect ice and data. Often the test airplane has to circle in a very small area to remain in icing conditions that a normal flight would exit in just a couple of minutes.
But it is the very capriciousness, and even rareness, of icing, that makes it a genuine threat. Because it can occur anytime there is cold air and moisture, we have to have a solution to escape before the drag of the ice accumulation overwhelms the ability of the airplane to fly.