Icing Folklore

Folklore: It's possible to collect ice at a temperature greater than zero degrees Celsius.

It's common to hear pilots say that they've picked up ice at temperatures above freezing---even as warm as 5 degrees Celsius. Induction icing, sure; but structural icing---it can't happen. Typically this is attributed to a faulty or inaccurate outside air temperature (OAT) probe that's reading a few degrees too warm.

Assuming the OAT probe is accurate, the other possible explanation is that the pilot descended into a supercooled liquid cloud while the temperature just above the cloud was a degree or two above freezing. It's not uncommon to see a temperature inversion (an increase in temperature with altitude) immediately above a cloud deck. The pilot may notice that the OAT is 2 or 3 degrees Celsius just before entering the cloud and jump to the quick conclusion that the remainder of the descent has to be above freezing. He or she is then astonished to witness ice form on the wings, being completely unaware that the temperature was actually colder in the cloud than above it.

Cold soaking is another possibility, though it's rare. Imagine an aircraft quickly descending from very cold conditions into a cloud deck where the temperature is slightly warmer than zero degrees Celsius. There may be just enough thermal momentum to keep part of the aircraft's surface below freezing, causing ice to accrete briefly.

Folklore: A layer of clouds a couple thousand feet thick isn't a serious icing hazard.

This definitely isn't the case for a stratocumulus cloud deck. These clouds are often found in the wake of strong cold fronts. Stratocumulus clouds can produce some nasty icing and typically are only a few thousand feet thick. They have characteristics of both stratus clouds and cumulus clouds. They can cover a large geographic area like stratus, but are associated with instability like cumulus. These clouds are "capped" by a strong temperature inversion, which acts as a lid on their growth. Instead of the smooth appearance of the tops of most overcast stratus decks, stratocumulus clouds have a quiltlike appearance.

On January 13, 2006, stratocumulus clouds crippled a Cirrus SR22 southeast of Birmingham, Ala. The pilot departed Birmingham and attempted to climb on top of this "thin" deck of clouds. Just as he broke out on top, he had to activate the Cirrus Airframe Parachute System (CAPS) after losing control of the aircraft because of significant ice accumulation.

According to the NTSB, "The airplane entered the clouds at 5,000 feet on autopilot, climbing at 120 knots. Upon reaching 7,000 feet, the airplane encountered icing conditions. The pilot informed the controller that he'd like to climb to 9,000 feet, which was approved. As the airplane reached the cloud tops in visual flight conditions at 8,000 feet, the airplane began to buffet."

It's common to have the highest liquid-water content at the very top of these clouds, where the drops are normally the largest and the temperature is the coldest. In this case, the cloud-top temperature was minus-10 degrees Celsius---perfect for a nasty icing event.

Folklore: Flying into a cloud that's producing snow isn't an icing risk.

Snow falling from a cloud base is a good sign that ice crystals exist in the clouds producing them. Thus, it's easy to conclude that supercooled liquid water doesn't exist in them, and therefore, there isn't an icing risk in these clouds.

While there are important exceptions, precipitation falling from a cloud can lessen the icing threat within that cloud. Moreover, if the precipitation is snow, the threat of icing is even less---but not zero. Snowfall can scour out the supercooled liquid water in the cloud, but a snow-producing cloud may still be a mixed-phase cloud. That is, it will contain both ice crystals and supercooled liquid water.

This is especially true when the snow falling is "showery." Aircraft flying into weather that's producing snow showers will often report rime ice accretion. Normally, the most significant accretion isn't from the clouds producing the snow, but from those around the snow-producing clouds. Those normally contain the highest liquid-water content.

Folklore: Climbing is usually the best option when flying through freezing rain or freezing drizzle.

If you're collecting ice and you're below the clouds or between layers, then you likely are flying in freezing rain or freezing drizzle. Pilots are taught to climb to a higher altitude if this happens. That's making a dangerous assumption that there's an altitude above you that contains air that's warmer than zero degrees Celsius.

In the case of freezing rain, there might actually be a warm layer above. Freezing rain is produced when snowfall from a cloud enters a layer of air that's above freezing. The warmer air in this layer melts the snow into rain drops, which then fall into a subfreezing layer below and deposit on your aircraft. In this particular case, a warmer layer aloft does exist.

For freezing drizzle, this often isn't the case. There likely will be warmer air above you, but often the temperature aloft doesn't rise above the freezing mark. Freezing drizzle is normally the result of an all-liquid process as small drops in the cloud collide and coalesce into drizzle-sized drops large enough to fall out of the cloud as freezing drizzle.

The best strategy in this case is to turn around and go back to where you were not accreting ice. If you don't know the temperature profile above you, it may not be wise to attempt a climb.

Folklore: Using the standard lapse rate is a good way to estimate the freezing level. For example, if the temperature at the surface is 4 degrees Celsius, the freezing level is 2,000 feet above ground level.

Many pilots apply the standard lapse rate of 2 degrees Celsius per 1,000 feet. This means that the temperature decreases 2 degrees Celsius for every 1,000-foot gain in altitude. Therefore, with a temperature of 4 degrees Celsius at the surface, flying in the clouds at, say, 5,000 feet AGL would be a bad idea. Correct?

Actually, you can't rely on these estimates, especially when Mother Nature is at her worst. In an unstable situation, such as that discussed with stratocumulus clouds, the lapse rate near the surface is often greater than standard---often reaching 3 degrees Celsius per 1,000 feet. At the other extreme, in a stable environment, there often is a temperature inversion that may put the freezing level as high as 10,000 feet AGL.

Estimating the freezing level using the standard lapse rate is like dividing 19 by 95 and getting 1⁄5 by crossing out the 9s. The method worked well for this particular example, but it's less likely to work when applied to other situations.

When it comes to icing, there are very few rules of thumb that a pilot can apply successfully. However, one of my personal favorites is called a "glory." As you fly over clouds, you may see your airplane's shadow cast on the top of the cloud, assuming the sun angle is perfect and there are no clouds above you blocking the sun. If there's liquid water in the cloud, you may also have witnessed a glory. A glory is a halo (or rainbow) around your aircraft's shadow. A glory is an excellent indicator that the cloud is dominated by liquid water. When the OAT is below freezing, it may be an excellent indicator that there's icing potential in the clouds below. If you see the shadow of your airplane and you don't see a glory, you may see what look like sparkles around the shadow. This typically means that the cloud is glaciated. In other words, the cloud is dominated by ice crystals.

My best advice with respect to icing is to put aside old wives' tales and avoid using rules of thumb. If you want to know the freezing level, for example, look at a temperature profile of the atmosphere using a forecast temperature sounding instead of guessing with a formula that works only when the atmosphere fits the formula. If you want to know if a snow-producing cloud is an icing threat, look for the presence of AIRMET Zulu or examine the Current Icing Product (CIP) probability analysis. If you want to know if you'll encounter freezing drizzle aloft, use the CIP SLD (supercooled large drop) analysis. In other words, attack the problem directly by utilizing weather products that depict the current icing environment instead of relying on folklore.