What Is Mixed Icing?

Answer: To answer that question, let’s look at the three icing types that pilots are asked to report. These include rime, clear, and mixed. What icing type accretes on your airframe depends on many environmental factors. Let’s briefly discuss each of these factors as it relates to the type of icing.  

Rime icing is a rough, milky, or opaque ice that is typically formed by the rapid freezing of supercooled liquid water drops onto the airframe. The rapid freezing helps to allow air to be trapped inside the ice, making it appear whiter. If you grew up with an old freezer that required regular defrosting, that ice buildup is similar to the appearance of rime ice. In other words, it has a frosty appearance.

First and foremost, rime icing is most common when temperatures are relatively cold, allowing the freezing process to occur rapidly. Small drop environments also tend to help with rapid freezing as do low liquid water contents. One place that this tends to occur is in stratiform clouds because the drops tend to be small and the water content tends to be low. But even in these clouds, if the temperatures are close enough to freezing, or the water content or drop size increases a bit, the icing could become more mixed or even clear. 

Keep in mind that the colder it gets, the more likely it is that any ice accreted would be rime. Remember, these are just tendencies. There’s no guarantee of what kind of ice you’ll get based solely on temperature or the type of cloud. There are many factors that come into play that are sometimes difficult to quantify or predict.

Clear icing is a glossy or translucent ice formed by the relatively slow freezing of supercooled liquid water drops. This tends to occur in clouds with a high liquid water content and larger drop sizes with rapid accretion like you might find in a cumuliform-type cloud. Clear ice also tends to occur in the warmer subfreezing temperature range and in  a large drop environment produced by freezing rain and freezing drizzle.

Moreover, larger drops such as those found in freezing rain and drizzle tend to exist at warmer subfreezing temperatures. Studies have shown that freezing rain only exists down to about minus 12 degrees Celsius, while freezing drizzle can exist at much colder temperatures, sometimes as cold as minus 21 degrees Celsius. However, the frequency of freezing rain and drizzle drops off sharply with decreasing temperature. In-flight studies suggest that the colder the situation, the smaller the drops tend to be outside of convective activity. 

Mixed icing can be thought of as a transition between clear and rime icing. Another way to get mixed icing is to fly through multiple icing situations, some that produce ice that’s more on the rime end of the spectrum and others that produce ice that’s more on the clear end of the spectrum. The overlap of these types can give it a mixed look. For mixed icing to build on its own, it comes down to that energy balance. If you’re somewhere between the energy balances that form rime and clear ice, then the resulting icing can have characteristics of both types.

Perhaps the most common occurrence of accreting mixed icing is during a climb or descent. For example, as the aircraft climbs, it may initially be accreting clear ice because of warmer temperatures. But as the temperatures get colder in the climb, rime ice begins to accrete over the clear ice, creating that mixed look. Essentially the altitude change takes the aircraft through multiple icing environments over a given time. Pilots will report this as mixed icing.

As shown in the pie chart above, rime is definitely the most common type reported. The reason rime ice is so common is because it occurs over a broad range of environmental conditions. Clear ice, on the other hand, occurs over a much narrower range of conditions, so it is observed less frequently. Mixed ice can be thought of as a transition from rime to clear ice, also occurring over a narrow range of conditions, so it is also relatively uncommon.  

Pilots are encouraged to report the type of icing they encounter. So, understanding where these types accumulate on the airframe can help you provide the best report. Rime icing tends to be closer to the immediate leading edges, thanks to the rapid freezing process. It’s the reason most ice protection systems are located on the leading edges of the airframe, where rime ice generally accumulates. Clear ice tends to extend farther back on the wing’s surface and sometimes well beyond the leading edge. If the aircraft has boots, then any ice accretion behind the protected surface can continue to accumulate, creating an ice ridging situation. Ice protection systems that employ TKS fluid do a wonderful job limiting runback ice since the fluid is dispersed well behind the TKS panels. These are generalities that hold true a lot of the time, but there are exceptions, especially as the complexity of the icing environment increases.

Making a good pilot weather report (PIREP) as it relates to airframe ice is critical. Reporting ice during a climb or descent without reporting the altitudes that you witnessed ice accretion is not helpful. Instead, provide the icing type along with the altitude range where icing was experienced. And be prepared to also provide the outside air temperature since it’s required anytime you report ice. It’s important to be sure you are reporting the static or outside air temperature and not the total air temperature—sometimes called the “ram” air temperature.  

The PIREP shown from the EZWxBrief progressive web app (ezwxbrief.com) is an example of a good icing report. The pilot of a Cessna 208 reported light, clear rime ice with a temperature of minus-10 degrees Celsius. But the remark in the report is the key. The remark (RMK) of “LGT CLEAR ICING 051-031” suggests that ice accretion was witnessed between 5,100 and 3,100 feet msl. About the only improvement I can suggest is to mention whether the icing was in the cloud or below the cloud within precipitation.