Icing

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This image displays the relative threat level of different types of clouds with respect to icing. Clouds filled with warm liquid droplets pose no icing threat unless they adhere to a very cold surface. Clouds containing uniquely supercooled liquid droplets are the most dangerous for icing conditions, since the supercooled droplets can readily freeze onto surface with which they come in contact. Mixed phase clouds are less a risk, since the frozen particles collect super-cooled water on the way down, creating larger precipitation. Ice clouds pose no threat for icing, as all available water is already in ice form, and will not stick to surfaces.

There are two main processes that can lead to icing conditions: precipitation falling and freezing on cold surfaces and supercooled liquid water freezing on cold surfaces in-flight. For the purpose of this term, only conditions which allow for supercooled liquid water accretion on surfaces will be discussed, and not accumulation of contaminants/precipitation on aircraft which may be removed during the ground de-icing process, or by in-flight de-icing equipment.

On the ground and in low-level flight, icing will occur in freezing precipitation; freezing rain will give severe icing conditions on contact with cold surfaces, while freezing drizzle is generally more moderate, mixed icing that could become severe if the freezing drizzle is long lasting. However, it should be noted that icing can also occur when precipitation (such as snow) falls onto the aircraft in-flight and freezes to the body.

The key ingredient required for in-flight icing conditions is the presence of supercooled liquid water. Supercooled liquid water is water that exists in a liquid state even when its temperature descends below 0^oC. This can happen for a variety of reasons, including droplets being free from imperfections on which ice could form, and low temperatures reducing energy in the droplet, making it difficult for the molecules to rearrange themselves into crystalline structures.

The factors that will impact the severity and extent of the icing are the abundance of supercooled liquid water present in the cloud, the temperature and the drop size. Generally, the greatest amount of supercooled liquid water, and thus icing, is in clouds that are between 0°C and -15°C. While icing conditions can exist outside of this range, the relative abundance of frozen particles increases as temperatures decrease, meaning that there is less super-cooled water available to freeze directly onto surfaces. Super-cooled water can exist down to temperatures of -40^oC, though it becomes less and less abundant at colder temperatures. Below -40^oC, the supercooled droplets will spontaneously freeze, and the risk of in-flight icing is eliminated.

Convective clouds also account for a significant source of in-cloud icing. Due to their significant updrafts, and generally warmer temperatures, supercooled water droplets can be injected higher into the atmosphere and distributed through a larger portion of the cloud. Because of this, supercooled droplets tend to be more abundant, and the risk of icing tends to be more, and higher in the cloud. Often, icing conditions tend to be worse near the convective cloud top, where any intensity of MXD icing, or even moderate clear icing, is possible.

Image Source: COMET^®Open a new window

Icing is also categorized as light, moderate or severe, with definitions of these being as follows:

Light (> 1 g/cm²/h) – The rate of accretion may create a problem if flight is prolonged in the icing environment (generally over 1 hour). Generally not a problem for aircraft that use de-icing/anti-icing equipment.
Moderate (> 6 g/cm²/h) – The rate of accretion is such that even a short encounter becomes hazardous. The use of de-icing/anti-icing equipment or a diversion is required.
Severe (> 12 g/cm²/h) – The rate of accretion is such that de-icing/anti-icing equipment fails to reduce or control the hazard, immediate diversion is necessary.

Convective clouds can also support in-cloud icing conditions up to much lower temperatures (near -25^oC), as they have strong vertical motion in the cloud, which can move supercooled droplets much higher in the atmosphere than they might otherwise naturally occur. Because of this, convective clouds always pose an icing risk.

This image shows areas around a low-pressure system in which icing is most likely. Some environments are more susceptible to icing, specifically ones that are moist, warm and have lift. If we relate this to general cloud structure around a low, environments that are more prone to icing are ahead of a warm front (also where freezing rain/ice pellets are likely to occur), with the trowal and in the convection associated with a cold front. Severe icing conditions can also be associated with mountain waves and thunderstorms, due to the strong lifting forces that push supercooled liquid water high into the atmosphere.

There are also cases of local freezing rain/ice pellets that occur in valleys where cold air is trapped, allowing rain to freeze prior to or on contact with the ground. The Ottawa and St-Lawrence River valleys are particularly susceptible to long periods of freezing rain but valleys in the Coast Mountains and Rocky Mountains are also susceptible to shorter duration freezing rain events in the valleys. Similarly, though infrequently, cold fronts bringing in very cold air at the surface can cause brief freezing rain at the surface.

With cases of freezing drizzle, mixed moderate icing can be expected below and within the stratus cloud that is producing the freezing drizzle. As with any icing environment, if an aircraft remains in a freezing drizzle environment for an extended period of time the ice can accrete to a level considered severe.

Image Source: Environment and Climate Change Canada

With cases of freezing rain/ice pellets, the icing will be most severe from the surface to the base of the above freezing layer (AFL). However, icing will also exist above the AFL within the cloud that is producing precipitation. This image is an example of an atmospheric temperature profile with an AFL. Above 4000ft AGL, temperatures are still cold enough to support supercooled water droplets. From 2000 - 4000ft, the cloud is above freezing, allowing precipitation falling into this layer to melt partially or completely, and reducing the risk of icing by supercooled droplets. Below this, temperatures fall again below freezing. Depending on the depth of this freezing layer, precipitation may entirely refreeze (pellets), or cool to near-zero, and freeze on contact with cold surfaces (freezing rain).

Image Source: Environment and Climate Change Canada

Icing is forecast and described as one of three possible types: Clear, Rime, and Mixed.

1. This is an example of clear icing forming on an aircraft surface. Clear Icing (CLR) – Water droplets do not all freeze instantly when meeting the leading edge of the aircraft, some of the supercooled liquid water spreads back along the aircraft before freezing. The outcome of this is a horn shaped cross-section of ice that destroys lift and rapidly increases drag. It is the least frequently reported and is generally associated with freezing rain (and sometimes ice pellets).

Image source: NASA - Lewis Research Centre

2. Rime Icing (RIME) – Small droplets at colder temperatures freeze on contact. Generally looks opaque or white in appearance and is granular or brittle. It is the type of icing most frequently reported. This image is an example of rime icing building up on cold surfaces.

Image Source: Météo France

3. Mixed icing (MX) - A mix of both clear and rime icing. Occurs in clouds where the temperature and droplet size vary. It is commonly reported in cloud, as well as with freezing drizzle. This is an example of mixed icing building on aircraft surfaces.

Image Source: NASA - Lewis Research Center

Duration

Icing conditions due to supercooled liquid water accreting onto an in-flight aircraft will only exist so long as there is supercooled liquid water, which can be up to several hours.

In the case of precipitation causing icing conditions on ground (FZRA, PL, FZDZ), conditions tend to be more brief over one particular region as FZRA and PL will move along with the warm front, usually making them last for a couple of hours at a time. However, if a warm front is slow moving or stalled, conditions can persist up to a day at a time. Stratus from which FZDZ falls can be quite long lasting, and requires a change in air mass, and conditions can last from hours to days, depending on the temperatures of the surface.

Dissipation

Icing conditions will dissipate only when an organized cloud with supercooled liquid water dissipates. When a cloud precipitates, this will generally encourage the presence of ice crystal growth and decrease the presence of supercooled liquid water. Additionally, falling precipitation can "catch" supercooled droplets, growing the size of falling precipitation at the same time. The longer a cloud precipitates the less likely supercooled liquid water droplets will survive, although precipitating clouds still pose an icing risk.

Alternatively, warming of layers of the air mass (daytime heating, warm air advection, etc) to above zero will reduce the risk of icing, as well as cooling the cloud/air to -40^oC where supercooled liquid water is no longer possible.

Image Source: Environment and Climate Change Canada

Naturally, icing conditions are found more predominantly in late fall, winter and early spring, when temperatures are coldest, especially when linked to freezing precipitation types.

In-cloud icing conditions in Canada can persist through most of the year, but as surface temperatures increase, one must ascend higher into the atmosphere to find temperatures between 0^oC and -15^oC.

In-cloud icing conditions can be difficult to forecast as the size and content of supercooled liquid water can vary greatly within a cloud in space and time. On the GFA you will see icing conditions most often forecast in organized cloud areas that meet the temperature range between 0°C and -15°C as well as with freezing drizzle, ice pellets and freezing rain.

More difficult is timing the exact timing and location of precipitation that will generate icing conditions on the ground, primarily FZRA and PL. Knowing the exact depth and placement of the above freezing level at altitude can be difficult for models to resolve, and observations from upper air soundings and AMDARs are not always well placed or timely, in order to properly capture the shape of this feature. This problem is further exacerbated in regions with few observations, few PIREPs, and no radar.

This GFA, valid at 1200Z on April 14, 2023, shows a warm front associated with a low-pressure system extending across northwestern Ontario. Various precipitation types are forecast in the vicinity of the warm front, with rain or rain showers along and south of the front, freezing precipitation along the northern edge and further north, and snow to the north only. The icing and turbulence panel captures how this precipitation and associated cloud cover translates into icing risk based on precipitation type, cloud thickness, icing parameters within the cloud, and the location of the freezing level at various heights. Note that it is not only organized areas of precipitation located around synoptic fronts that generate potential icing; low-level cloud over northern Quebec, though only forecast to be 3000ft thick, is expected to generate moderate mixed icing in the cloud.

The areas of organized freezing precipitation along the front are associated with regions of forecast severe clear icing that exist in sub-freezing layers within the first 6000ft above the surface. This is the layer in which melted snowflakes, now raindrops, become supercooled by the below-freezing air, and become capable of freezing on contact with a surface such as critical surfaces on an airborne aircraft, or the ground. Further aloft, only moderate mixed icing is forecast from 9000ft to 18000ft as this region is above the melting layer, and no precipitation is at risk of melting and refreezing at this height. Between those two layers is the above-freezing layer (or AFL). This layer will not be associated with any icing conditions as it is the layer in which temperatures are forecast to be above 0C.

Deciphering the GFA icing panel can become difficult when the freezing layer, the altitude at which air temperatures drop below 0C, varies through the depth of the cloud. The dashed lines in the panel indicate lines of equal height for the freezing levels.

For example, the northern-most dashed line across northern Quebec, through Moosonee, ON (CYMO), and west to just south of Muskrat Dam, ON (CZMD) is the freezing level that indicates that the 0C isotherm is at the surface. This means that the entire column of air is below freezing, and as seen in the clouds and weather panel this translates into a forecast of pure snow.

Just south of the warm front is a different cloud area, with broken layers forecast between 4000ft and 12000ft, in which no icing is forecast. This is largely due to where the freezing level is located. Looking at the dashed lines collocated with that cloud area, we see the freezing level quickly rise in altitude; from 2500ft (dashed line through Matagami, QC (CYMT)) to 10000ft (dashed line just north of Kapuskasing, ON (CYYU)). Any clouds below this altitude at that dashed line or south of it will be in temperatures above 0C, thereby eliminating the risk of icing.

Pickle Lake, ON (CYPL) is located in the area of the GFA that is forecast to see significant changes in precipitation type as the warm front moves across the region. Starting off in freezing and frozen precipitation and then transitioning to straight rain showers after 1900Z. In freezing precipitation, as indicated in the GFA, severe clear icing can be expected below the AFL in sub-0C air. In most other cases, however, icing risk will not necessarily be evident in a TAF. This is because icing is a function of ingredients through the layer of the atmosphere where clouds, and possibly precipitation, exist, but can also exist in-cloud when no precipitation is forecast. As such, it is important to assess various forecast products when planning a flight and to consult for a weather briefing as needed.

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Pickle Lake, ON (CYPL) is located in an area of the GFA where freezing precipitation is forecast. The atmospheric profiles from the RDPS, showing forecast conditions at 1500Z and 2100Z respectively on April 14^th, depict the calculated icing profile over the site. Please note that the x-axis temperatures are in degrees Celsius, even though the surface temperature and dewpoints shown in red and green are in degrees Fahrenheit.

At 1500Z, surface temperatures are expected to hover right around 0C, but above the surface until approximately 4800ft is the super-cooled layer of sub-0C temperatures in which severe clear icing is possible. Temperatures gradually rise with height in the inversion present through this layer until they are above 0C, which is the base of the AFL shown in the GFA. The top of the AFL is where temperatures once again drop below 0C, and where icing once again exists in-cloud, forecast here to be moderate mixed in intensity and type.

By 2100Z, both surface temperatures are forecast to have risen above 0C and precipitation to have dissipated considerably in the broken cloud layer environment shown in the profile. Forecasters will assess such profiles for icing risk, among other tools, and take in all factors listed in the science explained section when determining whether or not icing should be forecast over any given region.

Image Source: Pivotal Weather

Muskrat Dam, ON (CZMD) remains in the area of forecast snow throughout the passage of the low and its associated warm front. On the icing and turbulence panel, we see the site is located to the north of the freezing level dashed line tagged as “SFC”. This indicates that, at or north of this line, the entirety of the column of air will be below zero. This is the reason why the forecast icing layer starts so low in the atmosphere in the GFA, showing moderate mixed icing from 4000ft to 18000ft. In this way, assessing where the freezing levels are on GFA icing and turbulence panels can provide hints as to the phase of precipitation that may occur – while providing critical information on risks of both in-cloud and below-cloud icing.

Image Source: Pivotal Weather

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The RDPS forecast graphic for isobars and precipitation type, valid between 1200Z on April 14^th to 0500Z on April 15^th shows the precipitation phases associated with the warm front extending northward off the low-pressure system centered over northern Wisconsin. The anticipated areas of rain, freezing precipitation, and snow are shown, closely mirroring those seen in the 1800Z GFA clouds and weather panel.

Image Source: College DuPage

This icing severity graphic from the Aviation Weather Centre in the United States provides altitude-based icing information. For this specific event on April 14^th, a GIF of icing forecasts valid at 1400Z captures the risk seen with the frontal boundaries and associated low-pressure system. The forecast also captures a few other interesting points:

Starting at the surface, icing is seen at lower altitudes over northern Ontario. This is the result of the temperature variation north and south of the warm front as seen in the GFA icing and turbulence panel. North of the front surface temperatures are already below 0C, thus leading to icing possibility from the base of the cloud deck. Further south, the start of the forecast icing is instead based on where in the atmosphere temperatures drop below 0C. As altitude increases, additional areas of icing further south begin to show up – capturing this change in freezing level graphically.

Specific to the areas of freezing precipitation forecast in the 1800Z GFA panels, this forecast captures these areas as well with the highlighted “SLD (supercooled large droplets) threat”, albeit in slightly different geographical regions as this forecast is 4 hours ahead of the GFAs. These areas of red hatched marks indicate areas where especially dangerous icing potential is forecast and are collocated with areas of forecast freezing precipitation. Moreover, the SLD threat is mainly around 3000ft, which as per the GFA is below the AFL and in the area of forecast severe clear icing. This threat becomes much narrower and creeps north as we move to 5000ft and 7000ft, which can indicate the likelihood that the AFL is slightly higher here and the possibility of a deeper sub-0C layer resulting in ice pellets. Again, this risk is present in the GFA as well.

Image Source: Aviation Weather CenterOpen a new window

Radar imagery from northwestern Ontario valid 1200Z-1800Z on April 14^th shows the approach of organized precipitation over eastern Manitoba associated with the area ahead of the warm front closest to the low-pressure center shown on the GFA. Earlier images further north and east more intermittent precipitation exists. In-cloud icing, as indicated by the GFA forecast, is likely as cloud temperatures across the region. Due to radar coverage, no radar data is available at this time for Pickle Lake, ON (CYPL) or Muskrat Dam, ON (CZMD).

Image Source: Environment and Climate Change Canada

METARs for Pickle Lake, ON (CYPL) valid on April 14^th show both snow and freezing rain fell at the site as shown in the TAF. The presence of freezing precipitation at the surface increases the likelihood of severe icing in the lowest levels of the atmosphere, as was forecast in the GFA panel valid at 1800Z and shown previously.

Image Source: OGIMET

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