CHAPTER 2. TERMINOLOGY, ACRONYMS, AND ABBREVIATIONS
2-1. DEFINITIONS.
a. Adiabatic Cooling. A process by which a parcel of air cools. When a parcel of air is lifted, pressure is reduced due to the elevation increase. This reduction in pressure causes the parcel of air to expand in volume and in turn, the parcel cools to maintain an energy balance because no energy is added to the parcel.
b. Airman’s Meteorological Information (AIRMET). In-flight weather advisories concerning weather phenomena of operational interest to all pilots and especially to pilots of aircraft not approved for flight in icing conditions. AIRMETs concern weather of less severity than that covered by significant meteorological information (SIGMET) or Convective SIGMET.
AIRMETs may include moderate icing.
c. Automated Surface Observing System (ASOS)/Automated Weather Observation System (AWOS). A suite of sensors that measure, collect, and disseminate weather data to help meteorologists, pilots, and flight dispatchers prepare and monitor weather forecasts, plan flight routes, and provide necessary information for correct takeoffs and landings. There are many differences between these two automated weather systems and it is important for pilots to understand the strengths and limitations of the various configurations. The ASOS is comprised of a standardized suite of weather sensors and is a product of a National Weather Service (NWS), Department of Defense (DOD), and FAA joint venture. One of ASOS’s most important features is its ability to detect precipitation including intensity of rain, snow, and freezing rain. One ASOS limitation that currently exists is its inability to detect and report freezing drizzle without human augmentation. Pilots should be aware of this limitation at airports deemed service level C or D, as described in the Aeronautical Information Manual (AIM). A detailed description of ASOS’s capabilities can be found at the NWS ASOS Homepage:
http://www.nws.noaa.gov/asos/index.html.
d. The AWOS is a suite of weather sensors that are procured by the FAA or purchased by individuals, groups, airports, etc. There are seven different configurations of AWOS: AWOS A, AWOS I, AWOS II, AWOS III, AWOS III-P, AWOS-T, and AWOS P/T. Precipitation identification is only available with AWOS III-P or higher. It is important to note that the absence of reported precipitation does not mean that such conditions do not exist. The AWOS may not be configured to report this information. A detailed description of AWOS’s capabilities can be found in FAA AC 150/5220-16C, Automated Weather Observing Systems (AWOS) for Non-Federal Applications. The AIM also contains useful information on both ASOS and AWOS.
e. Aviation Weather Service Program. Aviation weather service provided by the NWS and the FAA that collects and disseminates pertinent weather information for pilots, aircraft operators, and ATC. Available aviation weather reports and forecasts are produced at NWS offices and displayed at FAA Flight Service Stations (FSS).
f. Center Weather Advisory (CWA). An unscheduled weather advisory issued by NWS meteorologists for use by ATC in alerting pilots of existing or anticipated adverse weather conditions within the next two hours. A CWA may modify a SIGMET.
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g. Clear Ice. A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets. The terms “clear” and “glaze” have been used for essentially the same type of ice accretion, although some reserve “clear” for thinner accretions which lack horns and conform to the airfoil.
h. Cold Front. Any nonoccluded front that moves in such a way that colder air replaces warmer air.
i. Convection. An atmospheric motion that is predominantly vertical, resulting in the transport and mixing of atmospheric properties.
j. Cumulus Clouds. Clouds in the form of detached domes or towers and are usually well defined. Cumulus clouds develop vertically in the form of rising mounds of which the bulging upper part often resembles a cauliflower; the sunlit parts of these clouds are mostly brilliant white. Their bases may be relatively dark and nearly horizontal.
k. Current Icing Product (CIP). A graphical planning product that combines sensor and numerical model data to provide a three-dimensional diagnosis of the probability and severity of icing, plus the potential for the presence of supercooled large drops (SLD). CIP is an unrestricted supplementary weather product.
l. Forecast Icing Product (FIP). The FIP examines numerical weather prediction model output to calculate the probability of encountering in-flight aircraft icing conditions. This icing product demonstrates the confidence that an atmospheric location, represented by a three-dimensional model grid box, will contain supercooled liquid water that is likely to form ice on an aircraft.
m. Freezing Drizzle. Drizzle is precipitation at ground level or aloft in the form of liquid water drops that have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle is drizzle that exists at air temperatures less than 0°C (supercooled), remains in liquid form, and freezes upon contact with objects on the surface or airborne.
n. Freezing Rain. Rain is precipitation at ground level or aloft in the form of liquid water drops which have diameters greater than 0.5 mm. Freezing rain is rain that exists at air temperatures less than 0°C (supercooled), remains in liquid form, and freezes upon contact with objects on the ground or in the air.
o. Front. The boundary between two air masses. A front can be classified as cold, warm, occluded, or stationary.
p. Hazardous Weather Information. Summary of SIGMETs, Convective SIGMETs, urgent pilot weather reports, CWAs, AIRMETs, and any other weather, such as isolated thunderstorms rapidly developing and increasing in intensity or low ceilings and visibilities becoming widespread, that is considered significant and is not included in a current hazardous weather advisory.
q. Heavy Icing. The rate of ice accumulation requires maximum use of the ice protection systems to minimize ice accretions on the airframe. A representative accretion rate for Page 4 Par 2-1
reference purposes is more than 3 inches (7.5 cm) per hour1 on the outer wing. Consider immediate exit from the conditions.2
r. Icing Conditions.
(1) Potential Icing Conditions. Atmospheric icing conditions that are typically defined by airframe manufacturers relative to temperature and visible moisture that may result in aircraft ice accretion on the ground or in flight. The potential icing conditions are typically defined in the airplane flight manual or in the airplane operation manual.
(2) Forecast Icing Conditions. Environmental conditions expected by a NWS or an FAA-approved weather provider to be conducive to the formation of in-flight icing on aircraft.
s. Ice Crystals. Ice crystals, often found in high concentrations near convective weather systems, can accrete within turbine engines and cause power loss. Ice crystals are not typically detected by either conventional ice detectors or airborne radar, and typically do not accrete on external airframe surfaces. Ice crystals found in stratus and cirrus clouds are usually in relatively low concentrations, and do not represent a threat to turbine engines.
t. Icing Envelopes. These icing envelopes, found in 14 CFR part 25, appendix C, are used for the certification of aircraft for flight in icing conditions. They specify atmospheric icing conditions in terms of altitude, temperature, liquid water content (LWC), and droplet size represented by the median volume diameter (MVD). (The envelopes use the term mean effective diameter (MED), but this equates to the median volume diameter for the instrumentation and assumptions current at the time the envelopes were established). There are two classes of icing envelopes: continuous maximum and intermittent maximum. The continuous maximum is for stratus-type clouds, and the intermittent maximum is for cumulus-type clouds.
u. Impinge. The striking and adherence of a droplet on an aircraft surface. The impingement rate is the rate at which droplets of a given size impinge a particular surface. In general, impingement rates are higher for larger droplets and smaller components, such as antennae.
v. Known, Observed, or Detected Ice Accretion. Actual ice that is observed visually to be on the aircraft by the flight crew or identified by onboard sensors.
w. Light Icing. The rate of ice accumulation requires occasional cycling of manual deicing systems 3 to minimize ice accretions on the airframe. A representative accretion rate for
1 These rates can be measured by a suitable icing rate meter.
2 It is expected that deicing or anti-icing systems will be activated and operated continuously in the automatic mode, if available, at the first sign of ice accumulation, or as directed in the AFM. Occasional and frequent cycling refers to manually activated systems.
3 See footnote number 2 Par 2-1 Page 5
reference purposes is 1⁄4 inch to one inch (0.6 to 2.5 cm) per hour 4 on the outer wing. The pilot should consider exiting the condition.5
x. Liquid Water Content (LWC). The total mass of water in all the liquid cloud droplets within a unit volume of cloud. LWC is usually discussed in terms of grams of water per cubic meter of air (g/m3).
y. Median Volume Diameter (MVD). The diameter is such that half the liquid water in a region of cloud is contained in smaller drops, and half in larger drops.
z. Mixed Ice. Simultaneous appearance of rime and clear ice or an ice formation that has the characteristics of both rime and clear ice.
aa. Moderate Icing. The rate of ice accumulation requires frequent cycling of manual deicing systems6 to minimize ice accretions on the airframe. A representative accretion rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour 7 on the outer wing. The pilot should consider exiting the condition as soon as possible.8 bb. Occluded Front. The front formed by a cold front overtaking a warm front and lifting the warm air above the earth’s surface. An occlusion (or frontal occlusion) forms when an air mass is trapped between two colder air masses and is forced to higher and higher levels.
cc. One-Minute Weather. The most recent 1-minute update weather broadcast based on ASOS/AWOS measurements and available to a pilot from an uncontrolled airport ASOS/AWOS.
dd. Orographic Cloud. A cloud that usually results from air flowing upslope from terrain and being cooled adiabatically.
ee. Outside Air Temperature (OAT). The measured or indicated air temperature (IAT) corrected for compression and friction heating. Also referred to as true air temperature.
ff. Pilot Briefing. A service provided by a Flight Service Station (FSS) to assist pilots with flight planning. Briefing items may include weather information, notices to airmen (NOTAMs), military activities, flow control information, and other items, as requested.
4 See footnote number 1.
5 It is assumed that the aircraft is approved to fly in the cited icing conditions. Otherwise, immediate exit from any of these intensity categories is required by regulations (14 CFR §§ 91.13(a), 91.527, 121.341, 125.221, and
135.227).
6 See footnote number 1.
7 See footnote number 2.
8 See footnote number 3.
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gg. Pilot Report (PIREP). A report from a pilot of meteorological phenomena encountered by aircraft in flight, usually transmitted in a prescribed format. The letters “UA” identify the message as a routine PIREP while the letters “UUA” identify an urgent PIREP.
hh. Rime Ice. A rough, milky, opaque ice formed by the instantaneous freezing of small, supercooled water droplets.
ii. Runback Ice. Ice that forms from the freezing or refreezing of water leaving protected surfaces and running back to unprotected surfaces.
jj. Severe Icing. The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and any other areas identified by the manufacturer.
Immediate exit from the condition is necessary.9 kk. Significant Meteorological Information (SIGMET). Information about in-flight weather of operational significance to the safety of all aircraft. SIGMETs may include severe icing. (See CWA and AIRMET.)
ll. Stagnation Point. The point on a surface where the local air velocity is zero. The region of maximum icing collection efficiency is near this point.
mm. Stationary Front. A front that has little or no movement because the opposing forces of the two air masses are relatively balanced.
nn. Stratus Clouds. Clouds that form layers with a uniform base. Stratus clouds can appear in ragged patches and may produce drizzle, rain, or snow.
oo. Sublimation. A process where ice turns directly into water vapor without passing through a liquid state.
pp. Supercooled Large Drop (SLD). A supercooled droplet with a diameter greater than 50 micrometers (0.05 mm). SLD conditions include freezing drizzle drops and freezing raindrops.
qq. Supplemental Weather Service Location. An airport facility staffed with contract personnel who take weather observations and provide current local weather to pilots via telephone or radio.
9 Severe icing is aircraft dependent, as are the other categories of icing intensity. Severe icing may occur at any ice accumulation rate when the icing rate or ice accumulations exceed the tolerance of the aircraft. Icing certification implies an increased tolerance to icing intensities up through heavy.
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Page 8 Par 2-1 rr. Telephone Information Briefing Service (TIBS). A telephone recording of meteorological and/or aeronautical information obtained by calling an FSS.
ss. Total Air Temperature (TAT). Kinetic heating causes the Total (or Ram) Air Temperature (TAT) to be warmer than the Static Air Temperature (SAT). TAT is close to the temperature of the wing leading edge, which also experiences ram rise.
tt. Trace Icing. Ice becomes noticeable. The rate of accumulation is slightly greater than the rate of sublimation. A representative accretion rate for reference purposes is less than 1/4 inch (6 mm) per hour on the outer wing. The pilot should consider exiting the icing conditions before they become worse. Pilots should be aware that any ice, even in trace amounts, could be potentially hazardous.
uu. Warm Front. Any nonoccluded front that moves in such a way that warmer air replaces colder air.
vv. Weather Advisory. In standard aviation weather forecast terminology, a warning of hazardous weather conditions not predicted in the forecast area that may affect air traffic operations. These reports are prepared by the NWS.
CHAPTER 3. ATMOSPHERIC CONDITIONS ASSOCIATED WITH ICING
3-1. AIRCRAFT ICING CONDITIONS.
a. Nearly all aircraft icing occurs in supercooled clouds. Liquid droplets are present at outside air temperatures below 0°C (32°F) in these clouds. At outside air temperatures close to 0°C (32°F), the cloud may consist entirely of such droplets, with few or no ice particles present.
At decreasing temperatures, the probability increases that ice particles will be found in significant numbers along with the liquid droplets. In fact, as the ice water content increases, the liquid water content tends to decrease since the ice particles grow at the expense of the water particles. At temperatures below about -20°C (-4°F), most clouds are made up entirely of ice particles.
b. The general rule is that the more ice particles and the fewer liquid droplets that are present, the less ice accumulation on the airframe. This is because the ice particles tend to bounce off an aircraft surface, while the supercooled droplets freeze and adhere. As a result, ice accumulation is often greatest at temperatures not too far below 0 °C (32°F), where liquid water content can be abundant, and is usually negligible at temperatures below about -20°C (-4°F).
c. An exception to the general rule just stated may be made for surfaces heated by a thermal ice protection system (or by aerodynamic heating near the stagnation point of an aircraft component at speeds in excess of perhaps 250 knots). For such surfaces, ice particles may melt upon impact and then run back to colder aft regions and refreeze.
d. The higher liquid water content associated with temperatures near freezing is not the only concern. Tests have shown that at outside air temperatures near freezing, the total air temperature may be above freezing. The result is no ice accretion near the stagnation, but the refreezing of the water running back on the airfoil, causing runback ice accretions, possibly behind the protected areas. The formation of a ridge is possible. Pilots should be vigilant at total temperatures (TAT) between -5°C (23°F) and +2°C (35°F). Pilots should know whether the temperature instrument they are reading in the cockpit is outside air temperature (OAT) or TAT.
e. The greater the liquid water content of the cloud, the more rapidly ice accumulates on aircraft surfaces. The size of the droplets also is important. Larger droplets have greater inertia and are less influenced by the airflow around the aircraft than smaller droplets. The result is that larger droplets will impinge on more of the aircraft surface than smaller droplets.
f. Every supercooled cloud contains a broad range of droplets, starting from between 1 and 10 micrometers (millionth of a meter) and usually not exceeding 50 micrometers (by comparison, the thickness of the average human hair is approximately 100 micrometers). A single droplet size must be chosen as representative, and in icing terminology this is the MVD, the diameter such that half the liquid water is in smaller drops, and half in larger drops. An icing-certificated aircraft is certificated for flight in stratus-type clouds with MVDs up to 40 micrometers and for cumulus-type clouds with MVDs up to 50 micrometers. The MVD distribution is such that aircraft that undergo icing certification are not evaluated for operations in clouds with a significant amount of liquid water in droplets with diameters larger than 100 micrometers. Such conditions are sometimes encountered, and accidents and incidents in Par 3-1 Page 9
Page 10 Par 3-1 such conditions have been documented. These conditions are referred to as freezing drizzle aloft in cloud or SLD in cloud. Table 1, Evolution of Icing Certification Standards, discusses some cues developed for aircraft with unpowered controls and pneumatic deicing boots, mainly relating to the location of the airframe ice, which the flightcrew can use in attempting to determine if such droplets may be present in a cloud.
g. An aircraft can also encounter SLD conditions in freezing drizzle (droplets with a diameter of 50 to 500 micrometers) or freezing rain (droplets with a diameter of 500 micrometers and larger) below a cloud deck. These droplets are, by definition, larger than those for which any aircraft is certificated, and accidents and incidents have occurred following flight in freezing drizzle or freezing rain. The larger drops can cause ice accretion behind the protected leading edge.
3-2. CLOUD TYPES AND AIRCRAFT ICING.
a. Air can rise because of many factors, including convection, orographic lifting (i.e., air forced up a mountain), or lifting at a weather front. As the air rises, it expands and cools adiabatically. If a parcel of air reaches its saturation point, the moisture within the parcel will condense and the resulting droplets form a cloud. Cloud water droplets are generally very small, averaging 20 micrometers in diameter, and are of such small mass that they can be held aloft by small air currents within clouds.
b. If rising air is moist (water vapor plentiful) and lifting is vigorous, the result can be clouds with substantial liquid water content and, sometimes, large droplets. The greater the liquid water content, the more rapid the icing; and the larger the droplets, the greater the extent of icing.
Tops of clouds often contain the most liquid water and largest droplets, because the droplets that reach the tops have undergone the most lifting. If the temperatures are cold enough at the tops (below around -15 °C (5 °F)), however, ice particles will usually start to form, which tends to deplete the liquid water.
c. Several types of clouds and the hazardous aircraft icing conditions that may be associated with them are discussed below.
(1) Stratus Clouds.
(a) Stratus clouds, sometimes called layer clouds, form a stratified layer that may cover a wide area. The lifting processes that form them are usually gradual, and so they rarely have exceptionally high liquid water contents. Icing layers in stratus clouds with a vertical thickness in excess of 3,000 feet are rare, so a change of altitude of a few thousand feet may take the aircraft out of icing.
(b) Lake-effect stratus clouds are exceptional in that they may have very high liquid water content because of the moisture available when they form over lakes. In the continental United States, lake-effect stratus clouds are most common in the Great Lakes region, particularly in early winter when cold northwesterly winds blow over the unfrozen lakes.
(c) Drizzle-size drops occasionally occur in stratus clouds, and pilots should always be on the lookout for cues that might indicate the presence of these drops (see Table 1 for a list of cues developed for aircraft with unpowered controls and pneumatic deicing boots).
(2) Cumulus Clouds.
(a) Cumulus clouds, which often form because of vigorous convection, can have high liquid water content. If an aircraft traverses them, the icing can be rapid. Because they tend to be of limited horizontal extent, it may be possible to avoid many of them. Because of the vertical development of cumulus clouds, icing conditions can be found in layers thousands of feet in depth, but with much less horizontal development than in stratus clouds.
(b) This class of clouds includes the cumulonimbus, or thunderstorm, clouds.
Updrafts in such clouds can be great and result in very large liquid water contents. Thus, a large icing threat can be added to the other excellent reasons to stay out of such clouds. The thunderhead anvil can spread out from the core for several miles and is composed mainly of ice crystals. These crystals will not adhere to unheated surfaces when they hit, but they may melt on a heated surface, run back, and refreeze. The ice content in the anvils can be high, and ingestion of the ice crystals has resulted in uncommanded thrust reductions.
(3) Orographic Clouds, Wave Clouds, and Cirrus Clouds.
(a) Orographic clouds form when moist air is lifted by flowing up the side of a mountain. As the parcel of air is lifted, it cools and forms a cloud. Such clouds can contain a large volume of water, and in some cases, large droplets.
(b) Wave clouds, recognized by their “wavy” tops, can have high liquid water contents. Continued flight along a wave may result in airframe icing.
(c) Cirrus clouds, found at very high, cold altitudes, are composed entirely of ice particles. Flight through these clouds should not result in structural icing, although the possibility exists for runback icing from the refreezing of particles that melted on thermally or aerodynamically heated surfaces.
d. Freezing Rain and Freezing Drizzle.
(1) Freezing rain forms when rain becomes supercooled by falling through a subfreezing layer of air. Ordinarily, air temperatures decrease with increasing altitude, but freezing rain requires a temperature inversion, which can occur when a warmer air mass overlies a colder air mass. This situation can occur along a warm front, where a warm air mass overruns a cold air mass. When flying in freezing rain, normally there is warm air (above 0 °C (32 °F)) above.
(2) Freezing raindrops are defined as drops of 500 micrometers (0.5 mm) diameter or larger. A typical diameter is 2 mm, and the few that grow much larger than about 6 mm tend to break up. Using 20 micrometers (0.02 mm) as a typical diameter for a cloud droplet, the diameters of rain and cloud drops differ by a factor of approximately 100, and the volume and mass differ by a factor of about 1,000,000. The size difference is shown in Figure 1. Droplet Par 3-2 Page 11
Page 12 Par 3-2 mass affects how far aft of the stagnation point (leading edge surfaces) droplets will strike the aircraft. The mass of freezing rain is typically 1,000,000 times that of cloud droplets. Because of this, freezing rain will result in ice forming in areas far aft of where it would form in ordinary supercooled clouds.
FIGURE 1. DROPLET SIZE COMPARISON Typical raindrop 2 millimeters Typical cloud droplet
0.02 millimeters
(3) Freezing drizzle also can form through the same process. It consists of supercooled liquid water drops that have diameters smaller than 500 micrometers (0.5 mm) and greater than 50 micrometers (0.05 mm).
(4) However, freezing drizzle is perhaps more commonly formed by a different process, known as the collision-coalescence process. When, through condensation, some droplets in a cloud grow to approximately 30 micrometers in diameter, they begin to settle, falling fast enough so that they collide with some smaller droplets. If the droplets coalesce, the result is a larger droplet, which now has an even better chance of capturing smaller droplets. Under favorable conditions, this process can produce drizzle-size drops in a supercooled cloud, usually near the top, where the larger droplets generally are found in any cloud. Statistics vary, but some studies have reported that freezing drizzle aloft forms more than 80 percent of the time by the collision-coalescence process in non-convective clouds. Thus, in freezing drizzle, the pilot cannot assume that a warm layer (above 0°C (32°F)) exists above the aircraft.
(5) The diameters of representative cloud and drizzle drops differ by a factor of about 10 and the volume and mass by a factor of about 1,000. The greater inertia and impingement efficiency of the drizzle drops will result in icing beyond the usual icing limits for typical cloud droplets. When drizzle drops are found within a supercooled cloud, they can result
in accretions that cause very rapid and dangerous stall speed and drag increases for some aircraft and roll control anomalies for others. These situations may be caused by the roughness, shape, and extent of the accretion that forms. This is an instance of SLD icing as discussed earlier in this paragraph. (See Table 1 for a list of cues developed for aircraft with unpowered controls and pneumatic deicing boots.)
3-3. FRONTS.
a. When air masses of differing temperatures, pressures, or relative humidity meet, a front is formed. If the front moves so that warmer air replaces colder air, it is called a warm front; if it moves so that colder air replaces warmer air, it is called a cold front. An occluded front forms when an air mass is trapped between two colder air masses and is forced to higher and higher levels. In all three cases, significant lifting occurs. If sufficient moisture and subfreezing temperatures are present, icing conditions are created.
b. Along a warm front, the warmer air tends to slide gradually over the cold front, forming stratus clouds conducive to icing (see Figure 2). In a cold front, the cold air plows under the warm air, lifting it more rapidly and resulting in the formation of cumulus clouds with high liquid water content if the lifted air is moist (see Figure 3). SLD in the form of freezing rain and freezing drizzle are sometimes found near fronts, as explained above.
FIGURE 2. WARM FRONT
FIGURE 3. COLD FRONT
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c. Because of the icing and other hazards associated with fronts, they are best avoided, if possible. When flying through a front, the shortest route through the front should be taken, instead of flying along the front, to reduce the time spent in potential icing conditions.
3-4. CONVECTIVE WEATHER AND ICE CRYSTALS.
a. Some turbine engine designs have shown a susceptibility to ice crystals that form in the atmosphere because of convective weather activity. Deep lifting (thousands of feet) and condensation of water vapor in an unstable atmosphere characterize convective weather. Some or all of the following can be found in areas of convection: strong wind shear, turbulence, lightning, and high-condensed water or hail. Convective weather can range from isolated thunderstorms or cumulonimbus clouds, to convective complexes or squall lines to tropical storms or hurricanes. It can extend hundreds of miles laterally and above 50,000 feet vertically. Convective weather systems, especially, those associated with tropical weather fronts can pump large quantities of moisture to high altitudes that freezes into ice crystals that can remain aloft. These ice crystals can remain as a cloud well after the convective system has decayed.
b. These clouds of ice crystals may not be detectable on conventional airborne radar.
Although these ice crystals do not typically accrete on airframe surfaces or trigger ice detectors, when present in high concentrations they can still accrete on warm surfaces within the flow-path of turbine engines. This accretion can ultimately result in engine upset, engine damage from shedding, or power loss.
c. Dry ice crystals, particularly those found above 25,000 feet altitude, do not provide good radar reflectivity. Dry ice particles have about 20 times less radar reflectivity than rain droplets, and therefore are difficult to detect. Clouds and temperatures less than ten degrees centigrade are better indicators of the possible presence of ice crystals when near convective weather. Follow AFM procedures.