Answer: When looking at the surface analysis chart issued every three hours by meteorologists at the Weather Prediction Center (WPC), you may have seen a tan dashed line with a label “OUTFLOW BNDRY” nearby. This is what meteorologists call a convective outflow boundary.

You may view North American surface analysis here.

Convective outflow boundaries emanating away from thunderstorms are generated as cold, dense air descends in downdrafts then moves outward away from the convection to produce a mesoscale cold front also known as a “gust front.” 

• READ MORE: Should I File an Initial Approach Fix?

Some gust fronts can be completely harmless or may be a precursor for an encounter with severe turbulence and dangerous low-level convective wind shear. The direction of movement of the gust front isn’t always coincident with the general motion of the thunderstorms. If the gust front is moving in advance of the convection, it should be strictly avoided. The pilot’s best defense is to recognize and characterize the gust front using METARs, ground-based radar, and visible satellite imagery.

According to research meteorologist and thunderstorm expert Charles Doswell, “cold, stable air is the ‘exhaust’ of deep, moist convection descending in downdrafts and then spreading outward like pancake batter poured on a griddle.” 

As a pulse-type thunderstorm reaches a point where its updraft can no longer support the load of precipitation that has accumulated inside, the precipitation load collapses down through the original updraft area. Evaporation of some of the rain cools the downdraft, making it even more dense compared to the surrounding air. When the downdraft reaches the ground, it is deflected laterally and spreads out almost uniformly in all directions, producing a gust front.

• READ MORE: How Do You Check NOTAMs?

Gust fronts are normally seen moving away from weakening thunderstorm cores. Once a gust front forms and moves away from the convection, the boundary may be detected on the NWS WSR-88D NEXRAD Doppler radar as a bow-shaped line of low reflectivity returns usually 20 dBZ or less. Outflow boundaries are low-level events and do not necessarily produce precipitation. Instead, the radar is detecting the density discontinuity of the boundary itself along with any dust, insects, and other debris that may be carried along with the strong winds within the outflow. The gust front in southwest Missouri shows up very well on the NWS radar image out of Springfield as shown below.

An important observation is to note the motion of the gust front relative to the motion of the convection. In this particular case, the boundary is steadily moving south while the thunderstorm cells that produced the gust front are moving to the east. This kind of outflow boundary is usually benign, especially as it gains distance from the source convection. On the other hand, a gust front that is moving in the same general direction in advance of the convection is of the most concern. These gust fronts often contain severe or extreme turbulence, strong and gusty straight-line winds, and low-level convective wind shear.

As mentioned previously, gust fronts are strictly low-level events. As such, even the lowest elevation angle of the radar may overshoot the boundary if it is not close to the radar site. 

• READ MORE: What Does It Take to Transition from a Jet to a Piston Airplane?

Shown below at 22Z, the NWS WSR-88D NEXRAD Doppler radar out of Greenville-Spartanburg, South Carolina, is the closest radar site and clearly “sees” the gust front (right image). However, the NEXRAD Doppler radar out of Columbia, South Carolina (left image), is farther away and misses the gust front completely. As the gust front moves away from the radar site, it may appear to dissipate, when in fact, the lowest elevation beam of the radar is simply overshooting the boundary. As a result, it is important to examine the NEXRAD radar mosaic image before looking at the individual radar sites.

Not all gust fronts are easy to distinguish on visible satellite imagery; the gust front could be embedded in other dense clouds, or a high cirrus deck may obscure it. It is also possible that the boundary may not have enough lift or moisture to produce clouds. In many cases, however, it will clearly stand out on the visible satellite image. When the gust front contains enough moisture, as it was in this situation, cumuliform clouds may form along the boundary and move outward as can be seen in this visible image below centered on Charlotte, North Carolina. This is very common in the Southeast and coastal regions along the Gulf of Mexico given the higher moisture content. 

As this particular gust front passed through my neighborhood located south of Charlotte, strong, gusty northerly winds persisted for about 10 minutes. As is common, the main core of the precipitation didn’t start to fall for another 10 minutes. When a gust front such as this appears on satellite or radar, it is important to monitor the METARs and ASOS or AWOS closely for the occurrence of high winds. Several airports in the vicinity reported wind gusts peaking at 30 knots. The sky cover went from being just a few scattered clouds to a broken sky with these cumuliform clouds shown below moving rapidly through the region.

These cumuliform-type clouds were the result of a strong convective outflow boundary that moved through Fort Mill, South Carolina.  [Courtesy: Scott Dennstaedt]

As mentioned earlier, a gust front moving away from thunderstorms is a low-level event that can contain strong updrafts and downdrafts. The graph shown below is a time series, plotting the upward and downward motion or vertical velocity in a strong gust front as it moves over a particular point on the ground. 

The top half of the graph is upward motion and the bottom half is downward motion. Time increases from left to right. As the gust front approaches, the vertical velocity of the air upward increases quickly over a one- or two-minute period. While the maximum vertical velocities vary with height in the outflow, a common maximum number seen is 10 meters per second (m/s) at about 1.4 kilometers or 4,500 feet agl (25 knots is roughly 12 m/s for reference). 

• READ MORE: Is a Medical Certificate Required for a Private Pilot Check Ride?

As the gust front moves through, the velocities abruptly switch from an upward to a downward motion, creating strong wind gusts at the surface. Most outflow boundaries don’t extend above about 2 kilometers or 6,500 feet agl. What is remarkable is that upward-to-downward motion changes in just about 30 seconds over the ground point where this was observed. But imagine flying an aircraft at 150 knots through this— the up-and-down exchange will happen in just a few seconds, producing a jarring turbulence event.

Just in case you were wondering, gust fronts are conveniently filtered out by your datalink weather broadcasts as shown below for XM-delivered satellite weather. This is because the broadcast only provides returns from actual areas of precipitation. Often outflow boundaries or gust fronts produce low reflectivity returns that fall below the threshold used to filter out other clutter not associated with actual areas of precipitation. 

When in flight, pay particular attention to surface observations, looking for strong, gusty winds, before attempting a landing at an airport when storms are approaching. 

The post What Is an Outflow Boundary Shown on a Surface Analysis Chart? appeared first on FLYING Magazine.

Answer: I advocate checking the weather and seeing what approaches are being supported by the conditions, then select an approach and file to an Initial Approach Fix (IAF) for that approach.

• READ MORE: How Do You Check NOTAMs?

The reason? Because you lose your comms en route or before you are cleared for the approach, you will be following the AVE F procedure, which states that in the event of loss of communication you will fly one of the four: the heading you were assigned, vectored to, told to expect or filed to. If you are operating on an IFR flight plan, you should have at least one of these. This is what ATC expects you to do, so they will be protecting that airspace at the fix you filed to.

If you simply fly to the airport and the airport has multiple instrument approaches and multiple IAFs, ATC is going to have a more difficult time protecting the airspace. It will be like Whac-a-Mole with airplanes. If you file to a particular IAF, and they see a target squawking 7600 at that fix, they will have a pretty good idea that’s you. Make sure you continue to transmit in the blind – this means you make appropriate radio calls and position reports although you cannot hear them reply.

• READ MORE: What Does It Take to Transition from a Jet to a Piston Airplane?

Bonus move: Adjust time en route by five minutes. For example, if it will take 23 minutes to get to the fix, file it as 17 minutes because that way you won’t have to wait for time to elapse in order to shoot the approach. 

Remember, you are requesting an approach when you file your flight plan. ATC is not obligated to grant your request, which is why you should have your approach binder with you (in either paper or electronic form). So if you are assigned something other than you filed, you will be prepared to fly what is offered.

The post Should I File an Initial Approach Fix? appeared first on FLYING Magazine.

But wait, is vertical visibility even a legitimate visibility? Actually, it’s a bit of a misnomer and not a true measure of visibility in the traditional sense. Vertical visibility is a close cousin to ceiling. That is, it represents the distance in feet a person can see vertically from the surface of the Earth into an obscuring phenomenon, or what is called an indefinite ceiling. What isn’t obvious is how vertical visibility is determined, and how this is different from a definite ceiling.

It’s arguable that an indefinite ceiling is perhaps the most misunderstood phenomenon reported in a routine (METAR) or special surface (SPECI) observation. Forecasters will add vertical visibility in a terminal aerodrome forecast (TAF) as illustrated in the image below for Bradford Regional Airport (KBFD) in Pennsylvania. Whether this occurs in a METAR or TAF, vertical visibility is coded as “VV” followed by a three-digit height in hundreds of feet above the ground level. For example, you may see “VV002,” which is a vertical visibility of 200 feet. While a definite ceiling can be broken or overcast, a vertical visibility always implies the sky is completely covered. Let’s explore the difference between a definite and indefinite ceiling and the operational considerations.

Automated Observations

In the early days, human weather observers used to employ what were called “pilot balloons” to estimate the ceiling height. Essentially the balloon was launched by the observer and, given the balloon’s known rate of ascent, they watched the balloon enter the base of the clouds and measured the time it took using a stopwatch to determine the ceiling height. Then new technology emerged called a rotating beam ceilometer that measured the height of clouds. While it was more effective than launching a balloon, this method was phased out around 1990 and replaced with the laser beam ceilometer, the technology still widely used today.

The task of walking outside and assessing the height of clouds is generally a thing of the past given that this technology is incorporated into the automated surface observing system (ASOS) or automated weather observing system (AWOS) present at many airports throughout the U.S. The trained observer simply logs in to the ASOS (or AWOS) and makes their observation based on the data gathered and reported by the automated system. Then the observation is edited and augmented by the observer as necessary. Depending on the airport, this process may be completely automated.

In all honesty, making an estimate of the height of the cloud base isn’t the difficult part. What’s difficult is to provide a representative description of the amount of cloud coverage (e.g., few, scattered, broken, or overcast) in the airport’s terminal area. A laser beam that points straight up may easily miss a scattered or broken cloud deck. To alleviate this issue, the automated systems process the data over a given amount of time since clouds are generally moving through the sensor array area. It was found that a 30-minute time period provided a representative and responsive observation similar to that created by a trained observer. The most recent 10 minutes of sky cover and ceiling height are double weighted using a harmonic mean. (A harmonic mean is used in the visibility and sky cover algorithms rather than an arithmetic mean because it is more responsive to rapidly changing conditions such as decreasing visibility or increasing sky coverage/lower ceiling conditions.) In the end, the goal is to provide an observation representative of the airport’s terminal area, which is the area within 5 sm from the center of the airport’s runway complex. Visibility, wind, pressure, temperature, etc., all have their own harmonic means accordingly.

In our everyday experience, we know that many cloud decks observed from the ground have a very well-defined base. For an untrained observer, it might not be a simple task to determine their height. However, it’s easy to pick out where the base of the cloud starts. Even in these cases, the cloud decks may vary in height and multiple cloud layers may exist. Visually, that may be more difficult to discern for the untrained eye, but automated systems do a reasonable job making that observation. In a convective scenario, it is not unusual to see multiple scattered and broken cloud heights. For example, at the West Michigan Regional Airport (KBIV) the following was observed:

KBIV 122353Z AUTO 08011KT 4SM RA BR FEW011 SCT048 OVC065 19/18 A2972

This observation includes three definite cloud layers, which are a telltale sign that a convective environment is in place even before the first lightning strike.

Nuts and Bolts

An ASOS continuously scans the sky. To determine the height(s) of the clouds, the backscatter returns from the ceilometer are put into three different bins. When there’s a “cloud hit,” the system identifies a well-defined and sharp signature pattern that you’d expect with the sensor striking the cloud base. Essentially this means most of the hits are aggregated around a particular height above the ground. Such a sharp signature is then incorporated into the 30-minute sky cover and cloud height harmonic average, and a new observation is born.

On the other hand, a “no hit” is recorded when there isn’t an ample amount of backscatter received, usually because there are no clouds below 12,600 feet agl over the sensor. Note that the ASOS (and AWOS) is designed only to detect clouds below 12,600 feet above the ground, although a trained observer can and does report higher clouds. Lastly, if the backscatter does not provide that sharp signature around a particular height, an “unknown hit” is recorded. It is this unknown hit that leads us down the path to an indefinite ceiling or vertical visibility.

Haze, Mist, and Fog, Oh, My!

So, isn’t an indefinite ceiling the same thing as a ground fog event? Not necessarily. Stratus is the most common cloud associated with low ceilings and reduced visibility. Stratus clouds are composed of extremely small water droplets or ice crystals (during the cold season) suspended in the air and may be touching the surface, so to speak. An observer along a coastal region or on the side of a mountain would likely just call this plain old fog. This is certainly understandable, since we grew up calling this kind of situation foggy.

Fog, however, is thought to be more of an obstruction to visibility from a surface observing standpoint. To understand the recording of obscurations, here’s how the ASOS automatically determines what to report. Once each minute, the obscuration algorithm checks the reported visibility. When the visibility drops below 7 sm, the current dew point depression (temperature-dew point spread) is checked to distinguish between fog (FG), mist (BR), and haze (HZ). If the dew point depression is less than or equal to 4 degrees Fahrenheit (~2 degrees Celsius), then FG or BR will be reported. Visibility will then be used to further differentiate between FG and BR.

Whenever the visibility is below five-eighth sm, FG is reported regardless of the “cloud” that produces it. So fog isn’t really about a cloud or ceiling as much as it is about visibility. Therefore, stratus and fog frequently exist together. In many cases, there is no real line of distinction between the fog and stratus; rather, one gradually merges into the other. Flight visibility may approach zero when flying in stratus clouds. Stratus over land tends to be lowest during night and early morning, dissipating by late morning or early afternoon. Low stratus clouds often occur when moist air mixes with a colder air mass or in any situation where temperature-dewpoint spread is small.

• READ MORE: Decoding the Weather

Moisture-Rich Environment

Essentially, an indefinite ceiling means there is something obscuring your view of the cloud base. When you look up, you won’t be able to see a well-defined cloud base like you would on a day where the sky isn’t obscured. According to the ASOS User’s Guide, “these ‘unknown hits’ are primarily caused by precipitation and fog that mask the base of the clouds.” The laser beam bounces off moisture at various heights, making it impossible to process this as a definite cloud hit. Instead, the ASOS identifies these unknown hits as a vertical visibility abbreviated as “VV” in the resulting routine or special observation.

Given the broad moisture field near the surface that scatters the laser beam signal, indefinite ceilings are guaranteed to be paired with low visibility situations. You are not going to see a surface visibility of 10 miles paired with a VV of 200 feet. Usually this means a low or very low IFR flight category anytime there’s an indefinite ceiling. Also keep in mind that an indefinite ceiling in a terminal forecast will result in a low visibility forecast.

In general, the higher the vertical visibility, the better the surface visibility. Therefore, a vertical visibility of 200 feet (VV002) is usually met with a visibility of one-half sm. Furthermore, a vertical visibility of 700 feet (VV007) will likely be associated with a visibility between 1 and 2 sm. While rare, you may even see a fairly high vertical visibility over 1,000 feet (e.g., VV012). In this case, the surface visibility may be over 3 sm. The really bad stuff, however, occurs with a visibility of one quarter sm (or even “M1/4 SM” denoting less than that) and a vertical visibility of zero feet (VV000) as illustrated in the image below for Bradford Regional Airport. This very low indefinite ceiling is not all that common unless you are stationed on the summit of Mount Washington in New Hampshire, where this low vertical visibility happens quite often throughout the year. It also occurs fairly often at airports along West Coast regions of the U.S., especially during their “May gray” or “June gloom” time frame.

As mentioned earlier, fog and precipitation are the two primary reasons the base of the cloud deck is obscured. Therefore, it’s common to see vertical visibility reported when light rain, drizzle, or even snow is falling from the cloud base.

Precipitation or not, it’s generally rare to see a single station reporting an indefinite ceiling. Most of the time, you will see indefinite ceiling reports embedded in a widespread area of low or very low IFR conditions, especially at coastal airports. Although airports such as Nantucket Memorial Airport (KACK) in Massachusetts can be reporting a low indefinite ceiling, at stations farther inland near Cape Cod the sky can be clear or nearly so.

It’s important to note that conditions producing an indefinite ceiling often take longer to improve. Normally there will be a transition from an indefinite to definite ceiling once the moisture begins to mix out with the help of the sun. However, the visibility may still be quite low for the next few hours. Keep this in mind when flight planning to an airport reporting an indefinite ceiling.

Operational Significance

From a practical standpoint, you should treat an observation or forecast for a vertical visibility the same as you’d treat a definite ceiling. Given the nature of conditions that produce an indefinite ceiling, you can expect a longer transition as you depart into such a ceiling under IFR. It’s easy to get spatial disorientation because of the gradual change.

An indefinite ceiling restricts the pilot’s flight (air-to-ground) visibility. Therefore, an instrument approach may be a bit more challenging even after you drop below the reported ceiling height because of the reduced visibility. Most importantly, a circle-to-land approach with an indefinite ceiling will make it quite difficult to keep the runway in sight, especially at night. And, as a final consideration, with an indefinite ceiling, don’t be surprised to see runway visual range also pop up in the observation for airports with such equipment.

This feature first appeared in the October 2023/Issue 942 of FLYING’s print edition.

The post Here’s the Lowdown on ‘Vertical Visibility’ appeared first on FLYING Magazine.

According to the MiseryMap as of Friday, there were noticeable delays at both San Francisco International (KSFO) and Los Angeles International (KLAX) airports to Seattle and Denver.

Ground travel  has also been impacted as the California Department of Transportation reports landslides on highways and power companies report thousands of people without electricity as a result of downed power lines and felled trees because of the saturated soil. In addition, law enforcement officials in parts of Southern California have reported downed trees that appeared to have been caught in a tornado.

There is so much rain that officials in Southern California issued a warning about excessive rainfall. According to data from the National Weather Service, record amounts of rain fell in a single day in multiple locations, including at several airports.

Van Nuys Airport (KVNY) has received a total of 9 inches and KLAX 5.68 inches, including 2.37 inches in one day. Other one-day, record-breaking totals include Long Beach Airport (KLGB) with 2.31 inches and Burbank’s Bob Hope Airport (KBUR) with 2.08 inches.

Earlier in the week, Santa Barbara Airport (KSBC) was closed to all traffic because of flooding. Travelers were urged to contact their airlines directly for more information.

In the San Francisco Bay Area, some residents have been without power for more than five days and are bracing for another strong storm set to move in over the weekend, bringing with it more heavy rain and wind gusts of up to 60 mph.

This story is evolving and will be updated as appropriate.

The post California Airports Reeling From Heavy Rain appeared first on FLYING Magazine.

Answer: There is an opinion in the aviation community that flying through snow is not only an icing hazard but also against FAA regulations for pilots in aircraft without a certified ice protection system. Keep in mind that each weather system is unique, and there are many exceptions to the general view presented here.

Let’s discuss some of the many factors associated with flying through snow.

Snow falling out of the base of a cloud means there are fairly deep, saturated conditions aloft. To produce snow typically requires that the cloud top temperature (CTT) be sufficiently cold. That usually means a CTT of minus-12 degrees Celsius or colder—the colder the air, the more likely the precipitation type is snow. In this situation, ice crystals can dominate any supercooled liquid water in the cloud and lead to the development of snowflakes in the cloud aloft. If you are flying through snow below the cloud base, does that imply icing conditions exist? Just to be clear, this is not a discussion of flying in the clouds producing the snow but below the cloud base. 

Snow is considered visible moisture. It can be mixed with other precipitation types that may include rain, freezing rain, or ice pellets. In general, snow falling from the base of a cloud doesn’t represent a significant airframe icing hazard unless it is mixed with other types of precipitation such as freezing rain. It can be an issue with induction icing but not airframe icing. In the unlikely case that snow does adhere to the airframe, an exit plan should be executed. 

• READ MORE: What Is Mixed Icing?

Outside of a mixed-precipitation scenario, snow is usually classified as wet or dry. Wet snow occurs when the static air temperature is at or above 0 degrees Celsius. That is, the snow falls into an atmosphere warmer than freezing and begins a melting process. Although liquid water doesn’t necessarily freeze at a temperature below 0 degrees Celsius, snow must begin to melt at a temperature warmer than that. If the temperature is warm enough, it will completely melt the snowflake into a raindrop before reaching the surface. You may have experienced this while driving in your car. You’ll see the wet snowflake splat on your windshield and quickly melt. Wet snow can begin to accumulate on grassy surfaces or other vegetation but usually melts quickly on other surfaces.

Moreover, because you are flying at an airspeed where kinetic heating occurs on the leading edge, even at a static air temperature of 0 degrees Celsius, snow will typically not accrete on the leading edges of the wings and horizontal stabilizer as a result of this kinetic heating driven by adiabatic compression. This is typically referred to as ram air rise. And certainly, with a static air temperature above 0 degrees Celsius, ice is very unlikely to accrete with the additional ram air temperature rise. In fact, even at a static air temperature of minus-1 or minus-2 degrees Celsius, accreting ice is difficult at best. Once the static air temperature gets colder than minus-3 C, then you are no longer dealing with wet snow since no melting is occurring.   

Certainly, wet snow can be problematic while taxiing. Or, if you pull your airplane out of a warm hangar, even dry snow will melt and begin to collect on some surfaces and may accumulate over time. It is recommended that you never depart with any of the aircraft surfaces contaminated, including wings and the horizontal stabilizer. Doing so may cause the aircraft not to develop the lift necessary to take off and climb, creating a risk of impact with terrain. 

Another metric to use is the Current Icing Product (CIP) found on the Aviation Weather Center website. CIP utilizes a recent three-hour forecast from the Rapid Refresh (RAP) model for parameters such as temperature, moisture aloft, supercooled liquid water content, and other useful model data. This is mainly to “seed” the forecast for these items, given that observational data is rather sparse throughout the atmosphere for these important parameters. Nevertheless, it combines this with surface observations, ground-based radar, pilot weather reports, satellite imagery, and lightning to produce an hourly analysis of icing probability, icing severity, and supercooled large-drop icing potential from the surface through 30,000 feet.  

CIP looks for information about the presence or absence of six precipitation types—freezing rain (FZRA), freezing drizzle (FZDZ), ice pellets (PL), rain (RA), drizzle (DZ), and snow (SN). A report of any of the first five means that altitudes below cloud base need to be considered for possible icing and SLD, because subfreezing liquid precipitation may be present. However, in an observation in which only snow is reported at the surface, ice crystals are clearly present beneath and within the lowest cloud layer, and those are not considered an icing threat, especially below the lowest cloud base. 

For example, if an airport is reporting an overcast sky at 2,500 feet and only snow is being reported, the CIP algorithm will remove any possible occurrence of icing from the cloud base down to the surface, regardless of what other sources may say. This is because snow not mixed with other precipitation types, such as freezing rain, is not seen as an icing hazard…even wet snow.  

Is it legal to fly through snow in an aircraft without a certified ice protection system? First you may want to read this letter from the FAA’s Office of the Chief Counsel. An excerpt  states: 

The formation of structural icing requires two elements: 1) the presence of visible moisture, and 2) an aircraft surface temperature at or below zero degrees Celsius. The FAA does not necessarily consider the mere presence of clouds (which may only contain ice crystals) or other forms of visible moisture at temperatures at or below freezing to be conducive to the formation of known ice or to constitute known icing conditions. There are many variables that influence whether ice will actually be detected or observed, or will form on and adhere to an aircraft. The size of the water droplets, shape of the airfoil, and the speed of the aircraft, among other factors, can make a critical difference in the initiation and growth of structural ice.

Yes, snow is definitely visible moisture, but will it adhere to the airframe? Dry snow is not going to adhere to the airframe while in flight. Wet snow, as mentioned above, is more of an induction icing or ground icing concern than airframe icing while in flight. 

Sometimes it’s not about airframe or induction icing. Flying through falling snow can also be very disorienting at times, especially when the snowfall is moderate or greater, or you are flying at night. It will often lower flight visibility to 3 sm or less and can make runways extremely slick. Landing while it is snowing on a snow-covered runway can lead to a flare at an altitude higher than normal, making for a hard landing.      

One last point. Often when snow falls into a fairly deep, dry layer below the cloud base, it can sublimate on its way down. This usually occurs with the onset of precipitation as a weather system approaches. Evaporation and sublimation are both cooling processes, and they will lower the temperature of the dryer air. An atmosphere that is a few degrees above freezing can lead to melting wet snow, and this process can quickly move the temperatures to below freezing, allowing for snow to reach the surface instead of melting.

The post Is Flying Through Snow an Icing Hazard? appeared first on FLYING Magazine.

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. 

The post What Is Mixed Icing? appeared first on FLYING Magazine.

A series of testing is underway at the McKinley Climatic Lab at Eglin Air Force Base, Florida, to verify T-7A system functionality during periods of extreme temperatures. During the tests, performance of the T-7’s propulsion, hydraulic, fuel, electrical, secondary power, and overall operations will be evaluated in conditions ranging from minus-25 degrees to 100 degrees Fahrenheit.

• READ MORE: First Air Force Pilot Flies T-7A Red Hawk

The Red Hawk is set to replace the 1960s-era T-38 trainer for Air Force fighter and bomber pilot flight training. Its iconic red-tail livery honors the Tuskegee Airmen of World War II, the U.S. Army Air Forces’ first Black aviation unit. 

Last month, the advanced trainer made its first cross-country flight to Edwards Air Force Base in California for flight testing.

• READ MORE: Air Force’s T-7A Trainer to Begin Next Phase of Flight Testing

“The Red Hawk must withstand a range of environments from sitting on the ground in the Texas heat to flying at altitude,” Troy Hoeger, Air Force Life Cycle Management Center’s T-7A chief developmental tester, said in a statement. “The climatic lab helps us do this in a deliberate and methodical way and will give us confidence that our new aircraft meets requirements.” 

The $9.2 billion Air Force program includes the purchase of 351 Boeing T-7A jets, 46 simulators, and support.

The post Air Force’s T-7 Red Hawk Undergoes New Round of Testing appeared first on FLYING Magazine.

A terminal aerodrome forecast, simply known as a TAF, is perhaps the most difficult forecast any meteorologist will ever make. Think about the challenge these forecasters face. A TAF is essentially an hour-by-hour forecast for conditions significant to aviation at an airport over the next 24 or 30 hours. This includes a forecast for details such as wind speed and direction, cloud coverage, ceiling height, prevailing visibility, and precipitation type.

When you think of a TAF, size matters. This forecast is difficult because of the relatively small diameter of the area they are attempting to cover. The U.S. definition of a terminal area is the region within five statute miles of the center of the airport’s runway complex. Thus, meteorologists refer to a TAF as a “point forecast,” and it’s critical to understand its limitations and how they affect the forecasts general aviation pilots ultimately use every day.

The Terminal Area

A five statute mile area is a tiny region to get all forecast elements right over a given period. In fact, the terminal area is often smaller than the resolution of the forecast guidance they are using to issue the TAF. It’s like placing a coin on a sidewalk and asking someone at the top of a five-story building to identify if the coin is a nickel, dime, or quarter using their naked eyes.

Here’s a way to visualize why it’s so hard to issue these forecasts. Let’s pretend for a moment that you are the forecaster and someone asks, “What are the chances there will be a thunderstorm reported somewhere in the conterminous U.S. in the month of July?” Certainly, there are a lot of thunderstorms in July, and the conterminous U.S. encompasses a huge area. Your forecast would likely be that there’s a 100 percent chance. And you would be 100 percent correct.

That was an easy forecast, and you didn’t even need a meteorology degree to get it right. Now, how about a slightly different question? What is the chance of a thunderstorm reported sometime during the month of July in the state of Oklahoma? Given a month is a long period and Oklahoma is a state with lots of thunderstorms during the summer, again, I’d bet your answer would be that there’s a 100 percent chance. Now, how about the chance of a thunderstorm being reported on July 14 at the Oklahoma City airport at 8 a.m.? Well, once again, there’s a pretty easy answer; you’d likely say it’s a zero percent chance.

When you narrow down the time and the location, you can see the swing from a near guarantee at 100 percent to a near guarantee at zero percent. Forecasting for a small five statute mile area is incredibly difficult, if not fundamentally impossible at times, but meteorologists at the local weather forecast offices are asked to carry out the impossible every day. They need to determine if that coin is a nickel, dime, or quarter.

There’s no doubt that TAFs are used by all pilots because of the significant detail they provide. Everyone from general aviation pilots to commercial air carriers utilize TAFs to anticipate weather conditions in the airport terminal area. Without question, TAF content can have a strong impact on fuel loads, the need for alternates, and other operational aspects because of their stringent regulatory nature.

Scheduling TAFs

Each weather forecast office in the conterminous U.S. is typically responsible for issuing a TAF for up to ten airports within its region of coverage called a county warning area or CWA. For example, the Greenville-Spartanburg forecast office in Greer, South Carolina, is responsible for preparing TAFs for six local terminal areas, including the Charlotte Douglas International Airport (KCLT).

It’s important that the TAFs are prepared and issued by local forecasters instead of forecasters sitting in some Washington, D.C., office. They often consider sub-synoptic local effects, and they are tuned into the local weather patterns since they deal with them every day. The difference between a low IFR ceiling and a clear sky can be just a matter of 10 miles at times.

Therefore, the size of the terminal area is a point (pun intended) that should not be overlooked. The TAF may or may not always be representative of an area or zone forecast. Additionally, locally derived forecast rules and outside pressure from the FAA or even the airlines can cause the TAF to be quite different than an area forecast.

Scheduled TAFs are issued four times daily (every six hours) at 00Z, 06Z, 12Z, and 18Z. In most circumstances, the TAF is transmitted between 20 minutes and 40 minutes prior to these times. Moreover, for high-impact airports such as Atlanta, Chicago and New York, TAFs may be routinely issued every three or even two hours. For now, those off-schedule issuances will still be released as amendments. So, if you see an amended forecast in these regions, it may not be because of a poorly aligned forecast with respect to the weather—it may be a new and improved forecast.

Precipitation events, especially thunderstorms, give meteorologists the most trouble. Forecasting convection in the terminal area is all about quantifying the uncertainty of the event. Even in reasonably dynamic situations with traveling weather systems, meteorologists can find it challenging to predict when convection will impact the terminal area over the forecast period.

Dealing with Uncertainty

Unfortunately, forecasters do not have a convenient way in a TAF to quantify their uncertainty. In the public forecast, you’ll see something like “a 30 percent chance of thunderstorms.”

Sure, forecasters can throw in a PROB30 forecast group into a TAF, but by NWS directives, PROB30 groups are not allowed to exist in the first nine hours of the forecast period. By the way, the NWS only uses PROB30, although you may see PROB40 in international TAFs or TAFs issued by the military. So, what can a forecaster do when there’s a chance of thunderstorms in the public forecast, but the uncertainty is high? In most cases, the forecaster will leave out any mention of thunderstorms given that it is just too uncertain and the likelihood is small that a thunderstorm will roll through that tiny forecast region. Forecasters are also pressured by the airlines to avoid placing thunderstorms in a TAF in these situations. A forecast for thunder may require filing an alternate, and the need to take on more fuel.

Perhaps in this uncertain situation, these are just the scattered variety of afternoon pulse-type thunderstorms. In this case, the forecaster has two possible solutions, neither of which will appear in your aviation textbook or ground school. First, they can add rain showers (SHRA) or showers in the vicinity (VCSH)

instead of thunderstorms. Showery precipitation is inherently a convective process. It’s not unusual to see forecasters include one of these two precipitation forecasts into the TAF when the uncertainty of thunderstorms is high. Essentially it becomes a placeholder for thunderstorms. When conditions eventually begin to evolve and it becomes clear thunderstorms will impact the terminal area, the forecaster will likely amend the TAF to replace SHRA or VCSH with TSRA (rain and thunderstorms within the terminal area).

Area Forecast Discussions

Second, forecasters may often explain their reasoning in the area forecast discussion or AFD. No, it’s not a discussion about the aviation area forecast that was retired in 2018. Instead, it’s a discussion about the weather expected in the local county warning area (CWA) for that weather forecast office. Every AFD contains an aviation section that discusses the TAFs for airports within that CWA. It is in the AFD that a forecaster can explain, contemplate, brood over, or even complain about why they didn’t include a forecast for thunderstorms, fog, or freezing rain.

In fact, most forecasters will do a pretty good job trying to quantify their uncertainty. In the case of thunderstorms, you may see words in the AFD like “including light rain showers to cover the unlikely threat of thunderstorms.” The AFD isn’t something you’d get in a standard briefing, but it certainly should be part of your preflight brief. I’ve always said that if you are not reading the AFDs, you are missing half the forecast.

You can find the full AFD for each county warning area by visiting the weather.gov website. If you visit weather.gov, in the upper-left corner, type in a location such as an airport, city and state, or Zip code, and you will be presented with a forecast that includes a link on that page labeled “Forecast Discussion” that is valid for that town or airport. That link will contain the entire discussion that includes the aviation section. The AFD is also included in some of the heavyweight aviation apps or using my EZWxBrief progressive web app.

Local Knowledge

So, the next time you pore over the TAFs along your route, remember these two points. First, never assume the weather forecast at one airport applies to a nearby airport. On some occasions when the weather is homogeneous across a region, it very well may be that a TAF is representative of the weather at airports close by. Forecasters have local knowledge and often make forecasts that take into consideration how terrain or the previous day’s weather can impact the weather at any particular airport.

Second, TAFs are not an area or zone forecast and should never be used as such. It’s often easy to look at all of the TAFs along your route and make a hasty decision. Just because the three or four TAFs along your proposed route do not mention thunderstorms doesn’t mean you won’t encounter them during cruise. Use TAFs for what they are intended to show. Therefore, if you have an emergency and need to land, knowing the potential weather at those airports along your route is important. TAFs can tell you if they are likely to be good alternates if you need one.

Lastly, keep in mind that a precipitation forecast in a TAF defines the type of precipitation expected to reach the surface. For example, a forecast for –RA (light rain) or –DZ (light drizzle) doesn’t imply there’s no chance of running into FZRA (freezing rain) or FZDZ (freezing drizzle). The precipitation forecast is based on what’s expected at the surface. If the temperature is forecast to be a degree or two above freezing at the surface, you will see a forecast for rain (or drizzle), but you may find that just 500 feet above the surface, there’s a nasty freezing rain (or freezing drizzle) event waiting for you.

The post The Point Forecast Sheds New Light on TAFs appeared first on FLYING Magazine.

Answer: As we start to move into the cold season over the next couple of months, you may begin to see freezing fog (FZFG) appear in a surface observation (METAR) or  terminal aerodrome forecast (TAF). So, let’s get ahead of the game and discuss some high-level details of what freezing fog is all about and whether or not it constitutes known icing conditions. 

In simple terms, freezing fog is nothing more than an obstruction to ground visibility.  When surface observing equipment such an ASOS measures the visibility to be less than 7 sm, a reason for what’s causing the decrease in surface visibility must also be included. Besides freezing fog, other common obstructions to visibility include HZ for haze, BR for mist and FG for fog. 

Haze or mist are the most common obstructions to visibility. They are added to the report when the surface visibility is less than 7 sm but greater than one-half of a statute mile. Once the visibility drops to one-half of a statute mile or less, fog becomes the dominant obstruction to visibility and FG is added to the report. In this case, when the temperature is also at or below 0 degrees Celsius, then freezing fog becomes an obstruction to visibility and FZFG is added to the report. 

The important thing to understand is that the software employed by the ASOS makes this determination solely by using the reported surface visibility and temperature independent of any ice accretion or if clouds are actually touching the surface. At times, airports with a trained human observer can override this as was done in this surface observation for Spokane, Washington (KGEG).  

KGEG 271353Z 19008KT 1/4SM -SN BR FEW001 OVC003 M03/M04 A2993   

    RMK AO2 SLP157 SFC VIS 2 VIS NW-N 1 P0000 T10331039

In this case, the reported visibility is one-quarter of statute mile and the temperature is minus-3 degrees Celsius. Both of these conditions meet the criteria for freezing fog. Therefore, if this were an automated report, the obstruction to visibility would have included freezing fog, or FZFG. However, notice in the remarks section, the observer notes that the surface visibility is 2 sm, and visibility to the northwest and north is 1 sm. Effectively the trained observer felt the obstruction was more representative of mist (BR) and not freezing fog.  

Is freezing fog considered known icing conditions? The short answer is no. According to the National Weather Service (NWS) directives, freezing fog is “consisting predominantly of water droplets at temperatures less than or equal to 0 degrees Celsius whether or not the fog is expected to deposit rime ice.” Once again, fog or freezing fog is an obstruction to visibility simply based on the measured surface visibility and temperature. Nevertheless, in a dense radiation fog event, ice may indeed accrete on the airframe while taxiing to depart.    

Similar to surface observations, freezing fog can also appear in a TAF. The same general rules apply. When a forecaster expects the visibility will be at or below one-half of a statute mile and the temperature will be at or below 0 degrees Celsius, they will ordinarily forecast FZFG. Based on the same NWS directives, freezing fog in a terminal forecast by itself is not a forecast for known icing conditions.

It is true that fog consists predominantly of water droplets when the temperature is warmer than 0 degrees Celsius. But to know if freezing fog is an icing hazard is a bit more complex and depends on many factors—not all of them are specifically discussed here. 

Freezing fog is primarily a ground icing issue, not an in-flight icing concern. The two environments have a great deal of differences and may depend where you are departing. For example, if you are flying out of Grand Forks, North Dakota (KGFK), which is landlocked, your chances of getting any significant liquid water content is pretty small, especially for radiation-type fog events that occur at warmer, subfreezing temperatures. In fact, the forecasters at the Grand Forks weather office don’t issue many TAFs with FZFG for that reason. The meteorologist in charge at the NWS Forecast Office in Grand Forks has mentioned to me,“…In our temperature regimes and winter air mass scenarios, we typically don’t have that high of true liquid water droplet concentrations but probably more suspended ice crystals…so we may get a bit of glaze but not a true rime-icing scenario.” 

• READ MORE: Fog: the Malignant Weather Ninja

Many years ago, I asked Dr. Marcia Politovich, an aviation icing expert and a deputy director at the University Corporation for Atmospheric Research in Colorado, weighed in on the topic. 

“It’s not likely that a surface temperature of minus-3 degrees Celsius or above could support mostly or only ice crystals, but history matters,” Politovich said. “Where the crystals came from is important [maybe they formed aloft in very cold air and are falling slowly through a warmer layer near the ground]. The local ice nucleus source might not be as important as what would be in the air, carried from some other location. Also cloud nuclei [for water drops] would also be sourced locally, so if the air is very clean, it’d be hard to produce anything very near the surface.”

Therefore, in a much colder temperature regime, it’s unlikely you’ll experience much in the way of rime since the fog will consist predominantly of ice crystals. If you are departing out of an airport near a large body of water or when the soil has been moistened after a precipitation event (other than snow), it is quite common to have some ice deposit onto the prop and other surfaces of the aircraft when the static air temperature is between 0 degrees and minus-10 degrees Celsius.  

However, if there’s no ice accreted after you finish your preflight and/or if the static air temperature is below minus-10 degrees Celsius, then the likelihood of ice rime on the airframe is minimal while the aircraft is waiting on the ramp or taxiing. Lastly, the most important rule is to never take off if the surfaces of your aircraft are contaminated with ice.

The post Is Freezing Fog Considered ‘Known Icing Conditions’? appeared first on FLYING Magazine.

I live in the Pacific Northwest, where fog—the lowest of the low clouds—is a nearly daily occurrence, especially in the fall and winter. I live close to the water, and on some days it never lifts. Other days, we get a few hours of flyable sunshine and visibility, and we watch the temperature and dewpoint spread very carefully because fog can sneak up on you ninja-like. And if you are not instrument rated and in an instrument-capable aircraft, instrument current, instrument proficient, and prepared to “go on the gauges,” you may have a really bad day. Know your enemy, as my father used to say.

• READ MORE: When Smoke Gets In Your Skies

What Is Fog?

Per the FAA’s Instrument Flying Handbook, fog is defined as a low cloud, which has its base within 50 feet of the ground, reducing visibility to less than five-eights sm. In order for fog to form, three basic conditions must be met:

• There needs to be condensation nuclei, such as smoke particles, salt, dust, pollen, etc., for the moisture to condense upon. In the Pacific Northwest, we have this in the form of salt from the ocean and smoke from all the wood stoves.

• There must be high water content and a low temperature/dew point spread. Dew point is the temperature the air must be cooled to in order to become saturated. When the air is cooled or moisture is added to it and the temperature and dew point are within 4 degrees Fahrenheit/2 degrees Celsius of each other, fog is likely, as it forms when the temperature and the dew point converge. As the day heats up, the temperature dew point diverges, and the fog ‘burns off.’ In the evening, the process reverses. In  the afternoon, especially in winter, the process reverses usually around 3 p.m. Keep this in mind when you head out on late afternoon flights.

• Fog forms when light surface winds are present as they cause surface friction to create an eddy, causing more air to contact the ground

Fog is basically a cloud at the surface, but like other clouds, there are varieties, and each has certain characteristics. For example, some need wind to form.

To recall the types of fog, use the acronym SURAPIF (Steam, Upslope, Radiation, Advection, Precipitation, Ice, Freezing).

Steam Fog

Steam fog, sometimes known as “sea smoke,” forms when cooler air moves over slightly warmer water. Steam fog is usually not very thick and needs wind to form. It is associated with a shallow layer of unstable air, so you can expect convective turbulence flying through it.

Upslope Fog

Upslope fog forms as moist, stable air is pushed up a hill or other sloping land mass. As the air moves up, it cools, and when the temperature and dew point converge, there is fog. Mountain fog is sometimes mistaken for smoke, which is how the Great Smoky Mountains in North Carolina and Tennessee get their name. Per the Aviation Weather Handbook, this type of fog is most commonly observed in the autumn and spring months and is the densest around sunrise when surface temperatures are often at their lowest.

Radiation Fog

Radiation fog, also known as “ground fog,” forms over low-lying, flat surfaces on clear, calm, humid nights, especially over a wet surface like ground after rain. As the surface cools, the adjacent air also cools to its dew point and fog forms. Radiation fog can vary in depth from a few feet to about 1,000 feet and usually remains in place.

A subset of radiation fog is valley fog, which, as the name suggests, forms in lower-lying areas. It is very thick and is sometimes referred to as “tule fog.” It can form when the air along ridgetops cools after sunset. As the air becomes more dense and heavy, it flows down the slope to the valley floor, where it continues to cool and becomes saturated to form fog.

Radiation and valley fog can drop visibility to near zero and make any kind of transportation hazardous because you can’t see in front of you and lack any depth perception. You may not even want to taxi in that kind of fog. Radiation fog is often a factor in chain-reaction accidents on highways in the winter months.

Advection Fog

Advection fog occurs when a low layer of warm, moist air moves over a cooler surface. This is very common along the West Coast in winter. Advection fog requires wind to form, and an increase in wind speed can make it thicken. It is also tenacious, moving over water and then inland, then back over water for days or even weeks at a time. The horizontal movement of advection fog helps distinguish it from radiation fog.

Precipitation Fog

Precipitation fog forms when rain evaporates as it falls through cold air. When the precipitation stops, the fog disappears. You can notice this when the objects at the end of the runway disappear under fog as it rains then reappear when the rain ends.

Ice Fog

Ice fog forms in polar and arctic regions—and other cold weather locations—when the temperature is minus-10 degrees or below, and the air is too cold for the air to contain supercooled water droplets, so it forms tiny ice crystals.

Freezing Fog

Freezing fog occurs when water droplets freeze on contact to a surface that is below 32 degrees. This means anything the freezing fog touches will become coated with ice—including all aircraft. Sometimes you can watch it form on cold mornings. As the fog rolls in, the aircraft on the ramp will slowly see their windows turn opaque, and their surfaces will appear to sparkle. This process is different from frost forming, which usually involves sublimation.

Respect the Fog

Even with an instrument certificate and an airplane loaded with the latest in technology, fog can quickly become too thick to operate in and destroy visibility. This is why instrument approaches have weather minimums in the form of ceiling and visibility printed on them. Respect the metrics, as sadly every year there are pilots who attempt to land in weather below the minimums and don’t live to tell the story because they misjudged their altitude or distance from terrain because of fog.

The post Fog: the Malignant Weather Ninja appeared first on FLYING Magazine.