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.

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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.

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