Updated: Nov 3, 2021
A pilot will encounter several different flavors of visibility. This includes flight visibility, ground visibility, prevailing visibility, runway visual range and vertical visibility just to name a few. But wait, is vertical visibility even a legitimate visibility? Actually it’s not a true measure of visibility at all. Vertical visibility is more of a close cousin to ceiling than it is to visibility. That is, it represents the distance in feet a person can see vertically from the surface of the earth into an obscuring phenomena or indefinite ceiling. Clear as mud?
There’s no doubt that an indefinite ceiling is perhaps the most misunderstood phenomenon reported in a surface observation (METAR). You’ll also find it mentioned in a terminal aerodrome forecast (TAF) as shown below in the EZAirport page of the EZWxBrief progressive web app. Whether in a METAR or TAF, vertical visibility is coded as VV followed by a three digit height in hundreds of feet (e.g., VV002). So let’s explore the difference between a definite and indefinite ceiling.
Human observers have used rotating beam and laser beam ceilometers for many years to measure the height of clouds. 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) located at many airports throughout the U.S. The trained observer simply logs into the ASOS and makes his or her observation based on the data gathered and reported by the automated system. Then the observation is edited and augmented by the observer as necessary.
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 period of time since clouds are generally moving through the sensor array most of the time. 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 are double-weighted using a harmonic mean.
In our everyday experience, we know that many cloud decks we observe 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 to the untrained eye, but automated systems do a reasonable job making that observation.
The nuts and bolts
Every minute the ASOS continuously scans the sky. To determine the height(s) of the clouds, the backscatter from the ceilometer is 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 that most of the hits are aggregated around a particular height above the ground. Such a sharp signature is then incorporated into the 30-minute cloud height 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 12,000 feet AGL over the sensor. Note that the ASOS is designed only to detect clouds below 12,600 feet above the ground although a trained observer can 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 to something referred to as an indefinite ceiling.
Moisture rich environment
Essentially, an indefinite ceiling means there is something obscuring your view of the base of the clouds. So 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 of 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 surface 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. 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 ½ statute mile. Furthermore, a vertical visibility of 700 feet (VV007) will likely be associated with a visibility between 1 and 2 statute miles. While rare, you may even see a fairly high vertical visibility over 1,000 feet (e.g., VV012). The really bad stuff, however, occurs with a visibility of 1/4SM (or even M1/4SM) and a vertical visibility of 0 feet (VV000) as shown below in the EZWxBrief EZAirport page for the Cheyenne Regional Airport. This really 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.
As mentioned earlier, fog and precipitation are the two primary reasons the base of the cloud deck is obscured. Therefore, it’s not unusual to see vertical visibility reported when drizzle or even snow is falling from the base of the cloud. 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.
Conditions that produce an indefinite ceiling take longer to improve. Normally there will be a transition from an indefinite to definite ceiling once the moisture begins to mix out. However, the visibility may still be quite low for the next few hours. Keep this in mind when flight planning to an airport that currently is reporting an indefinite ceiling.
Operationally 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. Moreover, an instrument approach may be a bit more challenging even after you drop below the reported ceiling height due to 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.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist