Prioritize...
This page details the atmospheric variable, visibility. Before leaving this page, make sure that you can do the following:
- Describe what visibility is and the types of atmospheric conditions that can affect visibility.
- Identify and decode the visibility observation on a station model (if displayed).
- Identify when an "obstruction to visibility" symbol (that is, present weather) must be listed along with the visibility measurement on a station model.
- Identify and decode the "present weather" symbol (if shown).
Read...
Meteorologists are very interested in horizontal visibility (the maximum distance away that an observer can see an object located near or on the ground), because it has major implications for transportation. Very poor visibility can cause major traffic accidents and airline catastrophes. Obviously, visibility is very important for pilots during take-off and landing, and if you want to see what it's like for a pilot to land in poor visibility, check out this video of a Boeing 737 landing in poor visibility in London (view from the cockpit).
Often, visibility can vary in the 360-degree panorama around a weather station. For example, there could be visibility-restricting snow showers just to the north and west of the station, disproportionately reducing visibility in those quadrants. To see what I mean, check out the panoramic view on a wintry day with recurrent, scattered snow showers in the vicinity of Penn State's main campus, and note that parts of the nearby ridges can't be seen because of snow showers. In such a situation, an observer reports a "representative visibility." On days when horizontal visibility dramatically varies over the 360-degree panoramic view around an airport or weather station, a trained weather observer determines a single visibility that reasonably describes more than half the 360-degree panorama. In more precise terms, a representative visibility is the greatest distance that objects can be observed and identified over more than 180 degrees of the panoramic view around an airport or weather station.
Horizontal visibility can run the gamut. On a perfectly clear day, you can't see forever, but visibility can reach approximately 100 miles in the mountainous West. On the other hand, visibility can lower to near zero in very dense fog, fierce blowing and/or falling snow, blowing sand/dust, smoke, etc. Automated weather stations, however, typically report the visibility as 10 miles when no obstructions to visibility are present.
Obstructions to visibility (so called "present weather") such as fog, haze, and smoke are considered non-precipitating obstructions to visibility, and their reductions to visibility can be very noticeable. For example, check out these photographs of the ridges south of Penn State's main campus on two different summer days -- one with a clean atmosphere and one with a hazy atmosphere. For a dramatic example of a non-precipitating obstruction to visibility, which may be of special interest to aviators in particular, check out the Explore Further section below.
Precipitation can also reduce visibility by varying degrees. The degrees to which precipitation reduces horizontal visibility gives rise to a hierarchy of qualifiers such as light snow, moderate snow, and heavy snow. Indeed, when dealing with snow, the qualifiers of light, moderate, and heavy are actually defined in part by horizontal visibility. Rain can also reduce horizontal visibility, but its qualifiers of light, moderate, and heavy are defined by rainfall rate (not horizontal visibility).
Now that you know the types of conditions that can reduce visibility, let's take a look at visibility is displayed on the station model, which is covered in the Key Skill section below. Before you dive into that section, one thing to note is that non-precipitating obstructions to visibility are displayed as present weather on the station model only if the horizontal visibility is less than or equal to seven miles. Why seven miles? Typically, a radio beacon that aircraft use while landing called the outer marker lies four to seven miles away from the start of the runway. So, if there's an obstruction to visibility that prevents pilots from seeing the runway from the outer marker, then the obstruction must appear of the station model.
Meanwhile, when precipitation falls at an airport, it is always depicted on the local station model as present weather no matter how light or how little it affects visibility. This protocol exists because pilots always need to know when precipitation occurs at an airport not only because it restricts horizontal visibility, but because it also lowers the heights of cloud ceilings, both of which come into play during take-off and landing. To summarize when the obstruction to visibility symbol is displayed on a station model, review this flow chart.
Key Skill...
Given that many of the primary weather stations are located at airports, horizontal visibility has a special place reserved on the station model. To locate horizontal visibility, look below and to to the left of the air temperature. The leftmost number (if present), represents the horizontal visibility reported in statute miles (one-and-a-half miles in the sample station model on the right). The symbol (again, if present) just below the temperature represents the present weather ("moderate rain" in the example on the right). Remember that present weather will always appear if it's precipitating, but it will only appear if it reduces visibility to seven miles or less when the obstruction to visibility is non-precipitating.
You should spend some time familiarizing yourself with all of these common symbols for present weather, but you can also see the entire table of international symbols for present weather if you're interested (some of them rarely get used). I also recommend practicing with the interactive station model tool below. The default value for visibility in the tool is one-and-a-half miles (the far left number), but you can change the visibility on the station model by altering the "Visibility" field in the Current Conditions panel. Give it a try! Next, examine the "Obstruction to Visibility" pull-down list in the Current Conditions panel. The default weather for the tool is rain showers (a single dot with a downward-facing triangle). Experiment with the various observations to see the symbols that they produce and notice that you can select both precipitating and non-precipitating types of weather (as long as the visibility is less than seven miles).
Finally, change the visibility to, say, 10 miles and then note how the Obstruction pull-down menu changes. First of all, the option "none" becomes available since you are no longer required to report an obstruction to visibility. Secondly, the non-precipitation types of weather are removed from the list because these are only reported if the visibility is less than seven miles.Explore Further...
Away from airports, pilots routinely report adverse flying conditions. Appropriately called Pilot Reports (PIREPs), these in-flight observations catalog turbulence, icing, and weather / sky conditions (website for PIREPs). In the United States, air-traffic controllers solicit pilot reports whenever any of the following are present or predicted for their area of responsibility: icing, turbulence, thunderstorms, wind shear, visibility lower than five miles, low ceilings, and volcanic ash. That's right...volcanic ash! As a dramatic example, in the spring of 2010, the Eyjafjallajökull volcano in southern Iceland erupted spectacularly (see photograph below), spewing large volumes of ash into the atmosphere (view of Eyjafjallajökull eruption from space) and temporarily brought commercial flights to a halt in the British Isles and other parts of Europe. That's because jet engines can fail when they ingest volcanic ash, which obviously poses a serious threat to aviation.
