This page details the atmospheric variable, visibility. Before leaving this page, make sure that you can do the following:
- You should be able to describe what visibility is and the types of atmospheric conditions that can affect visibility.
- You should be able to identify and decode the visibility observation on a station model (if displayed).
- You should know when an "obstruction to visibility" symbol (that is, present weather) must be listed along with the visibility measurement on a station model.
- You should be able to identify and decode the "present weather" symbol (if shown).
On December 8, 2005, Southwest Airlines Flight 1248 attempted to land at Midway Airport in Chicago during a snowstorm. Unfortunately, the plane crashed, and there were tragic consequences. At the time, visibility was less than one mile. Obviously, visibility is very important for pilots during take-off and landing...here's a video of a Boeing 737 landing in poor visibility in London (view from the cockpit).
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 directly below the slot for air temperature. The leftmost number (if present), represents the horizontal visibility reported in statute miles. The funky symbol (again, if present) on the right is a symbolic representation for the present weather. Check out the entire table of international symbols for present weather (precipitation and other restrictions to visibility).
Often, visibility can vary in the 360-degree panorama around a weather station (maybe there are visibility-restricting snow showers just to the north and west of the station, disproportionately reducing visibility in those quadrants). To see what I mean, click, hold and drag right/left to get a panoramic view on a wintry day with recurrent, scattered snow showers in the State College area. 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 heavy, "pea-soup" fog, fierce blowing and/or falling snow, blowing sand/dust, smoke, etc.
When precipitation falls at an airport, it is always depicted on the local station model as present weather. Even light drizzle or a snow flurry must be reported. Such a protocol exists because pilots always need to know when precipitation occurs at an airport. Again, precipitation restricts horizontal visibility and helps to lower the height of cloud ceilings, both of which come into play during take-off and landing.
There are degrees to which precipitation can reduce horizontal visibility, giving 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 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). You should now spend some time familiarizing yourself with all of the common symbols that represent precipitation types and their intensities on station models. The Weather Prediction Center (WPC) in Washington, D.C. provides a list of the most common symbols for present weather.
Precipitation does not have to occur for the weather symbol to appear on a station model. Fog, haze, and smoke are some of the non-precipitating types of obstructions to visibility that frequently appear on station models. For example, check out these photographs of the ridges south of University Park on two different summer days: one with a clean atmosphere and one with a hazy atmosphere. The key point to remember is that non-precipitating obstructions to visibility are displayed only if the horizontal visibility is less than or equal to seven miles. To sum up when the obstruction to visibility symbol is displayed on a station model, review this flow chart.
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.
In this page, you learned about visibility and the present weather symbol. Locate the visibility on the station model below by looking at the far left number. The default value for the tool is 1 and 1/2 miles. 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 7 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 7 miles.
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. In the spring of 2010, for example, 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 bringing a halt to commercial flights 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.