Aviation Weather Tips — Visibility

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Visibility conditions impact aviation operations in many ways. Poor visibility at a destination can reduce capacity of airports leading to ground delays, flight diversions, flight cancellations and extra operating costs just to name a few.

Below is an overview of what you need to know:

1. Visibility defined

Ground or surface visibility is the prevailing horizontal distance at which an object can be clearly discerned by a certified weather observer, under current light and weather conditions. Flight visibility is the average forward horizontal distance, from the flight deck, at which prominent unlighted objects can be seen and identified.

2. Measurements of visibility

Other than in the U.S. visibility is given in meters whenever reporting Runway Visibility Range (RVR) and Terminal Aerodrome Forecasts (TAFs). In the U.S. these values are provided in feet and miles respectively.

3. Calculation of visibility

Visibility values are determined by both human observers and the Automated Surface Observing Systems (ASOS). The human observer determines visibility by identifying objects and landmarks at known distances throughout a 360 degree circle around the observation point. The greatest visibility observed over 50% or more of the 360 degree area is the prevailing visibility. If, however, there’s a sector of the 360 degree area that significantly differs from prevailing visibility, the observer may add a remark. ASOS measures and converts sensor-driven values to visibility values corresponding to what the human eye can see. Pilots must be mindful that nearly half the area around an airport may have lower conditions than the reported prevailing visibility.

4. Reduced visibility

Atmospheric phenomena resulting in reduced visibility include rain, drizzle, thunderstorms, snow and blowing snow, ice pellets/crystals/ice fog, volcanic ash, fog, smoke, haze and airborne dust and sand.

5. RVR considerations

RVR is distance over which a pilot of an aircraft, on the centerline of a runway, can see delineated runway surface markings and centerline. RVR values are normally determined by the human eye or with an Instrumented Runway Visual range (IRVR) trasmissometer. RVR is important as it provides the main criteria used to determine category of visual aids operational at an airport as well as criteria/minima for instrument approaches.

6. Visibility vs. prevailing visibility

Visibility is defined as the greatest distance through the atmosphere, toward the horizon, that prominent objects can be identified with the naked eye. Prevailing visibility, in aviation terms, is measurement of the greatest distance visible throughout at least half of the horizon– and not necessarily continuous — as determined by light and weather conditions. Observers use a number of visual reference points including buildings, towers hills and assorted geographical features, to determine prevailing visibility.

7. Major impacts on visibility

Fog is often a significant factor in terms of lowered visibility values. Precipitation reduces visibility, depending upon intensity and droplet size, and light drizzle and/or snow can hinder visual flight rule (VFR) operations. Heavy rain, snow and thunderstorm activity, however, can significantly reduce visibility. Lowered visibility, when operating without instrument flight rule (IFR) certifications, may result in loss of aircraft control.

8. Obscuring phenomena and haze

Obscuring phenomena is any collection of particles – either aloft or in contact with the surface — that’s dense enough to be discernible to the observer. This includes everything from rain to fog to volcanic ash. Haze on the other hand are particles suspended in the air that reduce visibility by scattering light. Haze is usually formed by presence of condensation nuclei, such as aerosols, ozone, nitrogen oxides and hydrocarbons (air pollution).

9. Operating restrictions relating to visibility

Pilot certifications often determine under what conditions a pilot is able to operate. VFR flying, for example, mandates that the pilot be able to see outside the flight deck to control aircraft attitude, navigate and avoid obstacles and other aircraft. Flying in other than VFR conditions is considered an IFR operation.

10. Legal visibility minimums

Charter (non-scheduled commercial) operators generally have more operating restrictions in place compared to private non-revenue operations. Part 135 (IFR) operations must, for example, be capable of landing within 80% of runway length and this can affect operations to as many as 2400 smaller airports in the U.S. alone. Additionally, Part 135 (IFR) operations may not begin an approach to an airport that has no weather reporting facility, unless a designated alternate airport has approved weather reporting and is within one hour flight time. Although one mile forward visibility is usually a standard landing minimum requirement there are many factors that contribute to landing/take off minimums, including aircraft performance, operator certification and obstacle clearance. For VFR operations 1000 feet/3 miles is the technical legal minimum visibility requirement.

Be mindful that night VFR is more challenging than day VFR, as it’s more difficult to see clouds and to differentiate ground details. Be particularly careful in cases of marginal weather forecasts and lean more toward “deteriorating to” rather than “improving to” projections.

11. Provide your minimum visibility requirements

Operators often have individual and particular operating requirements in terms of visibility. Even with all legal minimums established by airports, local authorities and crew certifications, pilots will know what they’re comfortable with. Many VFR operators, for example, will not take a chance on departing at close to marginal visibility conditions. Remember that “just because something is legal doesn’t make it safe.” Always ensure that your 3rd-party providers are aware of your particular operating and visibility minimum requirements.

12. Additional tips

When looking for accurate visibility values it’s best to obtain this information no more than 24 hours prior to the estimated time of departure or arrival. While visibility predictions can be considered for planning purposes as much as 72 hours out, these forecasts are not as detailed or accurate. When operating to destinations with visibility under 1000/3 you should have at least two airport alternates with visibility of 800/2 or greater. Keep in mind that when operating to areas with limited alternates you may need to look some distance away to find suitable airport alternates that meets all of your operational and visibility requirements.

Conclusion

As weather forecasting is not an exact science, it’s recommended that operators utilize the services of certified and experienced meteorologists. It’s recommended that for planning purposes you obtain preliminary weather briefs, but also contact your weather provider within 24 hours of departure to obtain an updated overview of what to expect for your destination. Also, it’s important that you always provide your visibility preferences, so your service provider is able to offer the best solutions for your flight.

Questions?

If you have any questions about this article or would like assistance with your weather planning, contact me at stevearbogast@univ-wea.com.

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