Visibility in Meteorology

Defining Visibility in Meteorology

Visibility in meteorology is the maximum horizontal distance at which a large, clearly defined object can be seen and recognized against the sky. For aviation, this is usually reported in METARs and TAFs as the lowest value observed in any direction, measured in metres or kilometres. When visibility is 10 km or more, it's often recorded as '9999' in reports, indicating no significant operational limitation for VFR flights.

Types of Visibility

• Prevailing Visibility: The greatest distance at which objects can be seen over at least half the horizon circle. If visibility is significantly lower in a particular direction (below 1500 m or less than 50% of prevailing visibility and below 5000 m), this minimum value and its direction are also reported.
• Ground Visibility: The visibility measured at an aerodrome by an observer at ground level. This is typically what is reported in METARs.
• Flight Visibility: The distance a pilot can see forward from the cockpit during flight, which may differ from ground visibility, especially at altitude or when flying toward weather phenomena.
• Vertical Visibility: The distance an observer can see vertically into a surface-based obscuring layer (like fog or heavy precipitation) when the sky is not visible. This is measured or estimated when the cloud base cannot be determined and is reported in hundreds of feet (e.g., VV003 for 300 ft). If vertical visibility cannot be measured, it is reported as VV///.
• Runway Visual Range (RVR): The distance a pilot can see down the runway, measured by transmissometers and reported when visibility drops below 1500 m, crucial for low-visibility operations.

How Visibility is Measured

Observers use known landmarks at set distances to estimate visibility during daylight. At night, calibrated lights and optical devices with filters are used. Automated systems may also be employed. Factors reducing visibility include fog, precipitation, haze, dust, smoke, and other particulates.

Visibility Reduction Factors

• Fog and mist
• Precipitation (rain, snow, drizzle)
• Smoke, dust, sand, or volcanic ash
• Frost or condensation on windscreens
• Sun glare or low sun angle

Understanding these distinctions and measurement methods is vital for interpreting weather reports and making safe aviation decisions.