Visibility

Visibility

Visibility is the greatest distance at which an object on the ground is clearly identifiable. A key value for VFR (see Subject 010 SERA VMC minima).

Source: ICAO Annex 3 Meteorological Service; WMO; DWD.

Meteorological vs pilot visibility

The difference in visibility observations between ground weather observers and approaching pilots arises from the observer measuring horizontal meteorological visibility while the pilot assesses slant visibility downwards:

• Ground visibility: horizontal, measured at the surface.
• Pilot approach visibility: slant, affected by clouds, haze, sun.

Visibility in different air masses

Visibility is better in continental air than in maritime air; cloud formation is suppressed:

• Continental air: dry, few condensation nuclei → good visibility.
• Maritime air: humid, many aerosols (salt) → reduced visibility from haze.

Continental polar air usually has the lowest humidity often providing excellent visibility with minimal haze or fog.

Stable layering and haze

Stable atmospheric layering with strong inversion are especially beneficial to the formation of haze:

• Inversion traps aerosols and pollutants in near-ground air.
• Accumulation → visibility loss.

Daytime flight above inversion

During daytime flight about 1000 ft above a strong surface inversion, expect good flight visibility, poor slant visibility, and moderate to poor visual reference to the ground because the inversion traps haze and pollutants below, obscuring downward views while horizontal sightlines remain clear.

CAVOK

CAVOK = "Ceiling And Visibility OK" — standard abbreviation in METAR/TAF for ideal conditions:

• Visibility 10 km or more.
• No clouds below 5000 ft AGL or below the highest minimum sector altitude (whichever higher).
• No cumulonimbus clouds.
• No significant weather phenomena.

CAVOK relates to conditions including visibility of 10 km or above, no clouds below 5000 ft AGL or the highest minimum sector altitude, no cumulonimbus clouds, and no significant weather phenomena.

Visibility and precipitation

• Moderate to heavy snow: visibility < 1-2 km.
• Moderate to heavy drizzle: < 3 km.
• Rain: usually better than snow/drizzle.
• Hail: severe visibility reduction near Cb.

Moderate snow causes the most significant reduction of visibility.

Cloud-coverage terms in METAR

Cloud-base reporting in METAR

Cloud bases in a METAR report are specified in ft above AGL — more precisely: above aerodrome elevation (= AGL at the airfield).

Spread and visibility

The reduction of spread causes a reduction of visibility in otherwise unchanged conditions (see humidity lesson) — small spread = high RH = haze/fog = visibility down.