Visibility Restrictions
Understanding haze, smoke, dust, and other visibility reducers
Beyond Fog: Other Visibility Hazards
While fog gets most attention as a visibility hazard, pilots regularly encounter other phenomena that reduce visibility: haze, smoke, dust, sand, volcanic ash, and various forms of precipitation. Each has different characteristics, METAR codes, and operational implications.
Understanding these restrictions helps you interpret weather reports accurately, set appropriate personal minimums, and make safe go/no-go decisions.
Visibility Restriction Codes
| Code | Meaning | Description |
|---|---|---|
| FG | Fog | Water droplets, visibility < 5/8 mile |
| BR | Mist | Water droplets, 5/8 - 6 miles visibility |
| HZ | Haze | Dry particles (dust/pollution), < 7 miles |
| FU | Smoke | Combustion particles from fires |
| DU | Widespread Dust | Dust suspended over large area |
| SA | Sand | Sand particles in air (desert regions) |
| VA | Volcanic Ash | Pulverized rock/glass - extremely hazardous |
| SS | Sandstorm | Severe sand/dust, visibility < 5/8 mile |
| DS | Dust Storm | Severe dust, visibility < 5/8 mile |
| BLDU | Blowing Dust | Wind-raised dust, localized |
| BLSA | Blowing Sand | Wind-raised sand, localized |
Haze (HZ)
Haze is the most common non-precipitation visibility restriction. It consists of fine particles—dust, salt, pollution, or smoke—suspended in stable air. Unlike fog and mist, haze forms in relatively dry conditions (humidity below 80%).
Causes
- • Urban air pollution
- • Distant wildfire smoke
- • Agricultural dust
- • Sea salt near coasts
- • Industrial emissions
Characteristics
- • Brownish or bluish tint to sky
- • Gradual horizons
- • Worst in afternoons (mixing)
- • Trapped under inversions
- • Often seasonal (summer/fall)
Slant Range Visibility
Haze layers often concentrate near the surface. While looking horizontally you might see 5 miles, looking diagonally downward through the layer means seeing through more particulates. On approach, the runway may not be visible until much closer than the reported surface visibility suggests. Add extra cushion to your approach planning.
Example METAR
KLAX 161753Z 25008KT 4SM HZ CLR 24/12 A2992
Los Angeles reporting 4 miles visibility in haze. This is MVFR conditions despite clear skies. Typical summer LA basin conditions under marine inversion.
Smoke (FU)
Smoke from wildfires can dramatically impact aviation across vast areas. Western U.S. fire seasons regularly ground VFR operations across multiple states. Unlike haze, smoke can reduce visibility to near zero and extend to high altitudes.
Wildfire Smoke Hazards
- • Visibility: Can drop to <1 mile rapidly as smoke drifts
- • Health: PM2.5 particles cause respiratory issues—consider mask
- • Altitude: Smoke can reach FL200+ with active fires
- • Unpredictability: Wind shifts move smoke rapidly
- • Smell: Smoke in cockpit indicates you're in dense layer
Resources
- • AirNow.gov - smoke forecasts
- • NOAA Smoke Text Forecasts
- • AIRMETs for smoke (IFR)
- • Satellite imagery (GOES)
Operational Considerations
- • Plan alternates away from fire areas
- • Climb above smoke layer if possible
- • Monitor NOTAMs for TFRs near fires
- • Engine air filters may need inspection
Example METAR
KBOI 161853Z 18012G22KT 1SM FU BKN015 29/08 A2989
Boise reporting 1 mile visibility in smoke with broken clouds at 1,500 feet. This is IFR conditions from wildfire smoke—VFR flight is not possible.
Dust and Sand (DU, SA, BLDU, BLSA)
Dust and sand are common visibility restrictions in arid regions. The difference between codes depends on intensity and coverage:
DU/SA - Widespread
Dust or sand suspended over a large area, reducing visibility uniformly. Often associated with frontal passage or large-scale wind events.
BLDU/BLSA - Blowing
Wind-raised dust or sand, typically more localized and variable. Common near construction, plowed fields, or dry lake beds.
DS/SS - Storms
Dust storms or sandstorms with visibility below 5/8 mile. Severe weather that can ground all operations. Common in Middle East, Sahara, and American Southwest.
Dust Devil Hazards
While not reported in METARs, dust devils are common in desert areas on hot afternoons. They can contain updrafts of 40+ knots and can flip light aircraft. Avoid landing or taking off if dust devils are visible near the runway. They're hard to see from aloft— look for the dust column near the surface.
Volcanic Ash (VA)
AVOID ALL VOLCANIC ASH
There is no safe amount of volcanic ash for aircraft. Even concentrations too low to see can cause significant damage. All aircraft must avoid volcanic ash completely.
Damage Mechanisms
- • Jet engine flame-out (ash melts, re-solidifies)
- • Windscreen abrasion to opacity
- • Pitot/static port blockage
- • Erosion of compressor blades
- • Contamination of air systems
Detection Issues
- • Invisible to weather radar
- • Often in thin layers
- • Can drift thousands of miles
- • May persist for weeks
- • Night flight: sulfur smell first warning
Volcanic Ash Advisories
Volcanic Ash Advisory Centers (VAACs) track ash clouds worldwide. Check:
- • SIGMETs for volcanic ash
- • NOTAMs for affected airspace
- • VAAC advisories and graphics
- • Pilot reports of ash encounters
Precipitation Effects on Visibility
Precipitation restricts visibility in proportion to its intensity. METARs report visibility with the precipitation type:
| Intensity | Rain Impact | Snow Impact |
|---|---|---|
| Light (-) | Usually 3+ miles | Usually 1-3 miles |
| Moderate (no prefix) | 1-3 miles typical | 1/2 - 1 mile |
| Heavy (+) | <1 mile possible | <1/4 mile common |
Snow Considerations
Snow reduces visibility more than rain due to higher reflectivity and slower fall rate (particles stay airborne longer). Heavy snow can reduce visibility to near zero. Watch for SN in METARs with visibility below 3 miles.
Precipitation Codes
- -RA Light rain
- RA Moderate rain
- +RA Heavy rain
- -SN Light snow
- +SN Heavy snow
Visibility and Flight Categories
MVFR Warning
Marginal VFR (3-5 miles visibility) is technically legal for VFR flight in most airspace, but conditions can deteriorate rapidly. Many VFR accidents occur in MVFR conditions when pilots press on despite worsening weather. Treat MVFR as a serious caution requiring solid alternates and escape routes.
Key Takeaways
- HZ (haze) is dry particles; BR (mist) is water droplets
- Slant range visibility is often worse than reported surface visibility
- Wildfire smoke can reduce visibility across entire regions—check smoke forecasts
- AVOID ALL VOLCANIC ASH—there is no safe concentration
- MVFR (3-5 miles) warrants extra caution—conditions can deteriorate quickly