Pilots who focus exclusively on fog as the primary visibility hazard can be caught off guard by a broader family of atmospheric obscurants—haze, smoke, mist, blowing snow, dust, and more—that can reduce visibility just as dramatically and sometimes with far less warning. Unlike fog, which forms from condensation of water vapor close to the surface, many of these phenomena involve suspended solid particles or involve water droplets that behave differently from classic fog. The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 18, groups these together as obstructions to visibility, and every instrument-rated or aspiring instrument-rated pilot needs a working knowledge of each.
This article unpacks the key non-fog visibility restrictions: mist, haze, smoke, blowing snow, dust storms, sandstorms, and haboobs. For each, we examine what it is, how it forms, how it disperses, and—critically—what it means for your situational awareness in the cockpit.
Mist: The In-Between Obscurant
Mist occupies the middle ground between haze and fog. According to FAA-H-8083-28B Chapter 18, mist is a visible aggregate of minute water droplets or ice crystals suspended in the atmosphere that reduces visibility to less than 7 statute miles (sm) but at or above 5/8 sm (1 km). It appears as a thin, grayish veil over the landscape.
The key differentiator is relative humidity. Fog requires relative humidity at or near 100 percent. Mist, by contrast, forms when relative humidity is high, often above 95 percent, without the handbook fixing a precise upper bound that separates mist from fog. Because it does not obstruct visibility as severely as fog and the droplets are smaller in concentration, mist is less operationally debilitating. However, pilots should not dismiss it, particularly when it sits alongside other obscurants or when an instrument approach requires specific visibility minima. There is no hard line between mist and fog; the transition is gradual as humidity climbs toward 100 percent.
Haze: The Optical Illusion Maker
Haze is a suspension of extremely small, dry particles—too tiny to be seen individually—that collectively give the atmosphere an opalescent or milky appearance. The particles scatter the shorter (blue) wavelengths of light, which is why haze appears bluish against a dark background and yellowish or brownish against a bright background such as the horizon near the sun. This color shift is a diagnostic clue that distinguishes haze from mist; mist produces only a gray obscuration regardless of the viewing direction.
Haze forms and persists in stable air, where there is little vertical mixing to disperse the particles. It is typically a few thousand feet thick but can extend upward to 15,000 feet (4,600 m). This vertical extent has a critical operational implication: haze usually has a definite ceiling. Above that ceiling, air-to-air (in-flight) visibility is essentially unrestricted. Below it, slant-range (air-to-ground) visibility is poor—often far worse than the reported surface horizontal visibility would suggest. A pilot descending through a haze layer may find the runway environment much harder to acquire visually than expected, even when the ATIS reports several miles visibility.
Another practical variable: solar angle matters. Visibility in haze varies significantly depending on whether the pilot is looking into or away from the sun. Flying into sun through a haze layer can reduce effective forward visibility to near zero, while the same layer with the sun behind the aircraft may seem relatively clear. Plan approaches and departures with this in mind.
Finally, certain haze particles are hygroscopic—they grow larger as relative humidity increases—which can drastically and rapidly decrease visibility as moisture levels rise, even before fog officially forms.
Smoke: Haze's More Dangerous Cousin
Smoke is the suspension of small particles produced by combustion—wildfires, agricultural burning, industrial stacks, or other sources. Near the source, smoke can reduce visibility to zero. As smoke travels 25 to 100 miles (40–160 km) or more downwind, the largest particles settle out and the remainder disperses; at that point, the suspension essentially transitions into haze and is reported as such.
Beyond visibility, smoke carries a serious toxicological hazard. Its compounds include carbon monoxide—the most dangerous—as well as hydrogen cyanide and phosgene. Carbon monoxide is colorless and odorless, making it impossible to detect without a CO detector. Pilots operating near wildfire smoke areas should ensure aircraft are equipped with functioning CO detectors and consider using supplemental oxygen or avoiding the area entirely.
Smoke and haze share the same dissipation challenge: unlike fog, which evaporates when temperature rises above the dew point, smoke and haze particles do not evaporate. They must be physically dispersed by wind or removed by precipitation. On a sunny day with clear skies above, daytime heating can create convective mixing that spreads smoke or haze to a higher altitude, temporarily improving surface visibility—but the improvement is slower than fog clearing. If a thick cloud deck sits above the smoke or haze layer and blocks sunlight, convective mixing is suppressed and visibility may show little or no daytime improvement.
Blowing Snow and Whiteout
Blowing snow occurs when wind lifts snow from the surface to a height of 6 feet (2 m) or more and reduces horizontal visibility to less than 7 sm. Light, dry, powdery snow is most susceptible because its low density allows it to become airborne at lower wind speeds. When strong winds keep snow suspended to heights of up to 50 feet (15 m) or more and reduce surface visibility to near zero, the condition is called a whiteout—one of the most spatially disorienting environments a pilot can encounter because all visual contrast between the snow surface and sky disappears. The good news: visibility improves rapidly once the wind subsides, distinguishing blowing snow from precipitation-based restrictions.
Dust Storms, Sandstorms, and Haboobs
Dust storms form when strong winds of 15 knots or greater—sometimes 35 knots or more over compacted desert pavement—lift fine-grained clay and silt particles from barren surfaces such as dry lake beds and river floodplains. They average 3,000 to 6,000 feet in height but can reach 15,000 feet. Crucially, slant-range (air-to-ground) visibility in a dust storm is typically worse than the reported surface visibility, meaning a pilot overhead may be unable to locate an airfield even when the surface METAR shows 3 miles or more visibility.
Sandstorms are similar but more localized, involving heavier sand particles that rarely rise above 50 feet (15 m) and are mostly confined to the lowest 10 feet. A haboob is a special case: a dust or sandstorm generated by cold downdrafts from a thunderstorm, which turbulently loft dust and sand. A haboob wall can extend horizontally for more than 60 miles (100 km) and rise to the base of the parent thunderstorm. Visibility can drop to zero in seconds with essentially no warning.
After sunset, surface cooling stabilizes the lowest atmosphere, forming a temperature inversion that suppresses turbulence and allows dust to settle at approximately 1,000 feet (300 m) per hour. It can take many hours or even days for dust to fully clear—unless precipitation scrubs it from the atmosphere, which is highly effective.
Why These Phenomena Matter Operationally
Each of these obscurants can reduce visibility to values that trigger IFR or LIFR conditions, yet they behave very differently from fog in terms of onset, duration, and removal. Pilots planning cross-country or instrument flights must account for whether improving conditions are even possible without wind. A forecast of improving visibility after a smoke event depends entirely on wind, not temperature—a fundamental difference from fog that catches many pilots off guard.
For VFR pilots, any of these phenomena can result in inadvertent IMC. For IFR pilots, slant-range visibility reduction in haze or dust can make a technically legal approach operationally dangerous because the runway environment may not be visible even when the reported visibility meets minimums.
Key Numbers and Rules
- Mist visibility range: Less than 7 sm but at or above 5/8 sm; relative humidity typically high, often above 95%.
- Haze ceiling: Typically a few thousand feet, up to 15,000 ft; definite top above which air-to-air visibility is unrestricted.
- Smoke transitions to haze after traveling approximately 25–100 miles downwind.
- Dust storm surface wind threshold: 15 kt minimum; up to 35 kt over desert pavement.
- Dust settles at roughly 1,000 ft/hour after stabilization; precipitation clears it much faster.
- Blowing snow qualifies when lifted 6 ft or more above ground and visibility drops below 7 sm.
- Haboob wall can span 60+ miles horizontally and reach thunderstorm base altitude.
- Haze and smoke do not evaporate—they require wind dispersal or precipitation removal.
Common Test Traps
- Fog vs. mist vs. haze on a METAR: Fog (FG) is reported when visibility is below 5/8 sm; mist (BR) when visibility is 5/8 sm to less than 7 sm; haze (HZ) involves dry particles, not water droplets. Confusing the humidity thresholds is a frequent error.
- Slant-range vs. surface visibility in haze and dust: Reported visibility is surface horizontal visibility. A pilot at altitude looking down through a haze or dust layer will often find the actual slant-range visibility is far worse than the reported value—a runway may be invisible even when the report shows 3+ miles.
- Haze color clue: Haze appears bluish against a dark background and yellowish against a bright one; mist appears only gray. Exams test whether students can distinguish the two by appearance.
- Smoke/haze improvement mechanism: Smoke and haze do NOT clear the way fog does. They require wind dispersion. A thick overcast above can prevent daytime convective mixing, keeping visibility low all day—a common wrong-answer trap that assumes daytime always brings improvement.
- Carbon monoxide in smoke: CO is the primary toxicological threat from smoke; it is colorless, odorless, and acutely dangerous. This is a common oral exam question for pilots flying near wildfire areas.