Fog and haze are atmospheric phenomena that obscure visibility, but they originate from fundamentally different physical processes and consist of distinct airborne materials. Understanding these differences requires examining the specific composition and meteorological conditions that create each obstruction. This clarification separates water-based condensation (fog) from dry particulate matter (haze).
Defining Fog: Water-Based Composition and Formation
Fog is essentially a cloud that forms at or near the Earth’s surface. Its composition is entirely water-based, consisting of a dense visible aerosol of tiny liquid water droplets or, if cold enough, ice crystals suspended in the air. Fog forms when the air cools to its dew point, causing water vapor to condense into liquid water around microscopic particles called condensation nuclei.
Fog formation requires very high relative humidity, typically near 100% saturation. This saturation is often achieved through cooling processes, such as the ground radiating heat away after sunset, which chills the adjacent air layer to the dew point. Meteorologists classify this condition as fog when horizontal visibility is reduced to less than 1,000 meters (one kilometer). Different types of fog, like radiation or advection fog, are categorized based on the specific cooling mechanism that drives the air to saturation.
Defining Haze: Particulate Matter and Sources
Haze is composed of extremely small, dry solid particles or non-aqueous aerosols suspended in the atmosphere. These particles are typically less than 10 micrometers in diameter, often classified as fine particulate matter (PM2.5 or PM10). Unlike fog, haze does not require the air to reach the dew point and can occur in relatively dry air with low to moderate humidity levels.
The sources of these airborne particles are highly varied, stemming from both natural and human activities. Common origins include windblown dust and volcanic ash, as well as anthropogenic sources like smoke from wildfires, vehicle exhausts, and industrial pollution. Gases emitted from combustion, such as sulfur dioxide, can undergo complex chemical reactions to form secondary particles like sulfate aerosols. When these pollutant particles concentrate, they create a visible obscuration that reduces air clarity.
Key Distinctions in Visibility and Color
The fundamental difference lies in composition: fog is a wet, liquid-based suspension, while haze is a dry, solid-based suspension. This compositional difference results in distinct visual characteristics. Fog droplets scatter light uniformly across the visible spectrum due to their larger size, giving the fog a gray or white appearance that evenly obscures objects.
Haze particles are much smaller and scatter light selectively, which often creates a noticeable color tint. Depending on the particle composition and the angle of observation, haze can appear brownish, yellowish, or bluish. The way light is scattered by haze particles often makes distant objects appear less distinct and can enhance the brightness surrounding light sources (forward scattering).
While fog is defined by visibility falling below one kilometer, haze can reduce air clarity even when visibility remains greater than two kilometers. Fog requires the difference between air temperature and the dew point to be very small, indicating near-total saturation. Haze can form when the air temperature is significantly warmer than the dew point. Although haze does not require high moisture, its severity is amplified when humidity is high, as the dry particles absorb water and swell, increasing their light-scattering ability.