The atmosphere sometimes appears veiled in a milky or grayish layer, a common weather phenomenon known as haze. This condition significantly diminishes the transparency of the air, obscuring distant views and giving the sky a dull appearance. Haze is a product of tiny materials suspended in the air, a physical state that can signal important shifts in air quality.
Defining Atmospheric Haze
Haze is a suspension of extremely small, dry solid or liquid particles, known as aerosols, in the atmosphere. These particles are typically too minute to be individually seen by the naked eye but are numerous enough to collectively scatter and absorb light. The visual effect of this light scattering is a reduction in visibility and a loss of contrast, making objects like mountains or tall buildings appear faded or washed out.
The nature of this light interaction means that shorter wavelengths, such as blue light, are scattered more effectively by the fine particles, which can give the haze a grayish or bluish tint. This light scattering is responsible for the characteristic opalescent appearance of the sky during a haze event.
The Primary Sources of Haze
The particles that constitute haze originate from a diverse range of materials and processes, falling into both natural and human-caused categories. Naturally occurring sources include fine mineral dust lifted by wind from arid regions and volcanic ash injected into the atmosphere during eruptions. Biogenic aerosols, which are compounds released by vegetation like terpenes from trees, also contribute to the natural background haze.
Human activities are a major source of haze, primarily through the combustion of various fuels. Emissions from vehicles, industrial factories, and power generation introduce sulfates, nitrates, and carbonaceous particles into the air. Smoke from agricultural burning and from large-scale wildfires also releases significant amounts of fine particulate matter, which can travel thousands of miles to form haze over distant regions.
Meteorological conditions play a significant role in concentrating these particles, especially temperature inversions. Normally, air temperature decreases with altitude, allowing pollutants to rise and disperse. During an inversion, a layer of warmer air traps cooler air near the surface, preventing vertical air movement. This allows the haze-forming particles to accumulate in high concentrations, causing the haze to become denser and last longer.
Distinguishing Haze from Other Obscurants
Haze is often confused with other atmospheric conditions that reduce visibility, such as fog, mist, and smoke, but the key distinction lies in the composition of the particles. Haze is a phenomenon of dry particles, meaning it forms in air with relatively low to moderate relative humidity. The suspended particles are generally solid or non-water-based liquid aerosols.
Fog and mist are both composed of tiny liquid water droplets suspended in the air. These conditions require the air’s relative humidity to be near 100% for the water vapor to condense around nuclei. The difference between fog and mist is primarily a matter of visibility distance, with fog causing visibility to drop below 1,000 meters and mist allowing visibility to remain at or above that threshold.
While smoke is a source of haze, the two terms are not interchangeable, as smoke refers to the immediate product of combustion. When smoke disperses and the larger particles settle, the remaining fine particles form haze. The particles in haze are typically smaller than the water droplets found in fog or mist.