An aerosol in the atmosphere of sufficient concentration and extent to decrease visibility significantly when the relative humidity is below saturation is known as haze. Haze may contain dry particles or droplets or a mixture of both, depending on the precise value of the humidity. In the use of the word, there is a connotation of some degree of permanence. For example, a dust storm is not a haze, but the coarse particles may settle rapidly and leave a haze behind once the velocity drops.
Human activity is responsible for many hazes. Enhanced emission of sulfur dioxide results in the formation of aerosols of sulfuric acid . In the presence of ammonia, which is excreted by most higher animals including humans, such emissions result in aerosols of ammonium sulfate and bisulfate. Organic hazes are part of photochemical smog , such as the smog often associated with Los Angeles, and they consist primarily of polyfunctional, highly oxygenated compounds with at least five carbon atoms. Such hazes can also form if air with an enhanced nitrogen oxide content meets air containing the natural terpenes emitted by vegetation.
All hazes, however, are not products of human activity. Natural hazes can result from forest fires, dust storms, and the natural processes that convert gaseous contaminants into particles for subsequent removal by precipitation or deposition to the surface or to vegetation. Still other hazes are of mixed origin, as noted above, and an event such as a dust storm can be enhanced by human-caused devegetation of soil .
Though it may contain particles injurious to health, haze is not of itself a health hazard. It can have a significant economic impact, however, when tourists cannot see scenic views, or if it becomes sufficiently dense to inhibit aircraft operations.
[James P. Lodge Jr. ]
Malm, W. C. "Characteristics and Origins of Haze in the Continental United States." Earth-Science Reviews 33 (August 1992): 1–36.
Raloff, J. "Haze May Confound Effects of Ozone Loss." Science News 141 (4 January 1992): 5.
haze1 / hāz/ • n. a slight obscuration of the lower atmosphere, typically caused by fine suspended particles. ∎ a tenuous cloud of something such as vapor or smoke in the air: a faint haze of steam. ∎ [in sing.] fig. a state of mental obscurity or confusion: through an alcoholic haze. haze2 • v. 1. [tr.] force (a new or potential recruit to the military, a college fraternity, etc.) to perform strenuous, humiliating, or dangerous tasks: rookies were mercilessly hazed. 2. [tr.] drive (cattle) in a specified direction while on horseback.