Dry haze is a reduction in visibility caused by solid mineral dust particles suspended in the atmosphere, without significant moisture. It differs from mist (visibility reduced by microscopic water droplets) and from fog (visibility < 1 km due to saturation). Meteorologically, dry haze typically reduces visibility to 2-8 km, although severe episodes can drop it below 1 km.
Origin and impact in Spain
In Spain and the western Mediterranean, the main source is the Sahara Desert. Saharan dust intrusions arrive when a surface depression or an upper-level DANA generates a southerly flow that carries dust-laden air across the Mediterranean. The most intense episodes raise PM10 concentrations above 100-500 µg/m³ (the EU limit is 50 µg/m³).
The Canary Islands are the most affected region, with dozens of episodes annually (locally called "calima"), some severe enough to close airports. On the Peninsula, peak months are February-April and October-November. The sky takes on a whitish, yellowish, or orange tone, and direct solar radiation decreases while diffuse radiation increases.
Health effects are significant: increased hospital admissions for asthma, COPD, and cardiovascular problems; eye and respiratory irritation; and transport of allergens and microorganisms. When haze coincides with rainfall, mud rain occurs. See also: sirocco, sandstorm.