Haze (calima in Spanish) is a suspension of extremely fine dry particles in the atmosphere that reduces visibility and gives the sky a milky or yellowish appearance. Unlike fog, which consists of water droplets, haze is composed of dust, sand, salt crystals, smoke, or pollution aerosols. Visibility in haze typically ranges from 2–10 km (below 2 km it is classified as thick haze or dust storm).
In Spain and the Canary Islands, the most common haze is Saharan dust calima — plumes of mineral dust transported northward from the Sahara Desert by southerly winds. These events can reduce visibility to under 1 km, deposit a fine layer of orange-brown dust on everything, degrade air quality significantly (PM10 concentrations can spike to 200–500 µg/m³, far above the EU limit of 50 µg/m³), and produce spectacular orange or red sunsets. Severe calima episodes in the Canary Islands can close airports and trigger health warnings.
Saharan dust events typically occur when a depression sits over the western Mediterranean or Morocco, drawing hot, dusty air northward. They are most frequent in spring and summer and can transport dust as far as Scandinavia, the UK, and even the Caribbean. The dust serves as condensation and ice nuclei, potentially enhancing precipitation in some situations. For aviation, haze is significant because it reduces visibility — particularly in forward slant visibility on approach. Satellites track dust plumes in real time, enabling forecasts of calima events days in advance through models like the Barcelona Dust Forecast Center (part of the WMO Sand and Dust Storm Warning system).