boira

Boira is the Catalan term for fog, a meteorological phenomenon in which a cloud of tiny water droplets forms near the Earth's surface, reducing horizontal visibility. In meteorology, fog is typically defined as visibility of less than one kilometer (about 0.62 miles). Boira occurs when moist air near the ground cools or is cooled by contact with a cold surface, causing water vapor to condense into droplets that remain suspended in the air.

Boira arises through several mechanisms. Radiational fog forms on clear nights as the ground loses heat, cooling the air above it. Advection fog occurs when moist air moves over a cooler surface and loses enough heat to condense. Steam fog (also known as evaporation fog) happens when relatively warm air moves over a colder body of water, causing water vapor to condense. Upslope fog develops as moist air is forced to rise along terrain, cooling and condensing as it ascends. The prevalence and type of boira depend on local humidity, temperature, wind, and landscape.

Boira reduces visibility and can affect transportation, including driving, shipping, and aviation. It is common in winter in valleys, near rivers, and in other humid, low-lying areas within Catalan-speaking regions, though it can occur elsewhere and in different seasons. In weather forecasts and everyday speech, boira is described in similar terms to fog, with local conditions and expected dissipation often tied to sunrise, temperature changes, or wind shifts.