Mud rain is a meteorological phenomenon in which precipitation carries suspended mineral dust — usually of Saharan origin — depositing it on surfaces as the water evaporates or runs off. The visible result is a thin layer of fine mud of reddish, orange, or yellowish colour on vehicles, buildings, vegetation, and any exposed surface.
Formation mechanism
It occurs when two conditions converge: an air mass loaded with haze (suspended dust, typically of Saharan origin) and a precipitation system (front, DANA, or local convection). Raindrops act as "sweepers" of the dust suspended in the atmospheric column through a process called wet deposition.
In Spain it is a relatively common phenomenon, especially in the southern half and the Mediterranean, averaging 5-15 episodes per year. The most frequent months are February to June and October to November, when Saharan air intrusions toward the Peninsula are most common. In intense episodes, the particle concentration in the rain can give the water a milky or directly "muddy" appearance.
Although visually striking and annoying (cars end up covered in dust), mud rain has a positive ecological effect: Saharan dust delivers mineral nutrients (iron, phosphorus) to the soil, fertilising forests and ecosystems. However, fine particles (PM10, PM2.5) degrade air quality and affect people with asthma or allergies. See also: sirocco, sandstorm.