What is black rain? How oil fires, smoke, and atmospheric pollution can turn rainfall dark and potentially harmful

Recently, the rare scientific phenomenon received global attention after reports from Tehran in March 2026 indicated that residents observed ‘dark rain’ following airstrikes on oil storage facilities. This led to massive fires that released thick smoke into the atmosphere. When rain clouds formed over the city, the raindrops fell into the clouds, thus forming dark rain. This was witnessed as black droplets fell over the city.At the same time, residents reported headaches and breathing difficulties. Emergency officials in Iran indicated that the rain that fell after the fires could be dangerous or even acidic. Know the science behind black rain, its causes, environmental effects, and more.Black rain is an uncommon atmospheric phenomenon in which rain appears dark in colour due to large amounts of suspended matter, such as soot, ash, dust, or oil droplets. From a scientific perspective, black rain is rain that becomes contaminated during its passage through the atmosphere, particularly in highly contaminated atmospheric layers. The rain that falls from the atmosphere contains dark, often black, grey, or oily droplets.In early March 2026, various oil installations in Tehran were targeted during military operations. This caused massive fires that led to the formation of enormous clouds of smoke. However, with the formation of rain clouds in the region, the water droplets were able to collect the particles in the atmosphere as they fell. It was observed that the rain had a dark appearance, with oily marks being left behind. Environmental experts pointed out that contaminated rain is usually caused by extremely high levels of smoke in the atmosphere.Rain formation takes place when water vapour condenses around small particles called condensation nuclei. These particles can be dust, salt crystals, pollen, and pollutants. In normal circumstances, the quantity of material that the raindrops carry is negligible. However, in cases when the quantity of ash in the air is unusually high, the raindrops carry enormous quantities of the material as they fall through the air.This phenomenon is called atmospheric scavenging. In cases when the quantity of material in the air is exceptionally high, the rainwater will appear coloured. Such rain may look black, brown, or even reddish depending on the type of particles present in the atmosphere.Environmental events can lead to black rain:Factories, refineries, and coal-burning plants can release soot and carbon particles into the atmosphere. During rainfall, these pollutants may mix with water droplets and darken the rainwater.Large wildfires generate enormous smoke plumes filled with ash and burnt organic material. Rain falling through these smoke layers may appear grey or black due to the captured particles.Explosive eruptions release ash clouds composed of fine volcanic particles. Rain that passes through these ash clouds may turn muddy or dark.When petroleum burns, thick black smoke is produced, consisting of hydrocarbons and carbon particles. When rain falls in these conditions, the rain may absorb the oil residues and the soot, resulting in black or oily rain, as in the recent case in Tehran.Black rain may cause several environmental and health problems.Environmental problemsHuman health problemsBlack rain is a rare occurrence; it has been documented in the history of several major events. The most famous incident was the nuclear attack on Hiroshima. In this attack, a nuclear bomb was dropped that threw ash, debris, and radioactive material into the air. When clouds formed, the material mixed with the rain that fell from the clouds.Such a situation was also witnessed in the occurrence of massive pollution, wildfires, and burning oil fields.