Yellow Rain: Natural dust, scientific truth, and diplomatic tool

Yellow rain coats European cities every February and March, tinting cars and sidewalks with a golden hue.

This striking phenomenon occurs when Saharan dust, carried by wind, travels thousands of kilometres and is washed down by rain.

I learnt a lot about the yellow rain phenomenon through coffee chats at the World Meteorological Organisation (WMO), where my office is located. In 2021, when 180,000 tonnes of dust swept across Europe, a webinar with Dr Slobodan Nickovic, creator of the main ‘scientific dust model’, deepened my understanding of this interplay between nature, science, and diplomacy, leading to the following reflections. 

About yellow rain

Yellow rain consists of dust particles originating from the Sahara Desert, which releases between 60 and 200 million tonnes of fine particles annually.

These particles, including iron-rich minerals, are loose and dry, easily lifting them into the air. The iron content often gives the dust a yellowish or reddish hue, then colouring the rain.

Powerful winds, such as the Harmattan winds in West Africa, lift particles from the ground, often reaching altitudes of several kilometres.

Once airborne at higher altitudes, particles of dust and sand face less resistance to surface friction, which allows them to travel thousands of kilometres. This long-distance movement is why Saharan dust can reach places as far as the United Kingdom or even the Caribbean, though Europe is a common destination.

When the dust-laden air mass reaches Europe, it often encounters weather systems with rain. The raindrops capture the dust particles in a process called wet deposition. As the rain falls, it washes the dust out of the atmosphere and brings it down to the ground. The dust mixes with the rainwater, giving it a yellowish or muddy appearance—hence the term “yellow rain.” This is why you might notice a yellowish tint on cars, windows, or sidewalks after such an event.

Yellow rain is more common in spring, especially in March when the Harmattan winds are strong, favoured by dust transport to Europe.

Geopolitics and yellow rain

Yellow rain has an interesting geopolitical history. In the late 1970s, a sticky yellow substance fell from the skies in Southeast Asia, alarming the Hmong people and sparking fears of chemical warfare. The U.S. accused the Soviet Union of deploying a fungal toxin weapon against the Hmong, who had allied with the United States during the Vietnam War. This claim fueled Cold War tensions and diplomatic disputes.

Harvard biologist Matt Meselson’s 1983 investigation overturned these accusations. His analysis showed the “yellow rain” was likely bee feces—hollow pollen grains from giant Asian honey bees—not a bioweapon. Later studies confirmed bees swarm and defecate en masse on hot days, explaining the phenomenon. This science debunked a geopolitical narrative and exposed flaws in U.S. claims.

It remains as a potent historical example of an interplay between science and geopolitics.

Multilateral cooperation around yellow rain

No single nation can deal with the impact of yellow rain alone, as the movement of dust does not observe borders. Thus, countries under the auspices of the WMO established an international cooperation network to study dust composition, transport mechanisms, and ecological impacts.

For example, poor air quality during dust events can exacerbate respiratory illnesses, necessitating public health measures like advisories and air filtration systems. Ecologically, nutrient-driven algal blooms can affect marine ecosystems, impacting fisheries and communities dependent on them.

Dust as a light absorber particle in high latitudes (e.g. Arctic and Antarctic) and altitudes (e.g. Alpes) causes melt of snow and glaciers and thus significantly affects global climate. This is why IPCC, for the first time, recognizes dust aerosols as a short-lived climate forcer.

International organisations play a pivotal role in bridging national efforts to deal with the yellow rain phenomenon. The United Nations’ Sand and Dust Storms Coalition, launched in 2019, unites 19 members, including UN agencies and non-UN entities like the World Bank, to tackle sand and dust storms. This coalition supports joint strategies, such as improving forecasting models and implementing mitigation measures, and promotes innovation exchange, enhancing global resilience.

The most advanced centre for scientific cooperation on yellow rain is the WMO Barcelona Dust Regional Center, operated under the WMO’s Sand and Dust Storms Warning Advisory and Assessment System (SDS-WAS), which exemplifies this. 

Established in 2014, it integrates satellite data, atmospheric models, and ground observations from multiple countries to generate forecasts, enabling nations to anticipate and mitigate dust storms. This center’s work relies on data sharing, highlighting how multilateral efforts enhance predictive capabilities.

Scientific collaboration extends to studying dust composition, transport mechanisms, and ecological impacts. For instance, research has shown Saharan dust can carry microorganisms, potentially influencing biodiversity in distant regions. This finding underscores the need for a global scientific community to pool resources and expertise. This cooperative approach is crucial for unravelling complex environmental interactions—reinforcing the practical necessity of multilateralism.

Conclusion

The yellow rain phenomenon, historically and currently, can teach us a few lessons about science, multilateralism, and diplomacy:

Science provides rigorous evidence to correct misinformation, proving the power of data in dealing with natural phenomena. 

At a time when geopolitics gains new momentum, yellow rain examples show phenomena that are not and should not be part of geopolitical battles and tensions. 

Instead of geopolitics, international cooperation should prevail whenever countries have a common and shared interest in working together, as with the yellow rain phenomenon. 

Diplomacy is a tool of cooperation conducted in multilateral fora, such as the World Meteorological Organization where scientific evidence can help countries to nurture agreements around shared concerns and interests. 

The yellow rain saga shows science isn’t just a tool—it’s important for cutting through ideology to reveal reality and, ultimately, diffuse geopolitical tensions.