SGM Journals

Research explained · Atmospheric science

What makes rain turn red?

Rain can look red when it carries enough colored particles. In southern Spain, researchers traced red rain deposits to Saharan mineral dust. In a sample from Kerala, researchers cultured an alga from the colored rainwater. The color alone cannot tell you which material is present. [1][2]

See original studies View the SGM research record

How scientists tell the two apart

Southern Iberia, February 2017
Researchers analyzed the deposited minerals and compared them with satellite images and modeled air-mass trajectories.
The deposits contained Saharan dust with several iron-bearing minerals, including hematite and goethite. [1]
Changanasserry, July 2001
Researchers examined particles in rainwater and grew material from the sample in an algal culture medium.
The cultured alga belonged to Trentepohlia. A separate DNA staining study later addressed whether red rain cells contained DNA. [2][3]

Why can mineral dust leave a red deposit?

Rain can carry suspended dust to the ground. In the 2017 Iberian event, researchers found mixtures of clay and iron-bearing particles, including iron oxides. As water evaporates from a surface, the solid particles can remain as a visible reddish or brown residue. [1]

Do repeated red rains always come from the same place?

An 11-year study in northeastern Spain recorded 38 red rain events. Just two accounted for 62% of the dust deposited in those events. The researchers traced air masses back to multiple African source regions. Even at one monitoring site, events can differ substantially in how much dust they deposit and where it comes from. [4]

What evidence should you look for in a report?

Start with what was collected. A mineral analysis of the residue tests what the solid particles are made of. A culture can identify organisms that grow from a sample. Satellite imagery and wind trajectories help trace airborne material. A strong explanation connects the sample to the proposed source. [1][2][4]

Original research

  1. Rodriguez-Navarro, di Lorenzo, and Elert (2018), Atmospheric Chemistry and Physics. Mineralogy and physicochemical features of Saharan dust wet deposited in the Iberian Peninsula during an extreme red rain event. Compares the mineral evidence with satellite observations and modeled air-mass trajectories.
  2. Kumar et al. (2002), Eos. Colored rain falls in Kerala, India. Reports the Changanasserry rainwater culture and identification.
  3. Gangappa and Hogg (2013), Microbiology. DNA unmasked in the red rain cells of Kerala. Figure 5 compares the fluorescent signal in treated and untreated cells.
  4. Avila, Queralt-Mitjans, and Alarcón (1997), Journal of Geophysical Research. Mineralogical composition of African dust delivered by red rains over northeastern Spain. Tracks 38 red rain events and examines their deposits and likely source regions.