Why does the Sun turn red during wildfires? The science behind the eerie skies over Spain

Wildfires burning across Spain are doing more than scorching thousands of acres and stretching emergency crews to their limits. They are also transforming the sky itself.

In recent days, people across parts of the country have noticed the Sun taking on a deep red or orange hue, particularly in areas closest to the active fires. While the sight can look almost apocalyptic, the explanation lies in a well-understood phenomenon known as Mie scattering, which changes the way sunlight reaches our eyes.

Why does the Sun turn red during wildfires?

The effect occurs when sunlight passes through an atmosphere packed with large particles, such as the smoke produced by forest fires. Unlike the tiny gas molecules that usually fill the air, smoke particles scatter light in a different way, allowing the longer red and orange wavelengths to dominate.

As a result, the Sun loses its familiar bright yellow appearance and instead looks darker, deeper and much redder.

The phenomenon is named after German physicist Gustav Mie, who described it in the early 20th century. His theory explains that particles similar in size to the wavelength of visible light alter the path of the Sun’s rays. Dense wildfire smoke contains huge numbers of these particles, filtering the sunlight before it reaches the ground.

The same effect can also be seen during Saharan dust events or after major volcanic eruptions, when ash and other particles fill the atmosphere.

Why the sky changes color too

Mie scattering is only part of the story. Another process, known as Rayleigh scattering, is responsible for making the sky appear blue under normal conditions.

Small molecules in the atmosphere scatter blue light much more efficiently than red light, giving the daytime sky its familiar color. However, when the Sun sits low on the horizon at sunrise or sunset, its light has to travel through a much thicker layer of atmosphere. Much of the blue light is scattered away before reaching our eyes, allowing reds and oranges to become more prominent.

When wildfire smoke is added to the mix, the effect becomes even stronger. The Sun can appear blood red even well before sunset, while the sky often takes on gray, yellow or hazy orange tones.

Although the view can be spectacular, it is also a visible reminder of the high concentration of pollution particles released into the atmosphere by major wildfires.

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