In August 1883, a massive volcanic eruption occurred at Krakatoa in Indonesia. This was one of the most violent natural disasters in recorded history.
This catastrophic event destroyed two-thirds of the original volcanic island and triggered huge tsunamis that devastated nearby coastlines. The violent explosion resulted in the tragic loss of more than 36,000 lives and produced a shock wave that circled the earth seven times.
The blast expelled an estimated 21 to 25 cubic kilometers of rock, ash, and volcanic gases high into the atmosphere. While heavy fragments fell nearby, the finest dust and gas rose into the stratosphere and spread slowly across the globe. These volcanic materials remained suspended in the upper atmosphere for several years, blocking out sunlight and cooling the entire planet.
Shortly after this immense eruption, observers from widely scattered locations began reporting highly unusual colors in the twilight sky. In some tropical regions, the sun appeared as a glowing blue disc when it was near the zenith. Other witnesses described how the sunsets took on an eerie, opalescent green hue that seemed to prolong the evening daylight.
Terrified and amazed by these sights, people sent thousands of letters and postcards to scientific societies to describe the strange colors. Artists also tried to capture these inflamed evening skies in vivid watercolor paintings. These bizarre sights became a global topic of conversation and remained visible for up to three years after the disaster.
Why the Sun Turned Blue and Green
Scientists eventually solved this mystery by studying how light interacts with tiny volcanic particles floating in the sky. During the eruption climax, the volcano thrust massive amounts of sulfur dioxide gas high into the stratosphere. This sulfur gas combined with atmospheric water vapor to form a vast layer of tiny sulfuric acid droplets called aerosols.
For the sun to appear blue or green, these sulfuric acid aerosols had to meet very strict physical requirements. Computer simulations show that the aerosol particles needed a specific median radius of about 500 to 700 nanometers. Additionally, these particles had to have a very narrow size distribution to produce the rare effect known as anomalous scattering.
Under these unique conditions, the particles scattered away the longer wavelengths of red and orange light while letting shorter blue and green wavelengths pass through. This process is different from normal atmospheric scattering, which usually filters out blue light and makes sunsets look orange or red. This rare filtering of red light caused the sun and moon to appear blue, while turning the surrounding sky green.
Climate Effects
Besides creating beautiful colors, the stratospheric aerosol layer had a significant cooling effect on the global climate. By blocking about one percent of incoming sunlight, the volcanic veil cooled the planet by up to 0.5 degrees Celsius for about five years. This cooling even affected the oceans, causing sea levels to drop and not fully recover until the middle of the twentieth century.
These global sky phenomena also helped scientists make major breakthroughs in understanding the atmosphere. By tracking the path of the volcanic dust cloud, meteorologists gained their very first proof of a worldwide system of high-altitude winds. The Royal Society of London set up a special committee to gather these valuable reports from observers all over the world.
The legacy of the 1883 eruption continues to teach researchers how volcanic particles affect the earth's climate and weather patterns. While normal volcanic sunsets are red, the rare green and blue twilights remain a fascinating example of natural physics.
