Have you noticed purple sunrises or sunsets where you live? Submit your photographs to GLOBE or GLOBE Observer, especially as we get ready to start the Fall Cloud Challenge: What's Up in Your Sky (Oct 15 - Nov 15, 2019)!
People around the world have been noticing purple skies at dawn and at dusk, calling it the Raikoke Sunsets (see photographs submitted). The purple color is because of sulfurous gas that was launched into the stratosphere when the Raikoke volcano, located in Russia's Kirul Islands, erupted on June 22nd. Fine volcanic aerosols in the stratosphere, formed from this gas, scatter blue light, and when mixed with the orange colors you expect at sunrise or sunset, you get that purple hue.
If the volcano erupted in June, why is this plume still around? When particles like those from the Raikoke volcano go into the stratosphere, they can stay in the atmosphere for a long time causing things like purple sunsets.
Our atmosphere has many layers. Starting from the surface of the Earth, the layers of Earth's atmosphere are troposphere, stratosphere, mesosphere, and thermosphere. If you look closely, the temperature in these layers (red line in the graph) either gets colder or hotter as you move higher in altitude. Notice that in between these drastic changes in temperature, are sections where the temperature doesn't change at all with altitude. These sections are named by the layer below it with the word "pause" because it is a pause in the change of temperature with height (example tropopause). Those "pauses" keep things in that layer. Think of it as a lid that prevents things from one layer to easily move into the other layer.
Rainfall can "wash" all types of aerosols, including volcanic aerosols, but rain only happens in the troposphere, the layer of the Earth where all the weather happens. If these aerosols are lofted into the stratosphere, there is no process, like rainfall, to "wash" them out of that layer of the atmosphere.
Very strong volcanic eruptions or even thunderstorms, can break through the tropopause. That is what happened with the Raikoke volcano. Because layers don't mix together, those volcanic aerosols are still in the stratosphere causing purple sunsets and sunrises as the plume goes around the world.
The Fall Cloud Challenge is all about what is up in your sky! If you see purple sunrises or sunsets, send in your photographs and observations to GLOBE or GLOBE Observer app! You can also read and do activities associated with “What’s Up in the Atmosphere? Exploring Colors in the Sky” GLOBE Elementary book (free print and eBook) to learn more about the different colors in the sky.
Blog post adapted from Raikoke Sunrise: https://spaceweather.com/