Why is the sun red? During sunset, Northeastern US, Summer 2021 (Beginner)

In July 2021 around 8 pm when I got off work, my
co-workers and I all saw this red sun during the sunset. I live in the northeast
USA, around the PA/NJ/NY area. It was larger than a normal sun and blood red. I was
wondering what this sun was and what caused it. I'm really curious. Thank you.

Indeed right now (July 2021) the sun appears in a deep red during the sunset and sunrise here in the Northeastern United States. The cause of this is smoke from the wildfires burning on the west coast.

The color of the sun is very striking in person, below are some images from upstate NY

Red sun

Red sun

Satellite imagery from NOAA shows how the smoke of wildfires is spreading over North America

wildfire smoke

credit: NOAA, NASA Goddard Space Flight Center, CIRA

To understand why this is let us consider how sunlight interacts with our atmosphere. As light travels through it gets scattered by small particles in the atmosphere, this happens much stronger for blue light (shorter wavelengths) than for red light (longer wavelengths). This is the reason why the sky has a blue color. Close to sunset and sunrise when the sun is close to the horizon the light has a much longer path through the atmosphere, enhancing this effect. By the time the light reaches us most of the blue light is scattered away, giving us sunsets that are orange/red in color.

But this alone does not cause sunsets where the sun appears blood red like it is the case here. For that, you need aerosols in the sky which enhance the scattering even more. Aerosols are small particles suspended in the air, for example, the smoke released from the wildfires out west. This smoke reaches the upper levels of the atmosphere where high winds can carry it over large distances to the Northeastern US.

However, it is important to note that too much of these aerosols will actually mostly wash out the colors, especially when the pollution gets closer to the ground. There they also carry significant health risks for humans.

The large amount of wildfires in the western United States is enabled by long-standing policies of fire avoidance as well as intensifying droughts and heat waves that are linked to climate change

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About the Author

Lukas Wenzl

Lukas Wenzl

Lukas Wenzl is a graduate student at Cornell. He is working on Observational Cosmology, helping to prepare the next generation of surveys studying the origins and evolution of our universe.

Twitter:  @astrolukaswenzl
Website:  lukaswenzl.com

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