There are few situations that can cause a red moon. The most common way to see the Moon turn red is when the Moon is low in the sky, just after moonrise or before it’s about to set below the horizon. Just like the Sun, light from the Moon has to pass through a larger amount of atmosphere when it’s down near the horizon, compared to when it’s overhead.
The Earth’s atmosphere can scatter sunlight, and since moonlight is just scattered sunlight, it can scatter that too. Red light can pass through the atmosphere and not get scattered much, while light at the blue end of the spectrum is more easily scattered. When you see a red moon, you’re seeing the red light that wasn’t scattered, but the blue and green light have been scattered away. That’s why the Moon looks red.
The second reason for a red moon is if there’s some kind of particle in the air. A forest fire or volcanic eruption can fill the air with tiny particles that partially obscure light from the Sun and Moon. Once again, these particles tend to scatter blue and green light away, while permitting red light to pass through more easily. When you see a red moon, high up in the sky, it’s probably because there’s a large amount of dust in the air.
A third – and dramatic – way to get a red moon is during a lunar eclipse. During a lunar eclipse, the Moon passes behind the Earth’s shadow, which darkens it. If you could take a look at the Earth from inside its shadow, you would see that the atmosphere around the edge of the entire planet glows red.
Once again, this is because large amounts of atmosphere will scatter away the blue/green light and let the red light go straight through. During a lunar eclipse, the Moon passes fully into the shadow of the Earth and it’s no longer being illuminated by the Sun; however, this red light passing through the Earth’s atmosphere does reach the Moon, and shines on it.