Does anyone know a rough ballpark figure for the opacity of the Earth's atmosphere in the visible? Assuming an observer is at sea level looking to the horizon, I calculated the opacity due to Rayleigh scattering in the visible band, as shown in the attachment. I don't really require a high degree of precision, so if anyone knows what kind of ballpark figure it should be, please say! I'm finding it very hard to find anything using Google. My worry is that my values of approximately 0.05 are too low for the opacity. Surely when you look to horizon at sunset, almost 90% of the blue has been scattered since the sky is so red, not a mere 5%. Thanks.