Many times people have used the term “blue smoke” to refer to cannabis smoke, as if something strange was in everybody’s pot that colors its smoke blue. Someone who has never seen cannabis smoke might even think that it’s literally the color blue, but is it really?

The short is yes; cannabis smoke indeed has a blue tint to it, but the story is a little more complex than that, and it turns out cannabis isn’t the only herb that burns “blue.” That’s right; cigarettes also emit “blue” smoke from the end that burns, aka sidestream smoke.

In reality, none of the smoke emitted by either a joint or a cigarette actually has a blue color on its own. The napkin trick (the classic trick people employ to show tobacco smokers how nasty the smoke is) will tell you exactly what color your smoke is. Take a hit, and exhale through a white napkin, paper towel or piece of toilet paper. Cigarette smoke immediately turns the cloth brown; on the other hand, cannabis smoke will turn the napkin yellow only after repeated exhales on the same spot.

If the napkin trick tells us that cannabis smoke is yellowish-brownish (similar to the color of hash oil), why does it look blue? Interestingly enough, cannabis smoke, opal and the sky are all blue for pretty much the same reason: Rayleigh scattering.

Rayleigh scattering is a physical phenomenon by which small particles scatter blue wavelengths of light. As sunlight (which is composed of many photons of different wavelengths) travels into the atmosphere, the molecules that compose the atmosphere interact predominantly with photons of smaller wavelengths, the blue ones. As light with longer wavelengths travel straight through the atmosphere unheeded, the blue photons get bounced around in all directions. Since the blue light apparently originates from the atmosphere, the sky gets a blue color, though the gas the atmosphere is composed actually has no color on its own.

In the sidestream smoke coming off a joint or a bowl, a small portion of the very, very small particulates that make it up scatter blue light, but it is mostly opaque. You’ll notice that the cloud of smoke you exhale does not have this blue tint and is simply an opaque whitish like you’d expect of normal smoke, why? Interestingly enough, exhale smoke is white for the same reason the clouds are white: Mie scattering.

Clouds are aggregations of microscopic droplets of water. Unlike atmospheric gases and weed smoke, these droplets of water scatter all wavelengths of light equally due to their increased size as compared to nitrogen gas, or the small molecules in sidestream smoke. As sunlight hits the surface of a cloud, each droplet points the light equally in all directions and gives the cloud a white color, even if you’re in a plane flying straight though it.

Unlike the smoke coming off the tip of a joint, the smoke you inhale is incompletely combusted and is therefore composed of larger particulates. After you hold in the hit and exhale, the particulates in the cloud of smoke are large enough they scatter all wavelengths equally, just like the clouds in the sky. The particulates that compose cannabis smoke may be smaller than those in cigarette smoke, and might explain the slight difference in color between the two, either way, blue is definitely a relative term.