Questions: Is it possible to see farts?
Short answer: Yes.
Long answer: A few years ago, a viral video purported to show farts captured on infrared security cameras in China. This turned out to be a hoax. But do farts leave a visible trace?
Farts are made up of many gasses, most of which are simply the constituents of air that has been swallowed and expelled. Since the air contains water vapor, this will be expelled along with the rest, and it is of course possible to see water vapor on cold days. The cloud that appears when people exhale is a result of water molecules leaving the warm environment of the body and condensing upon entry into the cold air.
If water vapor is present in farts, then they should similarly be visible on cold days. Whether this is empirically observed is difficult to say, as we were unable to find any relevant scientific literature, and the internet is uncharacteristically silent on the topic. The few videos that purport to capture farts on cold days all struck as suspect.
Before we set out to answer the question definitively, we turned to our database of farts to find out whether farts actually contain water vapor. The figure below depicts the concentration of fart smell (VOCs) at different depths relative to an example fart.
Longtime readers of this blog will recall that our experimental protocol required subjects to remain seated upon the collection device for a period of time after the release of each fart, in order to ensure proper measurement of the relevant variables. Thus in the example we see that the smell near the top of the tube (1.0”, blue line) declines steadily over time. At the same time, the VOC concentration increases near the bottom of the collection tube (7.0”, yellow line). This is due to the velocity of the fart and the fact that hydrogen sulfide is denser than air.
In contrast, the humidity within the tube increases over time:
In particular, the humidity accumulates steadily near the top of the tube, while there is relatively little change close to the bottom. This is consistent with the fact that water vapor rises, as it is less dense than air.
These large increases in humidity indicate that farts contain a substantial amount of water vapor, which should in principle be visible when the air is cold enough. To be more precise about this, we need to determine the temperature at which condensation occurs – this is when the air can no longer hold the water vapor present in farts, or equivalently when the relative humidity reaches 100%. The temperature required for a given concentration of water to reach 100% relative humidity is called the dew point.
A handy way to calculate the dew point is with the simple approximation:
Here T is the ambient temperature, and RH is relative humidity. If we take the values of T and RH during the 10-second period following an average fart, we obtain the following estimates of the dew point at different distances from the subject:
These results suggest that a fart should be visible at a distance of a few inches from the source, when the temperature is about 8 deg. Celsius (about 46 deg. Fahrenheit), which is in fact similar to the conditions under which exhaled breath becomes visible. Nevertheless, our attempts to detect farts by eye have thus far been unsuccessful, even when the air temperature is cold. In this regard, visible farts are like the Higgs boson, predicted by theory but experimentally elusive. To be continued.