One could theorize that the poop particles expelled with the fart are better trapped in the moisture in the air, so as you breathe in the very humid air, and consequently your own poop particles, the sensory elements in your nose and sinus have an easier time detecting them. This is partly owing to how the moisture also makes them more likely to stick to surfaces inside your body as they travel your airways on the journey to your lungs, where your poop particles may end up. Hope this helps.