Question

does passing gas spread quicker in hot air or cold air

Answer 1

Short answer: generally hot air — a fart will disperse faster in warmer air.

Why:
- Molecular diffusion is faster at higher temperature (gas molecules move faster), so the gas spreads out more quickly on a microscopic scale.
- Buoyancy and convection: warm air is less dense and tends to rise, producing vertical motion that mixes the expelled gas into the surrounding air more quickly. In warm rooms, convective currents and turbulence (from heating, people moving, fans, etc.) also increase mixing.
- Cold, still air tends to be more stable and can trap odors near the source (temperature inversions near the ground make this worse).

Caveats:
- Wind and turbulence are usually far more important than temperature. A breezy cold day will disperse odors much faster than a calm warm room.
- In very confined spaces, ventilation patterns matter more than absolute temperature.

If you want to test it yourself, light an incense stick (easy visible plume) in two rooms at different temperatures and observe how quickly the smoke disperses.