Smoke is a complex aerosol, a suspension of fine solid particles and liquid droplets within a gas. Cannabis smoke is composed of a mixture of gases and particulate matter, including Total Particulate Matter (TPM) and various volatile organic compounds. The movement of this combustion byproduct is governed entirely by the laws of fluid dynamics. Whether this aerosol rises, falls, or hovers depends on a balance of forces, primarily thermal energy and external air currents.
The Physics of Initial Ascent
The initial upward movement of smoke is a direct result of thermal buoyancy, which dictates fluid movement based on density differences. Burning plant material releases heat, causing the smoke’s gases and particles to become significantly hotter than the surrounding ambient air. Heated air molecules expand, making the hot smoke less dense than the cooler air mass around it.
This temperature differential creates a buoyant force that drives the less-dense smoke upward in a concentrated stream known as a thermal plume. The plume rises because the surrounding, heavier air sinks and pushes the lighter, warmer smoke out of the way, a process known as convection. This heat-driven ascent is why the smoke column appears to shoot straight up immediately above the source of combustion.
Neutral Buoyancy and Rapid Cooling
The initial rise of the smoke plume is short-lived because the smoke rapidly cools as it mixes with the surrounding air. As the hot plume moves upward, it pulls in cooler air from the environment, causing the temperature difference that drives buoyancy to diminish quickly. This loss of thermal energy slows the upward momentum until the smoke reaches a state called neutral buoyancy.
Neutral buoyancy occurs when the density of the smoke aerosol becomes roughly equal to the density of the air around it. At this point, the primary upward force disappears, and the smoke stops rising or begins to spread horizontally. The smoke particles, still subject to gravity, then begin a slower process of diffusion and sedimentation, which causes the smoke cloud to eventually hover or settle.
Environmental Factors Dictating Trajectory
Once thermal buoyancy is lost, the smoke’s trajectory is primarily dictated by external forces, which often overpower any residual tendency to rise. The most significant factor is mechanical or natural airflow, such as wind, drafts, or ventilation systems. Air turbulence causes the rapid mixing and dispersal of the smoke plume, pushing it along the path of the prevailing air current.
Indoors, smoke movement is controlled by air pressure differences created by heating, ventilation, and air conditioning (HVAC) systems or open windows and doors. The phenomenon of smoke seemingly blowing toward a person is a demonstration of fluid dynamics. The person’s body blocks the airflow, creating a low-pressure zone that draws in air and the smoke from the opposite direction. Subtle factors like humidity and atmospheric stability also influence how far the smoke travels before it fully disperses.