The behavior of air and heat described in the question is related to convection, not the laws of thermodynamics. The laws of thermodynamics primarily deal with energy transfer and the behavior of systems as a whole. Convection, on the other hand, refers to the movement of fluids, such as air, due to density differences caused by temperature variations.
When it comes to containing the smell of weed or any other odor, placing a towel under the door can be somewhat effective, although it may not completely eliminate the smell. The idea behind using a towel is to create a barrier that restricts the flow of air between rooms, including the passage of odor molecules.
While it is true that warm air tends to rise and cold air sinks due to density differences, air movement in a room is influenced by various factors such as temperature differentials, airflow patterns, and ventilation systems. It's important to note that the behavior of air and heat can be complex, and the specific conditions of a given environment can affect their movement.
By placing a towel under the door, you are attempting to block or impede the movement of air and the odor molecules it carries. This can reduce the diffusion of odors into other areas of the house, but it may not be completely effective in all cases.
To further minimize the spread of odors, additional measures such as using air fresheners, maintaining good ventilation, or employing specific odor control methods can be considered.