Stillair

Stillair is a term used to describe air that is essentially at rest or has negligible bulk motion. In everyday language and in scientific contexts, still air refers to environments where air currents are minimal, and wind speeds are close to zero.

In meteorology and indoor settings, still air arises under calm weather conditions, quiet nocturnal cooling, or in enclosed spaces with little ventilation. Temperature inversions or stable atmospheric layers can also contribute to still air, allowing layers of air to resist vertical mixing. In buildings or laboratories, still air can occur when ventilation is limited or when airflow is intentionally reduced for experiments or processes.

The presence of still air affects how substances move and disperse. Transport is dominated by diffusion rather than convection, which can slow the spread of odors, pollutants, or airborne particles. Temperature and humidity gradients may persist longer in still air, influencing comfort, condensation, and microbial growth. Acoustic properties can differ in still air because there is little wind to carry or distort sound waves.

Measurements of still air typically refer to very low wind speeds. Common practical definitions place still air at wind speeds below about 0.5 meters per second (roughly 1 mile per hour), though thresholds can vary by field and context. In meteorology, terms such as calm or near-calm are used to describe similar conditions.

Applications and implications include ventilation design, where designers seek to avoid stagnant pockets of air, and experimental settings that require minimal air disturbance. Understanding still air helps in assessing diffusion-based processes, indoor air quality, and the behavior of scents, heat, and contaminants in calm environments.