Weather
In winter the air can become loaded with the products of incomplete combustion such as particulate matter (PM), volatile organic compounds (VOCs), carbon monoxide (CO) and nitrogen oxides (NOx). The severity of the resulting winter smog depends on the degree of atmospheric dispersion. The lower the level of atmospheric dispersion, the higher the level of winter smog. Atmospheric dispersion is mainly determined by wind speed and mixing height.Temperature inversion
1. Wind speed pushes and disperses the pollutants horizontally. No wind means stagnant air and allows levels of pollutants to build up in the air (smog).
2. The mixing height refers to the maximum height the pollutants can reach if dispersed vertically. In normal situations, the mixing height is enough to disperse the pollutants high into the atmosphere. The pollutants are carried up by the layer of warm rising air to the colder air higher up (see diagram).
In the case of temperature inversion, the pollutants are trapped at ground level where it causes most harm. This inversion occurs for example when ahead of a warm front or in broad surface ridge. Cold air becomes trapped under the layer of warm air that acts as a lid. The pollutants in the cooler layer cannot be dispersed and the pollutants stay concentrated at ground level (see diagram).