In the study of air pollution, dispersion is a crucial factor as it is the movement and direction of where air contaminants are spread. Many atmospheric factors influence the way air pollution is dispersed, including wind direction and wind speed, type of terrain and the effects of heat. In unstable conditions, ground level pollution is readily dispersed thereby reducing ground level concentrations. Elevated emissions, however, such as those released from a chimney, are returned more readily to ground level, leading to higher ground level concentrations.