The presence of mountains-or even hills-in the path of an airmass can have important effects on the characteristics of the air. Temperature, humidity, wind speed, and wind direction can all vary greatly across complex ter­rain, and the variation of any of these factors will directly affect fire behavior. Indirect effects, such as through the modification of fuel moisture content, will also result. In this article, we discuss aspects of terrain modification of airflows that are relevant to wildland fire man­agement.