Aspect is the compass direction a slope faces, and it decides how much solar radiation the ground receives. In the northern hemisphere south and southwest-facing slopes take the most sun, run hotter and drier, carry lighter and more cured fuels, and reach their fuel moisture minimum in mid-afternoon; north-facing slopes stay cooler and moister and often carry denser, heavier fuels. The hemispheres mirror. Elevation adds a second axis: temperature falls with height, so higher ground is generally cooler and moister, and it also greens up and cures later in the season, shifting the whole fire calendar. The two combine into large differences over a few hundred metres, which is why a fire can be running on one side of a ridge and creeping on the other. The confusion this resolves is treating a forecast for a location as describing the fuels there; the forecast describes the air, while aspect and elevation redistribute the drying. After this Concept you can predict where on a piece of terrain fuels will be driest at a given hour.
In wildland fire behavior, aspect (also called exposure) is the compass direction a slope face is oriented toward, and it governs how much solar radiation that slope receives and when, which in turn …