Which environmental factor that influences fire behavior is considered the least variable?

Topography is indeed considered the least variable environmental factor influencing fire behavior. This is because topographic features such as mountains, valleys, slopes, and ridges are largely static and do not change significantly over time. Unlike weather, which can fluctuate daily or even hourly with changes in temperature, precipitation, wind, and humidity, or fuels, which can vary due to seasonal changes, growth patterns, or human activities, the physical characteristics of the land remain relative constant.

An understanding of topography is crucial for predicting fire behavior, as it affects how fire spreads, where it will intensify, and how it will interact with other factors like wind. For example, fires tend to spread faster uphill due to the heat rising and catching more fuels. The stability of topography makes it a reliable predictor in contrast to the more dynamic nature of weather, fuels, and humidity conditions, which can significantly affect fire behavior but are subject to frequent and rapid changes.