The term spot fire describes a fire ignited outside of a burn unit as a result of a windborne ember (sometimes referred to as a firebrand). Prescribed fire practitioners should be diligent to watch for spot fires and extinguish them quickly before they reach unmanageable sizes. Understanding when and where spot fires are likely to occur is a useful skill when conducting prescribed burns. Sometimes this can be a costly learning curve. Relative humidity can be used to predict the likelihood of spot fires. For example, as relative humidity drops below 40% the probability of spot fires increases. As relative humidity approaches 25%, spot fires become likely. For more information about spot fires when planning prescribed fires see the following publication: Fire Prescriptions for Maintenance and Restoration of Native Plant Communities (.pdf, 1 MB).