Weather conditions and fuel conditions are large factors in the decisions made on a fire. Within the U.S., the Energy Release Component (ERC) is a scale relating fuel energy potential to area. The Burning Index (BI) relates flame length to fire spread speed and temperature. The Haines Index (HI) tracks stability and humidity of air over a fire. The Keetch-Byram Drought Index relates fuels to how quickly they could ignite and to what percentage they should burn. The Lightning Activity Level (LAL) ranks lightning potential into six classes.