Rain’s threatening to fall this weekend. Here’s why that’s not a bad thing. Tags: Fire Index and KBDI The Fire Index is the ability for spontaneous (from sparks from rail cars or form careless outdoor burning) fires. Even though it might seem obvious, the drier the conditions, the more easily fires can rapidly burn. The Keetch Bryam Drought Index (KBDI) is more about soil conditions. It’s very possible to see surface vegetation dry out quickly under the spring and summer sunshine, which is typically associated with low afternoon humidity levels. But soil moisture can still stay moist. This moist layer underneath can slow the spread of fires but more importantly reduce the sustainability of ongoing fires due to the higher soil moisture content.