The three RFS weather stations on this site display three measurements that help us understand how dry the landscape is and how favourable current weather conditions are for bushfire spread.
KBDI tells us how dry the soil has become over time.
DF tells us how dry and available forest fuels are for burning
Delta T tells us how quickly fuels are drying out
These are calculated estimates based on local weather observations. They are not official fire danger ratings or forecasts – they are calculated locally using data from the three sites and are intended as a guide.
1. Keetch–Byram Drought Index (KBDI)
How dry is the soil?
KBDI estimates the accumulated moisture deficit in the upper soil layers. It is expressed in millimetres, ranging from 0 to 200 in the Australian system. A higher number means more rainfall would be needed to bring the soil back towards saturation.
| KBDI | General interpretation |
|---|---|
| 0–50 | Moist soils. Generally limited accumulated drying. |
| 51–100 | Increasing soil dryness. Deeper vegetation and litter layers may be drying. |
| 101–150 | Substantial moisture deficit. Greater potential for deeper fuels to contribute to fires. |
| 151–200 | Very dry soils. Significant accumulated drought conditions. |
These ranges are illustrative guides, not official Australian KBDI danger categories.
Remember: KBDI changes gradually. It generally increases during extended dry and warm periods and decreases after sufficient rainfall – for example, a KBDI of 100 indicates an estimated soil moisture deficit of approximately 100 mm.
2. Drought Factor (DF)
How ready is the vegetation to burn?
Drought Factor estimates the availability of forest fuel for burning, taking into account longer-term soil dryness and recent rainfall. It ranges from 0 to 10.
| DF | General interpretation |
|---|---|
| 0–2 | Very low fuel availability. Recent moisture is limiting the amount of fuel likely to burn. |
| 3–4 | Relatively low fuel availability, although some fuels may still burn. |
| 5–6 | Moderate fuel availability. More forest fuel is becoming available to burn. |
| 7–8 | High fuel availability. A substantial proportion of forest fuel may contribute to a fire. |
| 9–10 | Very high fuel availability. Most available forest fuel may contribute to fire behaviour. |
These groupings are simplified explanations, not official DF categories.
Remember: Unlike KBDI, the Drought Factor can change quite quickly following significant rainfall, even when deeper soils remain relatively dry.
3. Delta T Graph
How quickly is the vegetation drying out?
The Delta T graph shows the difference between the air temperature and the wet bulb temperature, measured in degrees Celsius (°C). It provides a useful indication of how dry the air is and how quickly moisture may evaporate. A low Delta T (0–2°C) indicates moist air and slow evaporation, while 2–8°C generally represents moderate drying conditions. A Delta T above 8°C indicates increasingly dry air and faster evaporation. For firefighting, higher Delta T values can indicate greater potential for vegetation to dry out and fires to become more difficult to control, particularly when combined with high temperatures and strong winds.
Important information
The figures displayed on our Bellingen, Deer Vale and Nana Glen weather pages are independently calculated estimates, based on local automatic weather station observations and modelled drought conditions. They are intended for general weather monitoring and education, not for operational fireground decisions.
For official fire danger ratings, warnings and total fire bans, always refer to: