Fire Risk Index Explained
What the "Fire Risk" figure on our main weather dashboard means, where it comes from, and today's reading for the New Forest.
Current New Forest heathland fire risk
3
Low
Based on 1 consecutive dry day, a forecast high of 11°C today, and a forecast minimum humidity of 71%.
How this figure is worked out
The number above - and the same "Fire Risk" figure shown on our main weather dashboard - is our own estimate for the New Forest itself (the actual heathland fire-risk area, rather than this station's own back garden), built from three real, live factors:
| Factor | Contributes up to | Why it matters |
|---|---|---|
| Consecutive dry days | 60 points | The single biggest factor in real wildfire risk - how long it's been since the ground last had meaningful rain. |
| Today's forecast maximum temperature | 25 points | Heat dries out fuel further and helps a fire spread once started. |
| Today's forecast minimum humidity | 15 points | Low humidity lets vegetation dry out faster and fire spread more easily. |
This deliberately fixes a real limitation of the classic Chandler Burning Index reading this figure used to show (explained below): that method only ever looks at the current moment's temperature and humidity, with no memory of how long it's been since rain - which meant a humid, coastal station like ours could sit on "Low" for weeks even during a genuine dry spell. Consecutive dry days now make up most of this score for exactly that reason.
Rainfall & temperature - last three weeks
About the Chandler Burning Index
Southampton Weather's fire-risk figure was originally based on a standard known as the Chandler Burning Index (CBI) - still the origin of the 0-100 scale and the five bands used above. The classic CBI is calculated from a station's current temperature and relative humidity, and the six descriptions below still describe the general concept of relative wildland fire potential that both the old and new figures are built around:
| Low <50 |
Fuels do not ignite readily from small firebrands, although a more intense heat source, such as lightning, may start fires in woods. Fires in open cured grasslands may burn freely a few hours after rain, but woods fires spread slowly by creeping or smouldering, and burn in irregular fingers. There is little danger of spotting. |
| Moderate 50-75 |
Fires can start from most accidental causes, but the number of starts is generally low. Fires in open cured grasslands will burn briskly and spread rapidly on windy days. Timber fires spread slowly to moderately fast, and control is relatively easy. |
| High 75-90 |
All fine dead fuels ignite readily and fires start easily from most causes. Unattended brush and campfires are likely to escape. Fires spread rapidly and short-distance spotting is common; they may become serious unless attacked successfully while small. |
| Very High 90-97.5 |
Fires start easily from all causes and, immediately after ignition, spread rapidly and increase quickly in intensity. Spot fires are a constant danger, and long-distance spotting and fire whirlwinds can develop once a fire burns into heavier fuels. |
| Extreme 97.5+ |
Fires start quickly, spread furiously, and burn intensely - all fires are potentially serious. Direct attack is rarely possible and may be dangerous except immediately after ignition. The only effective, safe control action is often on the flanks, until the weather changes or the fuel supply lessens. |
Neither this figure nor the classic CBI is the official Government wildfire measure. That's the Met Office's Fire Severity Index, updated daily and used to trigger fire-prevention access restrictions on Open Access land - published only as a map, with no public raw-data feed. See the Natural England Open Access map or the Met Office FSI explainer for the authoritative picture. For the fuller New Forest conditions page - car parks, campsites, pony safety and more - see our New Forest Conditions page.
Where this data comes from: today's dry-day count, temperature and humidity are a live forecast for the New Forest (Lyndhurst area) from the free Open-Meteo service, cached for 30 minutes. This is our own weighted estimate, not the official Fire Severity Index.