Solar Radiation
Dieser Inhalt ist noch nicht in deiner Sprache verfügbar.
Unit: Watts per square metre (W/m²)
Solar radiation (also called shortwave radiation or insolation) measures the amount of energy from the sun reaching the ground surface. It is the primary energy source driving fire weather conditions.
Why it matters for fire weather
Section titled “Why it matters for fire weather”Solar radiation is the engine behind the daily fire weather cycle. It heats the ground and air, evaporates moisture from vegetation, and powers the atmospheric convection that influences wind and stability.
- Direct heating — south-facing slopes (in the northern hemisphere) receive more radiation than north-facing slopes, drying fuels faster. This aspect effect explains why south-facing slopes typically dry first and are the first to burn.
- Fuel temperature — dark-coloured fuels in direct sunlight can reach temperatures well above air temperature, lowering their moisture content beyond what the ambient relative humidity alone would suggest.
- Convective activity — strong solar heating drives thermals and boundary layer growth, which can increase wind gustiness in the afternoon and influence atmospheric stability (CAPE, Haines Index).
Solar radiation also feeds indirectly into the FWI system — it drives the temperature and humidity changes that the system uses as inputs. The FWI system accounts for day length (a proxy for solar radiation) in the DMC and DC calculations through latitude-adjusted day-length factors (Van Wagner, 1987).
How it works
Section titled “How it works”Typical values
Section titled “Typical values”| Radiation | Conditions |
|---|---|
| 0 W/m² | Night-time |
| 100–300 W/m² | Overcast or low sun angle (winter, early morning) |
| 300–600 W/m² | Partly cloudy or moderate sun angle |
| 600–900 W/m² | Clear sky, summer, midday |
| > 900 W/m² | Peak clear-sky radiation in southern Europe, midsummer |
Seasonal and aspect effects
Section titled “Seasonal and aspect effects”Summer peak radiation in Mediterranean Europe (~900 W/m²) is roughly 3× winter levels. This compounds with longer days to make summer the dominant fire season.
Aspect creates large differences in received radiation within short distances. In complex terrain, a south-facing slope may receive 2–3× more daily radiation than an adjacent north-facing slope, creating corresponding differences in fuel moisture. The FWI system does not capture this effect — it uses a single temperature observation regardless of local terrain.
How to read it in Wildflyer
Section titled “How to read it in Wildflyer”Solar radiation appears on the weather timeline. It interacts with all other variables: more radiation leads to higher temperature, lower RH, higher VPD, and drier fuels. Use it alongside cloud cover to understand how much heating energy is reaching the surface.
Sources
Section titled “Sources”- Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.
- World Meteorological Organization (2018). Guide to Meteorological Instruments and Methods of Observation. WMO-No. 8.