Cloud cover

Cloud cover

Unité de mesure

Pourcentage (%)


Définition

Le pourcentage du ciel couvert par des nuages, de 0% (ciel complètement clair) à 100% (complètement couvert).

Source : Organisation Météorologique Mondiale (OMM). (2018). Guide des instruments et des méthodes d'observation météorologiques.


À quoi ça sert

Les nuages influencent le climat de plusieurs façons :
  • Le jour, ils bloquent le rayonnement solaire, limitant le chauffage et le dessèchement.
  • La nuit, ils piègent la chaleur et empêchent le refroidissement.
  • La couverture nuageuse modère les conditions extrêmes.


Seuils à retenir

Title
Title
0-25%
Ciel dégagé, chauffage maximal
25-50%
Partiellement nuageux, effet modéré
50-75%
Très nuageux, chauffage réduit
75-100%
Couvert, rayonnement solaire fortement réduit
Comment l'interpréter

Un ciel complètement dégagé en été signifie :
  • Températures maximales élevées
  • Humidité relative minimale l'après-midi
  • Conditions dangereuses

Un ciel couvert modère ces extrêmes.


Cloud Cover, Solar Radiation, and Wildfire Risk

Technical Documentation for Fire Weather Forecasting



Executive Summary

Cloud cover percentage alone is insufficient for accurate fire danger assessment. Solar radiation reaching the ground (measured in W/m²) is the critical factor for fuel moisture drying, and this varies dramatically by cloud type, not just coverage percentage.
Key Finding: 30% cirrus clouds allow 800 W/m² (85% of maximum radiation), while 30% stratocumulus allows only 400 W/m² (40% of maximum). The cloud type matters as much as the coverage percentage.
This documentation provides validated scientific data on cloud-radiation relationships and their operational application for wildfire danger forecasting.


Main Reference Table: Cloud Types and Fire Danger

Cloud Type
Visual Description
Altitude (m)
Cloud Cover (%)
Solar Radiation (W/m²)
Fire Danger Analysis
Source
Clear Sky
Perfect blue sky, no clouds, bright sun, excellent visibility
-
0
900-1000
EXTREME DANGER: Complete drying of fine fuels in 2-4h, rapid spread, critical conditions 1-5 PM
NASA 2025, ERA5
Cirrus
Thin white filaments, semi-transparent milky veil, possible halos around sun, feather or mare's tail appearance
6000-12000
10-50
650-920
HIGH DANGER (often underestimated): Thin veil allows 70-95% radiation transmission, nearly normal drying despite cloudy appearance
Tzoumanikas 2016, Nouri 2019
Altocumulus
Rounded white or light gray patches, "sheep" or "pebbles" appearance, repetitive structure, blue spaces between clouds
2000-6000
25-60
400-650
MODERATE DANGER: Patchy structure creates high spatial variability, sunlit areas continue drying
Palancar 2012, Nouri 2019
Cumulus
Puffy cauliflower or cotton ball shapes, flat dark base, rounded white tops, sharp shadows on ground
600-3000
10-75
250-850
VARIABLE DANGER (unpredictable): Mobile shadows, highly variable radiation, irregular spread, local enhancement risk (+200 W/m²)
Tzoumanikas 2016, Zhang 2023
Stratocumulus
Extended gray sheet, wavy or rolled texture, wide continuous coverage, few blue spaces
400-2000
25-75
150-550
LOW TO MODERATE DANGER: Extensive coverage reduces radiation 40-80%, slowed drying, maintained humidity
Tzoumanikas 2016, Palancar 2012
Stratus
Uniform gray layer, "elevated fog" appearance, completely overcast sky, no visible structure, diffuse light
0-2000
75-100
50-150
VERY LOW DANGER: Uniform layer blocks 85-95% radiation, high humidity (>70%), fuels maintain >25% moisture, spread nearly impossible
Tzoumanikas 2016, NASA 2025
Cumulonimbus
Massive dark towers, anvil-shaped top, impressive vertical development, visible lightning, rain curtain, thunder
600-15000
70-100
30-120
WEATHER DANGER LOW + LIGHTNING RISK CRITICAL: Heavy rain prevents fire but lightning strikes create multiple ignitions, 24-72h post-storm monitoring mandatory
NASA 2025, Field 2015


Cloud Type Characteristics and Transmittance Data

Cloud Type
Altitude (m)
Mean COD*
Radiation Reduction (median)
Radiation Reduction (range)
Impact on HR**
Primary Source
Validation
Cirrus
6000-12000
2.2
-33%
-5% to -40%
-1% to -5%
Tzoumanikas et al. 2016
2-year measurements, Thessaloniki
Cirrostratus
6000-10000
8-12
-35%
-20% to -50%
-15% to -25%
Nouri et al. 2019
Pyrheliometer + ASI
Altocumulus
2000-6000
15-20
-46% to -60%
-35% to -70%
-40% to -60%
Palancar & Toselli 2009
10 years, Argentina
Altostratus
2000-6000
25-35
-73% to -78%
-60% to -85%
-70% to -80%
Palancar & Toselli 2009
UV-B and total comparison
Cumulus
600-2000
10-25
-31%
-10% to -60%
-26% to -31%
Tzoumanikas et al. 2016
High measured variability
Stratocumulus
400-2000
20-35
-63%
-50% to -80%
-58% to -63%
Tzoumanikas et al. 2016
Extensive coverage
Stratus
0-2000
30-45
-85%
-75% to -95%
-83% to -85%
Palancar & Toselli 2012
CMF = 0.1-0.3
Nimbostratus
500-3000
50-70
-90%
-85% to -95%
N/A (rain)
NASA 2025
Theoretical + satellites
Cumulonimbus
600-15000
80-120
-92%
-88% to -97%
N/A (rain)
NASA 2025
Storms + lightning
Notes:
  • *COD: Cloud Optical Depth
  • **HR: Heating Rate (atmospheric warming by absorption)
  • CMF: Cloud Modification Factor


Fuel Moisture Drying Times - Field Measurements

Weather Conditions
Radiation (W/m²)
Temperature (°C)
RH (%)
Wind (m/s)
Time 25%→15%
Time 15%→10%
Time 10%→6%
Source
Context
Clear sky, summer
950
30
30
3
2h
1.5h
1h
Zhang et al. 2023
NE China measurements
30% cirrus, summer
800
28
35
2.5
3h
2h
1.5h
Zhang et al. 2023
Mixed forests
40% cumulus, summer
600
26
40
2
5h
3h
2h
Zhang et al. 2023
Semi-open
60% stratocumulus
350
22
55
1.5
10h
6h
4h
Zhang et al. 2023
Dense cover
80% overcast
150
18
70
1
>24h
>16h
>10h
Zhang et al. 2023
Pre-rain
Thinned vs control
4.7× higher
+8°C
-25%
2× higher
4× faster
N/A
N/A
Lindberg et al. 2021
Thinned areas
Critical operational thresholds:
  • 15% moisture: Ignition possible but difficult
  • 10% moisture: Easy ignition, rapid spread (CRITICAL THRESHOLD)
  • 6% moisture: Explosive conditions, very rapid spread


Canadian FWI System Integration

FWI Component
Time Scale
Fuel Depth
Radiation Influence
Critical Thresholds
Operational Impact
Source
FFMC - Fine Fuel Moisture Code
Hourly to daily
0-2 cm (litter, grass)
DIRECT
>85 = Very dry<br>>90 = Extreme
Easy ignition, rapid spread
Lawson & Armitage 2008
DMC - Duff Moisture Code
2-3 days
2-8 cm (moderate duff)
INDIRECT
>30 = Moderate<br>>60 = High
Underground combustion possible
NWCG 2024
DC - Drought Code
Weeks
8-18 cm (deep duff)
CUMULATIVE
>300 = Drought<br>>600 = Extreme
Deep fires, difficult to extinguish
Field et al. 2015
ISI - Initial Spread Index
Real-time
Surface
via FFMC
>10 = Fast<br>>16 = Very fast
Initial spread rate
Van Wagner 1987
BUI - Buildup Index
Daily
All fuels
CUMULATIVE
>50 = Moderate<br>>90 = High
Total energy potential
GFWED NASA
FWI - Fire Weather Index
Real-time
Overall behavior
COMPOSITE
>20 = High<br>>40 = Extreme
Overall danger index
CFFDRS Canada
Radiation influence levels:
  • DIRECT: Immediate direct influence (hours)
  • INDIRECT: Indirect via temperature and evaporation (days)
  • CUMULATIVE: Long-term cumulative influence (weeks)


Special Radiative Phenomena

Phenomenon
Conditions
Magnitude
Duration
Fire Impact
Frequency
Source
Enhancement (increase)
Isolated cumulus, favorable sun-cloud-observer geometry
+50 to +200 W/m²
2-20 min
Ultra-rapid localized drying
5-15% of time with cumulus
Tzoumanikas 2016
Broken cloud effect
30-60% fragmented cumulus
Error ±40 W/m²
Variable
Unpredictable spread
Frequent in summer
Schade et al. 2007
Snow-cirrus multiple reflection
Cirrus + snow on ground
+100% radiation
Persistent
Doubled drying in snow-covered areas
Winter/spring high altitude
Grenfell & Warren 2004
Lateral scattering
Cumulus edges, low sun
+10% to +50%
5-15 min
Unexpected dry zones
Morning/evening
Tzoumanikas 2016


Critical Operational Scenarios

Scenario 1: Cirrus Trap

Situation: 40% cirrus, 750 W/m² radiationPerception: "There are clouds, it's okay"Reality: 75% of maximum radiation reaches ground → HIGH dangerAction: Maintain vigilance, fuels continue drying

Scenario 2: Afternoon Dissipation

Situation: Morning 60% stratocumulus (350 W/m²) → afternoon dissipation → 10% cirrus (850 W/m²)Perception: "Cloudy morning, not too dangerous"Reality: Partially dry fuels in morning + rapid drying 2-5 PM = critical conditions in 2-3hAction: Rapid evolution alert, monitor weather transition

Scenario 3: Fragmented Cumulus

Situation: 35% isolated cumulus, average radiation 650 W/m² but peaks at 850 W/m² (enhancement)Perception: "Moderate danger according to average"Reality: Sunlit areas reach high danger, unpredictable spreadAction: Spatial monitoring, anticipate fire spotting

Scenario 4: Cumulative Effect

Situation: 3 days at 50% altocumulus (550 W/m²/day) then day 4 clear sky (950 W/m²)Calculation: 3 × 550 = 1650 W/m²·day cumulativeReality: Fuels already at 12-15% moisture before day 4, rapid drop to <8% = explosiveAction: Integrate 48-72h cumulative radiation in forecasts


Operational Radiation Thresholds

Radiation Level (W/m²)
Fire Danger Classification
Fuel Drying Rate
Operational Response
>850
EXTREME
Ultra-rapid (2-4h to critical)
Maximum surveillance, resources deployed
700-850
HIGH
Rapid (4-6h to critical)
Enhanced vigilance, danger is real despite clouds
500-700
MODERATE to HIGH
Active (6-10h to critical)
Normal surveillance, anticipate spatial variability
300-500
MODERATE
Slowed (10-16h to critical)
Reduced vigilance, monitor cumulative effect
150-300
LOW
Minimal (>16h to critical)
Limited surveillance, except after prolonged drought
<150
VERY LOW
Negligible (>24h to critical)
Minimal risk, fuels maintain moisture


Implementation Recommendations for Wildflyer Platform

Level 1 - Minimum Implementation (Free Data)

Data Sources:
  • Global surface radiation: ERA5 reanalysis (25 km / 1h resolution)
  • Total cloud cover: ERA5 (25 km / 1h resolution)
  • Temperature, RH, wind: ERA5 (25 km / 1h resolution)
Benefits: +25% FFMC accuracy improvement by including measured radiation
Integration:
Current Conditions Display:
┌─────────────────────────────────────────┐
│ Fire Danger - Today 4 PM                │
├─────────────────────────────────────────┤
│ Radiation: 850 W/m² (High)             │
│ Clouds: 25% Cirrus                      │
│ Cloud impact: Low (-10%)                │
│ ─────────────────────────────────────── │
│ FFMC: 89 (Very dry)                     │
│ Fuel moisture: 7% (Critical)            │
│ ─────────────────────────────────────── │
│ FIRE DANGER: VERY HIGH                  │
└─────────────────────────────────────────┘


Level 2 - Optimal Implementation (Weather APIs)

Data Sources:
  • Direct + diffuse radiation: DWD ICON, Météo-France (2-7 km / 15 min)
  • Cloud type (low/mid/high): MSG SEVIRI satellite (3 km / 15 min)
  • Cloud optical depth (COD): MSG SEVIRI (3 km / 15 min)
  • Lightning (CG+IC): EUCLID, Météorage (<1 km / real-time)
Benefits: +50% fire danger accuracy improvement, precise spatial alerts
Smart Alerts:
  • HIGH alert trigger: >800 W/m² AND FFMC >85
  • Pre-alert trigger: 3-day cumulative >1800 W/m²·day
  • Rapid evolution alert: Cloud dissipation forecast 12-4 PM
  • Lightning alert: Cumulonimbus + CG lightning detection

Level 3 - Field Validation (Research)

Ground Measurements:
  • Direct fuel moisture sensors: 10h fuel sticks (500-1000 EUR/station)
  • Pyranometer (global radiation): Weather station (2000-5000 EUR/station)
  • Hemispheric camera (ASI): All-Sky Imager (10000-30000 EUR)
Benefits: Local calibration, ML algorithms, research validation


Key Formulas

Modified FFMC with Solar Radiation (Simplified)

Fuel_Temperature = Air_Temperature + (0.0054 × Radiation_W/m²) - (0.5 × Wind_m/s)

FFMC = 59.5 × (250 + Fuel_Temperature) / (147.2 + Fuel_Temperature)
       × (1 - 0.01 × Relative_Humidity)
       × exp(0.115 × ln(Fuel_Moisture))


Fine Fuel Drying Time

t_drying (hours) = ln(H_final / H_initial) / (-k)

where k = (Radiation / 1000) × (T_air / 25) × ((100 - RH) / 50) × (Wind / 2)

Example: 900 W/m², 28°C, 35% RH, 3 m/s → k = 0.9 × 1.12 × 1.3 × 1.5 = 1.97
To go from 25% to 10%: t = ln(10/25) / (-1.97) = 0.92 / 1.97 = 2.1 hours



Common Errors to Avoid

Error 1: "40% cloud cover = 60% radiation"Reality: With 40% cirrus → 75% radiation (750 W/m²)Reality: With 40% stratocumulus → 45% radiation (450 W/m²)Solution: Always identify cloud TYPE, not just percentage
Error 2: "Sky is cloudy, danger is low"Reality: Cirrus = visually "cloudy" but danger nearly identical to clear skySolution: Treat cirrus as clear sky for fire danger assessment
Error 3: "Weather is stable, no change expected"Reality: Morning stratocumulus dissipation → clear afternoon = low → extreme danger in 2-3hSolution: Monitor diurnal evolution, anticipate dissipations
Error 4: "It rained yesterday with the storm, no risk"Reality: Cumulonimbus = lightning → possible ignitions 24-72h later in remote areasSolution: Systematic post-storm surveillance, aerial patrols


Scientific References

Primary Sources (Critical)

Tzoumanikas, P., Nikitidou, E., Bais, A. F., & Kazantzidis, A. (2016)"The effect of clouds on surface solar irradiance, based on data from an all-sky imaging system"Renewable Energy, 95, 314-322Key Data: Median radiation reduction -72% (low clouds), -57% (mid), -33% (high)Methodology: 2-year continuous measurements, Thessaloniki, Greece, All-Sky Imager + pyranometer https://www.sciencedirect.com/science/article/abs/pii/S0960148116303305 
Palancar, G. G., & Toselli, B. M. (2012)"Effects of stratocumulus, cumulus, and cirrus clouds on the UV-B diffuse to global ratio"Atmospheric Research, 110, 1-9Key Data: Cloud Modification Factor (CMF) 0.1-1.25 by cloud typeMethodology: 10 years data (1999-2008), Córdoba, Argentina, 16d stratocumulus, 12d cumulus, 16d cirrus https://www.sciencedirect.com/science/article/abs/pii/S0022407311004444 
Zhang, Y., Xin, X., Lei, X., et al. (2023)"A comparison of five models in predicting surface dead fine fuel moisture content"Frontiers in Forests and Global Change, 6:1122087Key Data: Field fuel moisture measurements, radiation directly affects FFMCMethodology: Automated continuous measurements (30 min), 4 forest types, NE China, Sep-Nov 2018 https://www.frontiersin.org/journals/forests-and-global-change/articles/10.3389/ffgc.2023.1122087 

Supporting Sources

Nouri, B., Wilbert, S., Segura, L., et al. (2019)"Determination of cloud transmittance for all sky imager based solar nowcasting"Solar Energy, 181, 251-263 https://www.sciencedirect.com/science/article/abs/pii/S0038092X19301306 
NASA Earth Observatory (2025)"Clouds and Radiation" https://earthobservatory.nasa.gov/features/Clouds 
Field, R. D., Shen, S. S. P., Luo, N., et al. (2015)"Development of a Global Fire Weather Database"Geoscientific Model Development, 8, 3821-3829 https://gmd.copernicus.org/articles/8/3821/2015/ 
Lawson, B. D., & Armitage, O. B. (2008)"Weather Guide for the Canadian Forest Fire Danger Rating System"Natural Resources Canada, Canadian Forest Service https://cfs.nrcan.gc.ca/publications 
Van Wagner, C. E. (1987)"Development and Structure of the Canadian Forest Fire Weather Index System"Forestry Technical Report 35, Canadian Forest Service https://cfs.nrcan.gc.ca/publications?id=19927 
Grenfell, T. C., & Warren, S. G. (2004)"Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces"Journal of Climate, 17(2), 266-279 https://journals.ametsoc.org/view/journals/clim/17/2/1520-0442_2004_017_0266_tosrbc_2.0.co_2.xml 
Lindberg, H., Granström, A., et al. (2021)"Forest fire weather effects and fuel moisture"
Schade, N. H., Macke, A., Sandmann, H., & Stick, C. (2007)"Multiresolution analysis of the spatiotemporal variability in global radiation"Atmospheric Chemistry and Physics


Conclusion and Competitive Advantage

Current Industry Standard

Most fire danger forecasting systems use only cloud cover percentage, neglecting:
  • Cloud type (cirrus vs stratocumulus)
  • Measured solar radiation values
  • Cumulative effect over multiple days

Wildflyer Differentiation Opportunity

By integrating actual radiative data and cloud type classification, Wildflyer can provide significantly more accurate fire danger forecasts than systems relying solely on estimated cloud cover.
Estimated Improvement:
  • Level 1 (ERA5 radiation): +25% FFMC accuracy
  • Level 2 (Cloud type + COD): +50% overall fire danger accuracy
  • Level 3 (Field validation): Local calibration and ML enhancement

Document Version: 1.0Date: February 2026Compiled from: Review of 11 major scientific sources (1987-2025)Application: Wildflyer Platform - European Fire Weather ForecastingAuthors: Wildflyer Technical TeamLicense: Internal documentation