Cloud Cover

Unit: Percentage (%)

Cloud cover is the fraction of the sky obscured by clouds, expressed as a percentage (0% = clear sky, 100% = overcast).

Why it matters for fire weather

Cloud cover modulates fire weather in two ways:

Daytime heating. Clouds block incoming solar radiation, reducing surface heating and slowing fuel drying. Clear skies allow maximum solar energy to reach the ground, driving higher temperatures, lower relative humidity, and higher VPD.

Overnight recovery. Clear skies allow heat to radiate away at night, causing temperatures to drop and humidity to rise — conditions that help fuels recover moisture. Cloud cover traps heat, keeping nights warmer and drier, which reduces overnight fuel moisture recovery.

How it works

Cloud cover is reported as a percentage from weather models and satellite observations. For fire weather, the key distinction is between clear, partly cloudy, and overcast conditions:

Clear skies (0–20%) — maximum daytime heating and fuel drying. Best overnight moisture recovery (if winds are also light). Typical under high-pressure systems, which can persist for days or weeks.

Partly cloudy (20–60%) — moderate solar heating. Cumulus clouds in the afternoon may signal atmospheric instability — watch for boundary layer growth, wind shifts, and thunderstorm development.

Overcast (60–100%) — reduced surface heating and slower fuel drying. However, overcast skies with strong wind can still produce significant fire conditions — cloud cover alone does not determine fire danger.

Note

Dry thunderstorms — where the cloud base is high and rain evaporates before reaching the ground (virga) — are among the most dangerous fire weather events. Lightning reaches the surface while the expected rain never arrives. High CAPE values combined with a dry lower atmosphere signal this risk.

How to read it in Wildflyer

Cloud cover appears on the weather timeline as a percentage. Use it alongside solar radiation to understand how much heating energy is reaching the surface. Multi-day forecasts showing persistent clear skies under high pressure indicate sustained drying conditions.

Sources

• World Meteorological Organization (2017). International Cloud Atlas. WMO.