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.

Cloud cover and solar radiation

Solar radiation is the primary energy input to the Earth’s surface. The relationship between cloud cover and the radiation that actually reaches the ground depends on cloud thickness, type, and altitude:

Cloud cover	Approximate solar radiation reaching the surface
Clear sky (0%)	100% of potential — up to 850–950 W/m² in midsummer Mediterranean
25%	~80–85% — mostly direct sun between clouds
50%	~60–70% — intermittent direct sun
75%	~35–50% — mostly diffuse radiation
100% overcast	~10–30% — entirely diffuse, no shadows

Cloud type matters significantly. Thin cirrus at high altitude reduces radiation by only 10–20%. Thick cumulonimbus can reduce surface radiation by 90%. The same percentage of cloud cover reported by a model can represent very different reductions in actual solar energy depending on cloud type.

The fire weather consequence chain runs directly through solar radiation:

Solar radiation → surface temperature → relative humidity (inversely) → VPD → fuel moisture → FFMC → ISI and FWI

Every link in this chain depends on solar radiation. When clouds reduce radiation, temperatures rise more slowly, humidity drops less, VPD stays lower, fuels dry more slowly, and the FWI builds more slowly.

Quantitative example: On a clear July day in southern France, peak solar radiation may reach 900 W/m². With a full overcast, this drops to 100–150 W/m². Temperature may peak at 26°C instead of 36°C. Minimum RH may be 45% instead of 18%. FFMC may reach 86 instead of 94. FWI may peak at 12 instead of 40. The same location, the same date — but an order-of-magnitude difference in fire danger, solely because of cloud cover.

Why cloudy nights produce a dangerous next morning

This is one of the most common sources of operational confusion. The intuition that “cloudy = cooler and moister” is correct for the daytime, but reverses at night.

On a clear night:

• The ground radiates heat upward freely into space; air temperature drops steadily
• At some point, surface temperature falls below the dew point → dew forms on vegetation
• Fine fuels in direct contact with dew absorb moisture efficiently
• Minimum temperature may reach 12–15°C; maximum RH may reach 85–95%
• Fine fuels (FFMC) recover 15–25 points overnight

On a cloudy night:

• Clouds absorb outgoing longwave radiation and re-emit it downward, acting as an insulating blanket
• Surface temperature drops only 3–5°C instead of 10–15°C
• The dew point is never reached — no dew forms
• Minimum temperature remains 18–22°C; maximum RH reaches only 55–70%
• Fine fuels recover only 5–10 FFMC points overnight

The result: A fire that had an FFMC of 92 at 18:00 might recover to only FFMC 85 after a cloudy night, versus FFMC 72 after a clear night with dew. A cloudy night means poor overnight recovery — the morning danger window opens higher and earlier.

Caution

Do not assume a cloudy night automatically means a safer morning. Check the forecast minimum RH, whether dew is expected, and the cloud type — thin cirrus offers almost no insulating effect, while thick stratus traps heat effectively. The safest approach is to look at the FFMC forecast directly rather than inferring fuel moisture from cloud cover alone.

The clearing trap

One of the fastest fire danger escalation scenarios: cloudy overnight, then clearing in the morning.

When clearing occurs after a cloudy night:

1. Fuels have not recovered — the cloudy night provided poor moisture restoration
2. Full solar radiation suddenly returns to already-dry fuels
3. Temperature climbs rapidly, RH drops rapidly
4. The FFMC may reach its afternoon peak 1–2 hours earlier than after a proper overnight recovery

Afternoon convective cloud development (cumulus building vertically) does not mean lower fire danger. It signals increasing atmospheric instability — check CAPE, monitor for dry thunderstorm risk, and watch for gusty downdrafts. Afternoon convective cloud is a reason for increased operational vigilance, not reduced.

Multi-day clear sky patterns

Multi-day clear skies under a stable high-pressure system are the classic extended fire weather setup. Each day builds on the last:

• FFMC starts higher each morning (poor overnight recovery accumulates)
• DMC and DC continue to rise with no rain
• Deep fuels become progressively more available for combustion

A single clear day is a manageable event. Five to seven consecutive clear days represent a structural shift in fire danger across an entire landscape — even if the daily peak conditions look similar to a single high-danger day.

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:

• Persistent clear skies under high pressure — sustained drying conditions; watch the multi-day FFMC and DC trend
• Afternoon convective cloud — increased instability; check CAPE and boundary layer height
• Stratus or fog burning off in the morning — rapid heating onset once it clears; fuels may already be dry underneath

Sources

• World Meteorological Organization (2017). International Cloud Atlas. WMO.
• Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.