What Is Cloud Cover?
Cloud cover is the fraction of the sky obscured by clouds, as seen from a particular location. It’s one of the most familiar parts of any weather forecast: clear, partly cloudy, overcast. But the terms have specific definitions tied to how much of the sky is actually covered.
In forecasts, it’s mainly opaque clouds that matter. Thin, wispy cirrus that you can see the sun through is often excluded from coverage figures, though some definitions of total cloud cover include all visible clouds regardless of thickness.
How It’s Measured
Oktas
Meteorologists measure cloud cover in oktas: eighths of the sky. An observer mentally divides the sky into eight equal sections and counts how many are filled with cloud. The scale runs from 0 (completely clear) to 8 (completely overcast).
This is the standard unit defined by the World Meteorological Organization and is used in weather reports worldwide.
Automated Stations
Modern weather stations use ceilometers: upward-pointing lasers that detect cloud base height by measuring backscatter. The system samples a narrow column of air over a period (typically 30 minutes) and builds a statistical picture of sky coverage.
Ceilometers are reliable but have limitations. They only see directly above the sensor, so patchy skies can be harder to characterise accurately. Modern ceilometers can measure up to around 25,000 feet, though very high clouds above that range may still go undetected.
Forecast Categories
Weather forecasts translate okta measurements into plain-language terms. The table below shows the standard categories used by the National Weather Service and most meteorological services:
| Term | Sky Coverage | Oktas |
|---|---|---|
| Clear / Sunny | 0–5% | 0 |
| Mostly Clear (night) / Mostly Sunny (day) | ~6–25% | 1–2 |
| Partly Cloudy (night) / Mostly Sunny (day) | ~26–50% | 3–4 |
| Mostly Cloudy (night) / Partly Sunny (day) | ~51–69% | 5 |
| Mostly Cloudy | ~70–87% | 6–7 |
| Cloudy / Overcast | 88–100% | 8 |
A few notes on terminology:
- “Partly Cloudy” and “Partly Sunny” are not the same. “Partly Sunny” indicates more cloud cover (~51–69%) than “Partly Cloudy” (~26–50%).
- Day and night terms are paired: “Mostly Sunny” (day) becomes “Partly Cloudy” (night); “Partly Sunny” (day) becomes “Mostly Cloudy” (night).
- Exact breakpoints can vary slightly between meteorological services and forecast products.
Aviation Weather Codes
If you’ve seen weather reports with codes like SCT025 or BKN080, those are METAR cloud reports used in aviation. Each layer is reported as a coverage abbreviation followed by a height in hundreds of feet above ground:
| Code | Meaning | Oktas |
|---|---|---|
| SKC / CLR | Sky clear | 0 |
| FEW | Few clouds | 1–2 |
| SCT | Scattered | 3–4 |
| BKN | Broken | 5–7 |
| OVC | Overcast | 8 |
For example, SCT025 BKN080 means scattered clouds at 2,500 ft and broken clouds at 8,000 ft. The ceiling, the number that matters most for flying, is the lowest layer reported as broken or overcast.
How Cloud Cover Affects Temperature
Clouds have opposite effects depending on the time of day:
During the day, clouds reflect incoming sunlight back to space, keeping temperatures lower than they would be under clear skies. Low, thick clouds are especially effective at this.
At night, clouds absorb heat radiating up from the ground and re-emit some of it back downward. This slows heat loss and keeps overnight temperatures warmer. Clear nights cool faster and produce lower minimum temperatures.
The net result: cloud cover compresses the daily temperature range: cooler highs, warmer lows. A cloudy day-to-night cycle will feel more stable than a clear one.
Why Cloud Cover Matters
Cloud cover affects more than whether you need sunglasses:
- UV exposure: Overcast skies reduce UV, but thin or broken cloud can still let through most UV radiation. Scattered clouds can even briefly intensify UV through reflection.
- Feels like temperature: Cloud cover influences both solar heating and overnight cooling, which feed into how warm or cold it actually feels.
- Frost risk: Clear, calm nights lose heat fastest. Gardeners and growers watch for clear overnight skies as a frost indicator.
- Solar energy: Cloud cover directly determines solar panel output. Overcast skies can cut generation to 10–25% of clear-sky levels.
How Airpult Shows Cloud Cover
On Airpult, cloud cover is shown as a percentage on the forecast page alongside temperature and conditions. Use the explore page to search for any location and check its cloud cover forecast.
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