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 National Weather Service 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
Forecast services translate cloud cover into plain-language terms. The National Weather Service bases its sky-condition wording on the share of the sky covered by opaque clouds:
Term Opaque Cloud Cover Clear / Sunny 0/8 Mostly Clear / Mostly Sunny 1–2/8 Partly Cloudy / Partly Sunny 3–5/8 Mostly Cloudy 6–7/8 Cloudy / Overcast 8/8A few notes on terminology:
- “Partly Cloudy” and “Partly Sunny” describe the same broad range of cloud cover, not different categories. “Partly Sunny” is used only during daylight hours.
- “Mostly Sunny” is the daytime counterpart of “Mostly Clear.”
- Exact boundaries and wording vary between NWS products and other weather services. Airpult shows the percentage directly, which makes different forecasts easier to compare.
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:
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.
Cloud cover usually narrows the daily temperature range, with cooler highs and warmer lows than under clear skies.
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: On clear, calm nights, the ground loses heat quickly. Gardeners and growers watch for these conditions when assessing frost risk.
- Solar energy: Cloud cover reduces solar panel output, but the effect depends on cloud thickness, panel technology, and the position of the sun.
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.
