Cloud cover describes how much of the sky is covered by cloud at a given place and time. It can represent an observer’s view or a model’s estimate for an area. The percentage does not give cloud height, thickness, rainfall or the duration of a clear gap.
Use the value as a sky-amount description for that time, then check rain, visibility, ultraviolet radiation and wind separately. Oktas, METAR layers and aviation ceilings are related scales, not interchangeable numbers.
What a cloud-cover percentage means
From the ground, cloud amount is the fraction of the visible sky dome covered by cloud. A report of 60% means that roughly three fifths of that dome is covered from the observer’s position. In a numerical weather model, the value usually describes the estimated cloudy fraction of a horizontal grid cell. Airpult’s forecast value comes from this kind of model field and is interpolated to the requested coordinates.
Cloud cover is not the chance of cloud occurring. A forecast value of 40% does not mean a 40% probability of seeing a cloud, nor does it mean cloud will be overhead for 40% of the hour. It describes an amount of cover for that forecast time. The chance of rain in a precipitation forecast is a separate quantity. See What Is Precipitation?
The percentage also says nothing by itself about opacity. A thin sheet of high cloud and a low, dense deck may cover the same area while producing very different daylight, visibility and surface conditions. Cloud form and height require a separate description of cloud type.
Percentages and oktas are related, not identical
Human observers traditionally report cloud amount in oktas, or eighths of the sky. Zero oktas means no cloud is seen and eight oktas means the sky is completely covered. An okta is a discrete observational category, whereas a model percentage can take many values along a continuous scale.
The WMO reporting rules also use ranges at the ends of the scale. One okta means one eighth or less, but not a clear sky. Seven oktas means seven eighths or more, but not a completely covered sky. A few remaining gaps therefore stay in the seven-okta category rather than jumping to eight.
Nine oktas is not nine eighths. Observers use it when the sky is obscured, or when cloud amount cannot be estimated, for example in fog, heavy precipitation or blowing snow. In that case the report is not a cover fraction at all.
It is tempting to turn 50% into exactly four oktas or 25% into exactly two. Those fractions are useful for orientation, but they imply more precision than an eye estimate provides. The observer must judge an irregular three-dimensional sky, and the categories above already group amounts into ranges. Forecast services may also use different thresholds when they turn percentages into words such as “partly cloudy”. Check the legend used by the service rather than assuming one universal conversion.
Total cloud cover is the fraction of the whole sky hidden by cloud of any type. Layer amount is the fraction covered by one layer, including any part of that layer hidden by cloud below it. Adding layer amounts can therefore exceed eight oktas even when the sky is not overcast. The WMO International Cloud Atlas explains total cloud cover and the treatment of cloud layers that are partly hidden by other layers.
How aviation reports describe sky cover
METAR observations use standard abbreviations for the amount of cloud in each reported layer:
- FEW: more than zero through two eighths.
- SCT: three through four eighths, meaning scattered.
- BKN: five through seven eighths, meaning broken.
- OVC: eight eighths, meaning overcast.
SKC is used at manual US stations when no cloud is present. At a US automated station, CLR means no reported cloud below 12,000 feet; cloud may exist higher up. A group such as BKN025 reports broken cloud with a base at 2,500 feet above ground level.
For aviation, the ceiling is the lowest layer reported as broken or overcast, or the vertical visibility into a complete obscuration. Few and scattered layers do not form a ceiling, even if the total cloud-cover percentage looks high. Several scattered layers can still leave no ceiling. The FAA Aeronautical Information Manual gives the reporting definitions. A general cloud-cover percentage cannot replace a METAR, TAF or an operational aviation briefing.
Why cloud layers cannot simply be added
Total cloud cover combines clouds at every height as seen overhead or underneath. Suppose a low layer covers half an area and a high layer also covers half. If both lie over the same half, total cover remains 50%. If they occupy opposite halves, the total can reach 100%. Adding the two layer values would therefore give the wrong answer in many cases.
Models need an overlap rule to estimate how clouds on different vertical levels line up. ECMWF calculates total, low, medium and high cloud cover from its three-dimensional cloud field using assumptions about vertical overlap. Its cloud forecast guide notes that total cover is no greater than the sum of the separate layer values. This is why a total-cloud forecast can differ from what you might infer by adding the low, middle and high fields.
How cloud cover is observed
A trained observer scans the visible sky, estimates the total amount and separates layers by base height. Terrain, buildings or a lower layer can hide part of the sky. The result represents one position, not the whole surrounding region.
Automated airport systems commonly use a ceilometer. It sends a narrow laser pulse upwards and detects cloud bases above the sensor. Because it samples a column rather than the whole sky, the system estimates layer amount from successive “cloud hits” as clouds move across the beam. That time-averaged column is not the same as a human okta estimate of the full sky dome.
In the US ASOS network, an algorithm time-averages recent samples. The National Weather Service description gives a 12,600-foot measuring range and 12,000-foot reporting range, and notes that some thin cloud or moisture layers may not be reported.
Why a forecast can differ from the sky
A model grid cell covers more space than a garden, trail or airport sensor. Small cumulus clouds, narrow fog banks, coastlines and mountain slopes can produce sharp changes within it. Small forecast errors in humidity, temperature and vertical motion also alter cloud formation. Low cloud trapped beneath an inversion is a recognised modelling challenge. Humidity and Dew Point Explained and Temperature Explained cover those related fields.
Timing matters as much as the percentage. A model may predict the right band of cloud but move it through an hour too early or late. Cloud can also grow or dissolve between the forecast times shown. Check the hours on either side of your planned activity, and compare a fresh observation with the forecast when the start time approaches. How Weather Forecasts Work gives wider context on models and uncertainty.
Using cloud cover in practice
The percentage is a starting point for outdoor, night-sky and solar decisions. It does not replace rain, visibility, UV or irradiance forecasts.
Outdoor plans
Use the percentage as a description of the expected sky, then check the weather elements that matter for the activity. Cloud cover alone does not establish rain, poor visibility or safe ultraviolet exposure. Thin or broken cloud can still allow strong UV, so consult UV Index Explained. Low cloud touching the ground becomes fog and can sharply reduce visibility even when conditions a short distance away are clearer. Fog Explained and Visibility Explained cover those conditions.
Astronomy
Lower cloud cover usually gives more opportunities to see the night sky, but the percentage is only a first filter. Patchy cloud may leave one horizon open and hide your target; high thin cloud may blur faint objects even when bright planets or the full moon remain visible. Look at neighbouring hours, the direction of approaching cloud and recent satellite imagery. For a timed observation, a forecast gap is more useful than a low daily average. See What Is a Full Moon?
Solar energy
More or thicker cloud generally reduces direct sunlight, yet cloud percentage alone is not a power forecast. Optical thickness, the Sun’s height, gaps near the solar disc and the balance between direct and diffuse radiation all matter. Use a solar-irradiance or photovoltaic forecast when output matters; the European Commission’s PVGIS provides solar-radiation and photovoltaic information.
Reading cloud cover on Airpult
Airpult uses model cloud-cover data for a location’s sky summary and icon. Treat it as the best estimate for the displayed time, then use the hourly sequence to see whether cloud is increasing, clearing or changing quickly. Open Airpult Explore to find your forecast.
