A forecast showing 70% cloud cover looks straightforward, yet it is easy to misread. It does not mean rain for 70% of the day, exactly 70% of the time without sunshine, or clouds over 70% of your town. It is an estimate of the fraction of the sky covered by cloud over the stated point or area and during the stated period.
Read it with the time, location, product type and other weather variables. Cloud cover helps describe how the sky may look, but it does not replace precipitation probability, visibility, UV information or an official warning.
What the percentage represents
Imagine the sky dome divided into one hundred parts as seen from one place. Around 25% means mostly clear with some cloud; 50% suggests cloud and clear sky occupy similar fractions; close to 100% means almost or completely overcast. This is primarily a spatial description, not a stopwatch.
Weather observers also use oktas, dividing the sky into eighths. The Met Office guide to measuring cloud runs from 0 oktas for no cloud to 8 for complete cover. Code 9 means the sky is obscured by fog or another phenomenon; it is not an amount beyond overcast.
| Approximate percentage | Indicative oktas | Practical reading |
|---|---|---|
| 0% | 0 | Clear |
| 10–25% | 1–2 | Few clouds |
| 35–65% | 3–5 | Cloud and sunny spells |
| 75–90% | 6–7 | Mostly cloudy, some breaks |
| 100% | 8 | Overcast |
This table is a guide, not a universal conversion for every provider. An app may round values, summarise several hours or average a model grid cell.
Total cloud and separate layers
High cirrus may sit above low stratus while gaps remain in parts of both layers. Total cloud cover is the fraction obscured by cloud of any type when viewed from below. Layer amounts cannot simply be added because clouds may overlap vertically. Two layers at 60% therefore do not necessarily make 120%, or even guarantee overcast conditions.
Height and thickness matter as much as area. Thin high cloud can transmit substantial daylight; a thick low layer can make the surface gloomy at the same percentage. For aviation, mountain trips or photography, cloud base, ceiling and type may matter more than the total.
Why overcast does not guarantee rain
Clouds consist of water droplets or ice crystals, but not every cloud produces precipitation that reaches the ground. You can have 100% cover from dry stratus or high cloud without rain. Conversely, one isolated cumulonimbus can produce a heavy shower when total cover is low.
Check the chance of rain and its time window, forecast rainfall amount and, close to the event, radar. Cover, probability, amount and intensity answer different questions.
It is not the same as sunshine duration
Fifty per cent cloud cover over six hours does not promise exactly three hours of sunshine. Cloud may remain in one half of the sky, repeatedly pass across the sun, or form a thin veil that dims rather than blocks it. Solar position, transparency and cloud movement all change conditions at the ground.
For light-sensitive plans, also inspect UV or radiation information, the hourly trend and cloud animation if available. Relevant ultraviolet radiation can still reach the surface through cloud.
How to read a cloud-cover map
- Read the legend. Is it total, low, medium or high cloud?
- Check time and date. Separate satellite observation from model forecast.
- Follow the sequence. Animation reveals whether breaks widen, a layer advances or cloud stalls.
- Consider scale. A grid-cell value may summarise an area, so the sky over one street can differ.
- Combine variables. Add precipitation, visibility, wind and warnings as your activity requires.
Cloud maps make more sense in the wider weather pattern. Our guide to reading isobars and fronts helps relate cloud bands to pressure systems without assuming automatic impacts.
A decision example
Suppose an outdoor photo session from 17:00 to 19:00 has 80% cloud cover, a 10% chance of rain and mainly high cloud. A sensible reading is “few clear gaps and probably filtered light”, not “rain for 80% of the time”. If the product instead shows low cloud, poor visibility and rising rain probability, the practical situation changes even though total cover remains 80%.
Common mistakes
- Confusing cloud cover with rain probability.
- Adding percentages from different layers.
- Treating a daily average as a constant value.
- Assuming every cloud blocks the same amount of light.
- Ignoring timestamp, resolution and update time.
Sources and responsible use
This is an educational guide, not a current warning. The AEMET Visual Meteorological Glossary and the Met Office observation guide provide further background. For safety-sensitive decisions, use forecasts and warnings from the competent national weather service.