Traditionally cloud cover observations are made by dividing the sky into 8 regions (4 azimuthal quadrants and 2 zenithal elevations) and assessing whether each region contains any clouds. As a result cloud cover is generally reported in octas (eighths).
Simple observations are sufficient to provide an estimate of cloud cover in octas. Zero octas corresponds to a clear sky and eight octas corresponds to a completely cloud-covered sky. It has been noted that many observers over-estimate cloud cover in near clear-sky conditions, for various reasons.
| Octa | Percentage cover | Notes |
|---|---|---|
| 0 | 0% | Clear sky |
| 1 | 12% | |
| 2 | 25% | |
| 3 | 38% | |
| 4 | 50% | |
| 5 | 62% | |
| 6 | 75% | Broken cloud |
| 7 | 87% | |
| 8 | 100% | Unbroken cloud |
Cloud cover exerts a large influence on the earth's climate, and directly or indirectly affects the habits of many species. This includes both short-term (eg. activity levels, behaviours) and longer-term (flowering season, migration) effects.