The sky condition algorithm is calculated every 5 seconds based on data collected during the last 30 minutes. The last 10 minutes are double-weighted to make the algorithm more responsive to variations in cloudiness.
The following figure shows the operation of the sky condition algorithm. Instead of forming a histogram of hit heights, the algorithm uses both height and timing information to find hits close to each other. These hits are combined into clusters and a height value is calculated for each cluster. This height gives the base height of a cloud or cloud layer represented by the cluster. The algorithm reports layers by combining clusters whose heights are close to each other and then selecting clusters that cover the greatest amount of sky.
The sky condition algorithm reports the total sky cover in octas with values between 0 and 9. (One octa equals one eighth.) Value 9 is reported only in a vertical visibility condition. In addition, the sky cover can attain the value −99. The value −99 is reported after the start-up of the ceilometer indicating there is not enough data for algorithm processing.
| Volume | Sky cover |
|---|---|
| 0 – 8 | Cloudiness in octas |
| 9 | Vertical visibility (VV) |
| −99 | Not enough data |