Sky condition algorithm description
The SkyVue PRO sky condition algorithm follows guidance from ICAO and WMO documents1 and is based on 30 minutes of data. For this reason, sky condition is not available for 30 minutes after power cycling the sensor, a reboot, or changing measurement parameters. The sky condition algorithm is as follows:
Each measured hit comprising the lowest cloud height, (vertical visibility + onset of backscatter) / 2 and a weighting of 1 or 2 is stored in a running half hour buffer.
The weighting is 2 for hits in the last 10 minutes and 1 for hits in the previous 20 minutes.
If >50% of the hits in the latest 10 minutes are vertical visibility, then the vertical visibility averaged over the last 10 minutes is reported. Only hits below the vertical visibility height limit set are used.
The buffer is sorted in height order and hits combined into bins with the following widths:
|
Height |
Bin width |
|---|---|
| 0 – 5000 ft (0 – 1524 m) |
100 ft (30.5 m) |
| 5000 – 15,000 ft (1524 – 4572 m) |
200 ft (61 m) |
| 15,000 – 26,250 ft (4572 – 8000 m) |
500 ft (152 m) |
For each bin a mean height is calculated by the following:
Hi = ∑(hiwi)/∑(wi)
Where wi is the weight (1 or 2) of the hit with height hi
Bins are then reduced. A distance (D) between adjacent bins is calculated by the following equation:
D = NiNj (Hi-Hj)2/Ni+Nj
Where Ni and Nj are the sum of hits of each bin and Hi and Hj the respective heights.
Find the pair with the minimum distance D and combine into one bin with the height of the lowest and number of hits of the combined total.
This process is repeated until 5 or less bins remain. These are now taken as cloud layers.
Finally, if cloud layers are closer than the distances below, then they are merged with the height of the lowest and combined total number of hits.
|
Height range |
Distance (D) |
|---|---|
| Height ≤ 300 m (1000 ft) |
≤ 90 m (300 ft) |
| 300 m < Height ≤ 900 m (1000 ft < Height ≤ 3000 ft) |
≤ 120 m (400 ft) |
| 900 m < Height ≤ 1500 m (3000 ft < Height ≤ 5000 ft) |
≤ 180 m (600 ft) |
| 1500 m < Height ≤ 2400 m (5000 ft < Height ≤ 8000 ft) |
≤ 300 m (1000 ft) |
| Height > 2400 m (8000 ft) |
≤ 480 m (1600 ft) |
Weights per layer are used to calculate number of oktas per layer.
If the total possible is Wmax, then the cover is (Wi/Wmax) x 8.
Cloud amounts are rounded up to the next highest whole oktas.
For layers above the lowest this is based on the maximum possible weights. If the lower layers have W1 hits and the next layer has W2 hits, then the cover of the higher layer is given by:
(W2/(Wmax-W1)) x 8
For example, taking weighting into account, if lower layers of cloud only allow 4/8 of clear sky above and the cover is 2/8 out of those 4/8, then the layer above is taken as 4/8.
Finally, only layers meeting the following criteria are reported:
Layer 1 ≥ 1/33 oktas
Layer 2 ≥ 3 oktas
Layer 3 ≥ 5 oktas
Layer 4 ≥ 7 oktas
Layer 5 ≥ 7 oktas
Any layer > (8 – 1/33) oktas is reported as 8 oktas