0 0 ON DIRECT SUN
1 1 ON DIRECT MOON
2 2 ON BLUE ZENITH SKY
3 3 ON ZENITH CLOUD (UNIFORM STRATIFIED LAYER OF SMALL OPACITY)
4 4 ON ZENITH CLOUD (UNIFORM OR MODERATELY VARIABLE LAYER OF MEDIUM OPACITY)
5 5 ON ZENITH CLOUD (UNIFORM OR MODERATELY VARIABLE LAYER OF LARGE OPACITY)
6 6 ON ZENITH CLOUD (HIGHLY VARIABLE OPACITY, WITH OR WITHOUT PRECIPITATION)
7 7 ON ZENITH CLOUD (FOG)
8 8 ON ZENITH HAZE
9 9 ON DIRECT SUN THROUGH THIN CLOUD, FOG OR HAZE
15 15 MISSING VALUE
