By: Bob Jackson, MIC Seattle CWSU

1. Cloud Heights and Sky Cover Observed And Perceived By: Bob Jackson, MIC Seattle CWSU

2. Observation is based on what the Observer Sees (perceives)the sky to be.

3. There are significant differences between human and machine OBS systems.

4. The OBS look similar,but the user mustunderstand the differences

6. Most Observation sites are not ideal

9. Cloud Height Indicator (CHI)uses aLaser Beam Ceilometer Same as used to get cloud heights before Automated OBS.

10. A human Observer sees from horizon to horizon.

11. The CHI looks straight up. It assumes that the clouds are moving. It performs well when clouds are moving.

12. Cloud Height/Cloud Coverage • Laser Beam Ceilometer (LBC) • Takes cloud reading every 30 seconds. • Reports up to three cloud layers. • Reports clouds up to 12,000 ft.

13. Important Cloud Height Points • No ‘Partial Obscurations’ reported • Beam may be attenuated in heavy precip, so • a ‘False’ scattered layer may be reported. • Continuous monitoring • Rapid response to changing conditions, which surpasses most human observers.

14. Important Cloud Height Points • Stationary or slow moving SCT clouds may be reported as BKN or OVC. • May detect moisture layers or temperature inversions, and give false ceilings when sky is actually clear. • Night-time OBS are usually better due to human’s night vision problems.

15. Reporting FOG Patches • ASOS reports higher visibility in clear air. • ASOS reports lower visibility in fog “patch”. • ASOS will report continuous low visibility in a stationary fog bank, while • Manual visibility will often be much higher in a stationary fog bank.

16. The observation is based on what the ObserverSees(perceives) the sky to be.

17. OVC010 Cloud layer Seen as a STRATUS deck to someone in the valley.

18. 1/2SM FG VV002 Cloud layer Same layer seen as FOG to someone on the mountain.

19. OVC020CB Human

20. OVC020 Machine

21. Human SCT020CB OVC250

22. Machine CLR

23. OVC020CB Human

24. Machine OVC020

25. A Ceiling is when more than 1/2 of the sky is obscured.

27. Human would observe: SCT020 Advancing cloud layer 2,000 ft above ground. o 90 Horizon

28. Machine sees no clouds in this example. Machine would observe: CLR Advancing cloud layer 2,000 ft above the ground. o 90

29. Machine would observe: Machine sees no clouds. CLR Advancing cloud layer 2,000 ft above the ground. o 90 Horizon

30. Machine would observe: Now Machine sees clouds. BKN020 Advancing cloud layer 2,000 ft above the ground.

31. Machine would then observe: Machine sees clouds and will take 10 minutes to go between CLR and OVC. OVC020 Advancing cloud layer 2,000 ft above the ground.

32. A human would report: SCT020 SCT080 8,000 ft Solid Layer 2,000 ft Solid Layer O 90

33. A machine would report: CLR 8,000 ft Solid Layer 2,000 ft Solid Layer O 90

34. A human would report: SCT020 OVC080 8,000 ft Solid Layer 2,000 ft Solid Layer

35. A machine would report: CLR 8,000 ft Solid Layer IF clouds are not moving. 2,000 ft Solid Layer

36. What would a HUMAN report?? SCT020 BKN080 8,000 ft Solid Layer 8,000 ft Solid Layer 2,000 ft Solid Layer

37. What would a MACHINE report?? BKN080 8,000 ft Solid Layer 8,000 ft Solid Layer 2,000 ft Solid Layer

38. What would a MACHINE report?? After 10 min. OVC080 8,000 ft Solid Layer 8,000 ft Solid Layer 2,000 ft Solid Layer

39. What would a MACHINE report?? SCT020 OVC080 8,000 ft Solid Layer 2,000 ft Solid Layer

40. What would a MACHINE report?? OVC020 8,000 ft Solid Layer 2,000 ft Solid Layer

41. What would the METAR be? SCT020 BKN040 Scattered Layers 4,000 2,000

42. What would the METAR be? 8,000 Scattered Layers SCT040 BKN080 4,000

43. What would the METAR be? 8,000 Scattered layers If clouds are moving, ASOS reports cloud levels quite accurately. 4,000 SCT020 BKN040 OVC080 2,000

44. Observation is based on whatthe ObserverSees(perceives)the sky to be. (A Subtle Reminder)

45. What would the METAR be? If LESS than 1/8, a human would observe: OVC030

46. What would the METAR be? If MORE than 1/8, a human would observe: BKN030

47. ASOS would report: SCT030

48. Scattered Cloud Layers Opaque SCT020 BKN040 OVC080 Transparent Transparent clouds are treated the same as opaque clouds. Transparent

49. Fog An Indefinite Ceiling of 200 ft is encoded as Vertical Visibility or ‘VV’ 1/2SM FG VV002 Visibility 1/2 Mi 200 ft Horizon?

50. Part of the sky can be seen, or the sky is ‘Partially Obscured’ (A ‘Partial Obscuration’) But there is still a cloud, Where is it ??? If > 0 - 2/8 of the sky is obscured, 1/2SM FG FEW000 Visibility 1/2 Mi Horizon?