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

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 must understand the differences

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6 Most Observation sites are not ideal

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

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 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. Important Cloud Height Points

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 Observer Sees (perceives) the sky to be.

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

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

19 Human OVC020CB

20 Machine OVC020

21 Human SCT020CB OVC250

22 Machine CLR

23 Human OVC020CB

24 Machine OVC020

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

41 Scattered Layers 2,000 4, 000 What would the METAR be? SCT020BKN040

42 Scattered Layers 4,000 8,000 What would the METAR be? SCT040BKN080

43 Scattered layers 4,000 8,000 2,000 What would the METAR be? SCT020BKN040OVC080 If clouds are moving, ASOS reports cloud levels quite accurately.

44 Observation is based on what the Observer Sees (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 Opaque Transparent Scattered Cloud Layers Transparent clouds are treated the same as opaque clouds. SCT020BKN040OVC080

ft Horizon ? Fog Visibility 1/2 Mi An Indefinite Ceiling of 200 ft is encoded as Vertical Visibility or ‘VV’ 1/2SM FGVV002

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

51 Horizon ? Part of the sky can be seen, or the sky is ‘Partially Obscured’ (A ‘Partial Obscuration’) Visibility 1/2 Mi If 3/8 - 4/8 of the sky is obscured, 1/2SM FGSCT000 But there is still a cloud, Where is it ???

52 Horizon ? Part of the sky can be seen, or the sky is ‘Partially Obscured’ (A ‘Partial Obscuration’) Visibility 1/2 Mi If 5/8 - 7/8 of the sky is obscured, 1/2SM FG BKN000 But there is still a cloud, Where is it ???

53 Visibility 1/2 Mi 1/2SM FGFEW000SCT040 What would the METAR be? 4,000

54 What would the METAR be? 4,000 8,000 1/2SM FGFEW000BKN080 Visibility 1/2 Mi SCT040

55 Transparent Opaque Remember, transparent clouds are treated same as opaque clouds. Visibility 1/2 Mi 1/2SM FGSCT000OVC080

56 Summary The Observation is based on what the Observer Sees (perceives) the sky to be.

57 Summary The OBS look similar, –but the user must understand the differences

58 Summary There are significant differences between human and machine OBS systems.

59 There is no such thing as bad weather, Just different types of good weather

60 Don’t Ignore Mother Nature’s Warning signs. Don’t bother me with the facts.

61 The End