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

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By:Bob Jackson, MIC Seattle CWSU

Cloud Heightsand

Sky CoverObserved

AndPerceived

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Observation is based on whatthe Observer Sees

(perceives)the sky to be.

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There are significant differences between

human and machine OBS

systems.

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The OBS look similar,but the user must

understand the differences

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Most Observation sites are not ideal

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Same as used to getcloud heights beforeAutomated OBS.

Cloud Height Indicator (CHI)uses a

Laser Beam Ceilometer

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A human Observer seesfrom horizon to horizon.

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The CHI looksstraight up.

It assumes that the clouds are moving.

It performs well whenclouds are moving.

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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.

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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.

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• 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

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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.

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The observation is based on what the Observer Sees (perceives)

the sky to be.

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Cloud layer

OVC010

Seen as a STRATUS deck to someone in the valley.

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Cloud layer

Same layer seen as FOG to someone on the mountain.

1/2SM FG VV0021/2SM FG VV002

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Human OVC020CBOVC020CB

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Machine OVC020OVC020

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Human

SCT020CB OVC250SCT020CB OVC250

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Machine

CLR

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Human OVC020CBOVC020CB

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Machine

OVC020OVC020

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

obscured.

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Advancing cloud layer2,000 ft above ground.

90Horizon

Human would observe:

o

SCT020

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Advancing cloud layer2,000 ft above the ground.

90o

Machine sees no clouds in this example.

CLR

Machine would observe:

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90Horizon

o

Machine sees no clouds.CLR

Advancing cloud layer2,000 ft above the ground.

Machine would observe:

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BKN020Now Machine sees clouds.

Advancing cloud layer2,000 ft above the ground.

Machine would observe:

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OVC020Machine sees clouds and will take 10 minutes togo between CLR and OVC.

Advancing cloud layer2,000 ft above the ground.

Machine would then observe:

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8,000 ft Solid Layer

2,000 ft Solid Layer

90O

A human would report:

SCT020 SCT080

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8,000 ft Solid Layer

2,000 ft Solid Layer

90O

A machine would report:

CLR

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8,000 ft Solid Layer

2,000 ft Solid Layer

A human would report:

SCT020 OVC080

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A machine would report:

CLR

IF clouds are not moving.8,000 ft Solid Layer

2,000 ft Solid Layer

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8,000 ft Solid Layer

2,000 ft Solid Layer

8,000 ft Solid Layer

What would a HUMAN report??

SCT020 BKN080

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8,000 ft Solid Layer

2,000 ft Solid Layer

8,000 ft Solid Layer

What would a MACHINE report??

BKN080

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8,000 ft Solid Layer

2,000 ft Solid Layer

8,000 ft Solid Layer

What would a MACHINE report??

OVC080

After 10 min.

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SCT020 OVC080

2,000 ft Solid Layer

8,000 ft Solid Layer

What would a MACHINE report??

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2,000 ft Solid Layer

8,000 ft Solid Layer

What would a MACHINE report??

OVC020

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Scattered Layers

2,000

4,000

What would the METAR be?SCT020 BKN040

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Scattered Layers

4,000

8,000What would the METAR be?

SCT040 BKN080

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Scattered layers

4,000

8,000

2,000

What would the METAR be?

SCT020 BKN040 OVC080

If clouds are moving,ASOS reports cloudlevels quite accurately.

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Observation is based on what

the Observer Sees (perceives)

the sky to be.(A Subtle Reminder)

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

OVC030

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What would the METAR be?

If MORE than 1/8,a human would observe:

BKN030

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ASOS would report:

SCT030SCT030

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Opaque

Transparent

Transparent

Scattered Cloud Layers

Transparent cloudsare treated the sameas opaque clouds.

SCT020 BKN040 OVC080

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200 ftHorizon ?

Fog

Visibility1/2 Mi

An Indefinite Ceiling of 200 ft isencoded as Vertical Visibility or ‘VV’

1/2SM FG VV002

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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

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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 FG SCT000

But there is still a cloud,Where is it ???

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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 ???

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Visibility 1/2 Mi

1/2SM FG FEW000 SCT040What would the METAR be?

4,000

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What would the METAR be?

4,000

8,000 8,000

1/2SM FG FEW000 BKN080

Visibility 1/2 Mi

SCT040

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Transparent Opaque

Remember, transparentclouds are treated same as opaque clouds. Visibility

1/2 Mi

1/2SM FG SCT000 OVC080

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Summary

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

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Summary

• The OBS look similar, – but the user must

understand the differences

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Summary

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

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There is no such thing as bad weather,

Just different types of good weather

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Don’t Ignore Mother Nature’s Warning signs.

Don’t bother mewith the facts.

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The

End