By bob jackson mic seattle cwsu
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Cloud Heights and Sky Cover. Observed And Perceived. By: Bob Jackson, MIC Seattle CWSU. Observation is based on what the Observer Sees (perceives) the sky to be. There are significant differences between human and machine OBS systems.

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By: Bob Jackson, MIC Seattle CWSU

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By bob jackson mic seattle cwsu

Cloud Heights

and

Sky Cover

Observed

And

Perceived

By:

Bob Jackson,

MIC Seattle CWSU


By bob jackson mic seattle cwsu

Observation is based on what

the Observer Sees (perceives)the sky to be.


There are significant differences between human and machine obs systems

There are significant differences between human and machine OBS systems.


The obs look similar but the user must understand the differences

The OBS look similar,but the user mustunderstand the differences


By bob jackson mic seattle cwsu

Most Observation sites

are not ideal


Cloud height indicator chi uses a laser beam ceilometer

Cloud Height Indicator (CHI)uses aLaser Beam Ceilometer

Same as used to get

cloud heights before

Automated OBS.


By bob jackson mic seattle cwsu

A human Observer sees

from horizon to horizon.


By bob jackson mic seattle cwsu

The CHI looks

straight up.

It assumes that the

clouds are moving.

It performs well when

clouds are moving.


Cloud height cloud coverage

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.


Important cloud height points

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.


Important cloud height points1

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.


Reporting fog patches

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.


By bob jackson mic seattle cwsu

The observation is based on what

the ObserverSees(perceives) the sky to be.


By bob jackson mic seattle cwsu

OVC010

Cloud layer

Seen as a STRATUS deck

to someone in the valley.


By bob jackson mic seattle cwsu

1/2SM FG VV002

Cloud layer

Same layer seen as FOG to

someone on the mountain.


By bob jackson mic seattle cwsu

OVC020CB

Human


By bob jackson mic seattle cwsu

OVC020

Machine


By bob jackson mic seattle cwsu

Human

SCT020CB OVC250


By bob jackson mic seattle cwsu

Machine

CLR


By bob jackson mic seattle cwsu

OVC020CB

Human


By bob jackson mic seattle cwsu

Machine

OVC020


A ceiling is when more than 1 2 of the sky is obscured

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


By bob jackson mic seattle cwsu

Human would observe:

SCT020

Advancing cloud layer

2,000 ft above ground.

o

90

Horizon


By bob jackson mic seattle cwsu

Machine sees no clouds

in this example.

Machine would observe:

CLR

Advancing cloud layer

2,000 ft above the ground.

o

90


By bob jackson mic seattle cwsu

Machine would observe:

Machine sees no clouds.

CLR

Advancing cloud layer

2,000 ft above the ground.

o

90

Horizon


By bob jackson mic seattle cwsu

Machine would observe:

Now Machine sees clouds.

BKN020

Advancing cloud layer

2,000 ft above the ground.


By bob jackson mic seattle cwsu

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.


By bob jackson mic seattle cwsu

A human would

report:

SCT020 SCT080

8,000 ft Solid Layer

2,000 ft Solid Layer

O

90


By bob jackson mic seattle cwsu

A machine would

report:

CLR

8,000 ft Solid Layer

2,000 ft Solid Layer

O

90


By bob jackson mic seattle cwsu

A human would

report:

SCT020 OVC080

8,000 ft Solid Layer

2,000 ft Solid Layer


By bob jackson mic seattle cwsu

A machine would

report:

CLR

8,000 ft Solid Layer

IF clouds are not moving.

2,000 ft Solid Layer


By bob jackson mic seattle cwsu

What would a HUMAN report??

SCT020

BKN080

8,000 ft Solid Layer

8,000 ft Solid Layer

2,000 ft Solid Layer


By bob jackson mic seattle cwsu

What would a MACHINE report??

BKN080

8,000 ft Solid Layer

8,000 ft Solid Layer

2,000 ft Solid Layer


By bob jackson mic seattle cwsu

What would a MACHINE report??

After 10 min.

OVC080

8,000 ft Solid Layer

8,000 ft Solid Layer

2,000 ft Solid Layer


By bob jackson mic seattle cwsu

What would a MACHINE report??

SCT020 OVC080

8,000 ft Solid Layer

2,000 ft Solid Layer


By bob jackson mic seattle cwsu

What would a MACHINE report??

OVC020

8,000 ft Solid Layer

2,000 ft Solid Layer


By bob jackson mic seattle cwsu

What would the METAR be?

SCT020

BKN040

Scattered Layers

4,000

2,000


By bob jackson mic seattle cwsu

What would the METAR be?

8,000

Scattered Layers

SCT040

BKN080

4,000


By bob jackson mic seattle cwsu

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


Observation is based on what the observer sees perceives the sky to be

Observation is based on whatthe ObserverSees(perceives)the sky to be.

(A Subtle Reminder)


By bob jackson mic seattle cwsu

What would

the METAR be?

If LESS than 1/8,

a human would observe:

OVC030


By bob jackson mic seattle cwsu

What would

the METAR be?

If MORE than 1/8,

a human would observe:

BKN030


By bob jackson mic seattle cwsu

ASOS would report:

SCT030


By bob jackson mic seattle cwsu

Scattered Cloud Layers

Opaque

SCT020

BKN040

OVC080

Transparent

Transparent clouds

are treated the same

as opaque clouds.

Transparent


By bob jackson mic seattle cwsu

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?


By bob jackson mic seattle cwsu

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?


By bob jackson mic seattle cwsu

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 3/8 - 4/8 of the sky is obscured,

1/2SM FG

SCT000

Visibility

1/2 Mi

Horizon?


By bob jackson mic seattle cwsu

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 5/8 - 7/8 of the sky is obscured,

1/2SM FG

BKN000

Visibility

1/2 Mi

Horizon?


By bob jackson mic seattle cwsu

What would

the METAR be?

1/2SM FG

FEW000

SCT040

4,000

Visibility

1/2 Mi


By bob jackson mic seattle cwsu

8,000

8,000

1/2SM FG

FEW000

SCT040

BKN080

4,000

What would

the METAR be?

Visibility

1/2 Mi


By bob jackson mic seattle cwsu

Opaque

Transparent

Remember, transparent

clouds are treated same

as opaque clouds.

1/2SM FG

SCT000

OVC080

Visibility

1/2 Mi


Summary

Summary

  • TheObservation is based on what the ObserverSees(perceives) the sky to be.


Summary1

Summary

  • The OBS look similar,

    • but the user mustunderstand the differences


Summary2

Summary

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


By bob jackson mic seattle cwsu

There is no such thing as

bad weather,

Just different types of

good weather


By bob jackson mic seattle cwsu

Don’t Ignore

Mother Nature’s

Warning signs.

Don’t bother me

with the facts.


By bob jackson mic seattle cwsu

The

End


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