Estimation of cloud amount

The total cloud cover is the fraction of the celestial dome covered by all the clouds observed. The term cloud amount in reference to a genus, a species, a variety, a layer, or a certain combination of clouds indicates the fraction of the sky cover by that genus, species, variety, layer or combination.

This fraction is represented by a figure equivalent to how many eighths or oktas of the sky is covered.

Oktas

Description

0         

sky completely clear

1         

from a trace of cloud up to 1/8

2         

more than 1/8 but not more than 2/8

3         

more than 2/8 but not more than 3/8

4         

more than 3/8 but not more than 4/8

5         

more than 4/8 but not more than 5/8

6         

more than 5/8 but not more than 6/8

7         

more than 6/8 but not total coverage i.e. if there is any sky visible then use 7/8

8         

sky completely overcast (no breaks or openings)

Note that cloud amounts are generally round up to the next okta. For example ‘2 and a bit’ oktas is rounded to 3 oktas. The exception is when more than 7 but less than 8 oktas is observed – in this instance cloud amount is rounded down to 7 oktas.

Dividing the sky into oktas (eighths) is fairly easy; one can imagine the sky divided into half, so that each half is equal to 4 oktas.  Dividing into half again gives an area of 2 oktas. It is then easy to see how much of the sky is equal to 1 okta. When the cloud is in a more or less continuous sheet or patch it is not difficult to assess its amount; however, if the cloud consists of several separated elements, it is necessary to imagine the amount of sky covered as if all the separate pieces were joined together.

An animation of the concept: (file size: 14MB )

Owing to the effects of perspective, gaps existing between clouds near the horizon may not be visible. When estimating amount in this instance, take into account only those gaps visible from the observing point.

Observing when the sky is partly obscured by fog, haze, etc

If cloud can be seen, estimate the amount as well as circumstances permit. If there is no evidence of cloud and the sun or stars can be seen through the fog, consider the sky to be completely clear of cloud.

Several layers of cloud

When clouds exist at different heights it may be necessary to determine the amount of cloud present at each level, even though the lower clouds may obscure some of the clouds in a higher layer.

With practice, and knowledge of the nature of the various cloud forms, the estimation of cloud amounts at different heights becomes relatively easy.  For example, if ragged low clouds exist below a layer of Nimbostratus it would be safe to assume the amount of Nimbostratus cloud as 8 oktas.  This can usually be confirmed by watching the sky for a short time, when the movement of the lower cloud will usually reveal any breaks or gaps that may exist in the higher layer. However do not make unconsidered guesses. The amount of cloud (or if necessary, the amount of each individual genera of cloud) at each level is determined as if no other clouds are present.

Assessing the amount during the night hours

Before commencing an observation at night, allow the eyes to become accustomed to the surrounding light.  The observation is then made in the same way as during the daylight hours.  On bright moonlight nights the observations can be made without difficulty.  When there is no moon it is more difficult but after conditioning the eyes to the dark, silhouettes of low cloud can be seen.  The blotting out of stars indicates the presence of clouds and clouds can be assumed with the presence of precipitation.  At times of thunderstorm activity, lightning may provide sufficient illumination to enable an estimate to be made.

Laser Ceilometer

Refer to the Laser Ceilometer section for details on the ceilometer's capability to provide cloud amount information.

Cloud amount for varying reporting requirements

While manual cloud observations performed for inclusion in METAR/SPECI reports consider all cloud within the celestial dome, information contained in other report and broadcast mechanisms may only consider the area associated with the probable arrival and departure flight paths of aircraft operating at the aerodrome. Observers must be familiar with any local requirements regarding their observations.

For dissemination to aeronautical users via the METAR/SPECI code and ATIS broadcast, the following conversions are applied for manually observed cloud amounts:

Oktas

Code

Decode

1 – 2

FEW

Few

3 – 4

SCT

Scattered

5 – 7

BKN

Broken

8

OVC

Overcast