SMHI-SAF Cloud Mask
Cloud Mask:Description and examples
Goal of the Cloud Mask product
This product attempts to delineate all absolutely cloud-free pixels in a
satellite scene with a high confidence. In addition, it will identify cloud
free snow or ice contaminated pixels when illumination allows.
Cloud Mask algorithm description
The cm algorithm is based on
a multi-spectral threshold technique
applied to each pixel of the satellite scene. The thresholding
algorithm, utilising all 5/6 spectral channels of the
avhrr/2 or
sensor (or the corresponding channels from the
sensor), nwp
short range forecast data, 1 km
gis (digital
elevation model and landuse) data and land emissivity maps. The scheme
makes use of off-line radiative transfer simulations (6S and
of cloud free atmospheres, to estimate, prior to the reception of
satellite data, the optimal thresholds valid for the given satellite
List of inputs for Cloud Mask
Satellite data:
0.6 microns, 0.9 microns, 11 microns,
12 microns and (1.6 or 3.7 microns),
from avhrr
or from viirs.
From viirs
also channel 8.5 microns.
Auxillary data:
surface temperature
950 hPa temperature
Column Integrated water vapour
Sun zenith, satellite zenith, sun-satellite azimuth difference
angle and the sun and satellite azimuth angle
1 km landuse data
1 km digital elevation map
Sea-ice data (from the Ocean and Sea ice
(optional)
Land emissivity data
Coverage and resolution
This product is available in full imager resolution and for the
whole field of view.
Description of Cloud Mask outputs
The Cloud Mask output consists of:
a six category main output
a quality flag
a dust/volcanic plume flag
In addition to the above, the output includes a threshold test flag,
for internal use only, identifying the decisive threshold tests.
Main output
Cloud Mask Classes
Class number Class name
Non-processed
Cloud Free
Cloud contaminated
Cloud Filled
Unclassified
The main output is given by the six categories listed above. Cloud
free land and cloud free sea are one and the same category, so in order
to make an output image like the above the user will have to apply a
land/sea mask herself, or use the information available in the quality
flag (bit number 0 - see below).
Quality flag
The quality flag occupies sixteen bits, and provide the user with
valuable information on the conditions under which the cloud mask
processing was performed, and on the quality of the thresholding. The
exact outline of the quality flag is given below.
Seven bits to describe the illumination and environmental conditions
under which the cloud mask was derived - numbers refer to the
individual bits:
Illumination & Environmental conditions
Bit number Meaning of the bit
Land/Not land
Coast/Not coast
Night/Not night
Twilight/Not twilight
Sunglint/No sunglint
High terrain/low terrain
Inversion/No inversion: Low level inversion present or not.
Two bits to describe the use/availability of
nwp data and
the availability of the
Missing data
Bit number Meaning of the bit
data not used.
One or more
channels missing.
Two bits to describe the quality of the thresholding - if the
measurement was close to one of the thresholds of the active test the
result will be assigned a low confidence:
Thresholding quality
Bit number Meaning of the bit
Low quality/High quality
Very low quality - Pixel has been reclassified after spatial smoothing
State before spatial smoothing was cloud contaminated
State before spatial smoothing was cloudy
Bit number nine is set (meaning low quality) when the value of a pixel
in some feature is close to the threshold determining the output.
Bit number ten is set when an isolated pixel has been changed from
cloudy to cloudfree (or vice-versa) after applying spatial
smoothing. This spatial smoothing will only be applied when the T11-T37
test was the one determining the output value.
Three bits for describing the sea ice:
Sea ice data
Bit number Meaning of the bit
External sea-ice information
used (e.g.
ice maps).
Ice information derived from
Sea-ice present according to
sea ice maps
(osisaf or
The sea ice schemes are used in case the ice concentration is higher then 30%.
Dust and Volcanic plume flag
The information on excesive aerosol content due to dust clouds or
volcanic plumes have been excluded from the main output, due to poor
separability using the spectral channel of the
Instead this
information is put in a separate output flag:
Dust and Volcanic Plume
Bit number Meaning of the bit
Non-processed (containing no data or corrupt data)
Not contaminated
Dust cloud
Unclassified (due to known separability problems)
Example of Cloud Mask visualisation
Unprocessed
Cloud free land
Cloud free sea
Cloud contaminated
Cloud filled
Snow/Ice contaminated
Unclassified
Low quality
Here are shown example images of the
Cloud Mask
product over northern Europe.
To the left it is the cloud mask only,
and to the right also the cloudfree pixels with low quality,
according to the processing flag bit number 9, are shown. The low
quality pixels are shown in purple.
The data are from a noaa19 overpass (orbit 16826) May 14, 2012,
11:25&UTC, as received and processed at
Norrk?ping, Sweden. The corresponding
rgb images using
channel 0.6&microm,0.9&microm,11&microm and
3.7&microm,11&microm,12&microm are shown below for comparison with
the Cloud Mask.
The cloud mask images shown here have been generated with the
nwcsaf/pps
software. They display only part of the information available in the
product. The pps
Cloud products are most appropriately visualised using a
dedicated Graphical User Interface
(gui) tool
allowing the display of all relevant information available in the
hdf5 file,
including flags, geographical location, etc.
has developed such a tool running under Unix and freely
available upon request. Otherwise the
hdf-viewer
(hdfview) available from
is recommended.Cloud | Definition of Cloud by Merriam-Webster
often attributive
: a white or gray mass in the sky that is made of many very small drops of water
: a large amount of smoke, dust, etc., that hangs in the air
: a large number of things (such as insects) that move together through the air in a group
Source: Merriam-Webster's Learner's Dictionary
Full Definition of cloud
a visible mass of particles of condensed vapor (as water or ice) suspended in the atmosphere of a planet (as the earth) or moon2
something resembling or suggesting a cloud: as a
a light filmy, puffy, or billowy mass seeming to float in the air &a cloud of blond hair& &a ship under a cloud of sail& b (1)
a usually visible mass of minute particles suspended in the air or a gas
an aggregation of usually obscuring matter especially in interstellar space
an aggregate of charged particles (as electrons) c
a great crowd or multitude :
&clouds of mosquitoes& 3
something that has a dark, lowering, or threatening aspect &clouds of war& &a cloud of suspicion&4
something that obscures or blemishes &a cloud of ambiguity&5
a dark or opaque vein or spot (as in marble or a precious stone)6
the computers and connections that support cloud computing &storing files in the cloud&
defined for English-language learners
defined for kids
Examples of cloud in a sentence
The sun is shining and there's not a cloud in the sky.flying high above the cloudsIt stopped raining and the sun poked through the clouds.a cloud of cigarette smokeThe team has been under a cloud since its members were caught cheating.There's a cloud of controversy
hanging over the election.
Illustration of cloud
Origin and Etymology of cloud
Middle English, rock, cloud, from Old English clūd; perhaps akin to Greek gloutos buttock
First Known Use: 14th century
Related to cloud
Synonyms , , Related Words ; , , , , , ; , ; , ,
Other Climate/Meteorology Terms
: to confuse (a person's mind or judgment)
: to make (something, such as an issue or situation) difficult to understand
: to affect (something) in a bad way
Source: Merriam-Webster's Learner's Dictionary
Full Definition of cloud
—usually used with over or up &clouded over before the storm&2 a
of facial features
to become troubled, apprehensive, or distressed in appearance &her face clouded with worry& b
to become blurry, dubious, or ominous —often used with over &the outlook is clouding over&3
to billow up in the form of a cloud1 a
to envelop or hide with or as if with a cloud b
to make opaque especially by condensation of moisture &steam clouded the windows& c
to make murky especially with smoke or mist &smoke clouded the sky&2
to make unclear or confused &cloud the issue&3
&a clouded reputation&4
to cast gloom over &cloud prospects for success&
defined for English-language learners
Examples of cloud in a sentence
greed clouding the minds of menThese new ideas only cloud the issue further.The final years of her life were clouded by illness.
First Known Use of cloud
Related to cloud
Synonyms , , , , , , , , , , , , , , , , , , Antonyms , , , , Related Words , , , , , , , ; , , , , , , Near Antonyms , , ,
Rhymes with cloud
, , , , , , ,
Phrases related to CLOUD
Related Phrases
CLOUD Defined for Kids
a visible mass of tiny bits of water or ice hanging in the air usually high above the earth 2
a visible mass of small particles in the air &a cloud of dust& 3
a large number of things that move together in a group &a cloud of mosquitoes& 4
an overwhelming feeling &The news cast a cloud of gloom.& 5
the computers and connections that support cloud computingcloudless \-l?s\
cloudedclouding
to make or become cloudy &The sky clouded up.& 2
to have a bad effect on &Involvement in the crime clouded his future.& 3
to make confused &Doubts clouded her judgment.&
Learn More about cloud
Thesaurus:
Spanish Central:
Britannica English:
Seen and Heard
What made you want to look up cloud? Please tell us where you read or heard it (including the quote, if possible).
a native of the northern United States
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