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| Washington, D.C. The
U.S. Weather Bureau station (~1941) at the
National Airport. Cloud heights being measured
by means of a daylight ceiling projector. |
NWS Cloud Chart
Back in the olden days', prior to the availability of
high-resolution satellite images, the weather observer
would identify the types of clouds present and estimate
their height as part of his/her weather observation.
From those sky condition observations, symbols
representing cloud types were plotted on weather maps
where the forecaster would analyze the data to determine
the location of various weather systems.
Knowing the type of cloud(s) provided vital information
about the state of the atmospheric at each location.
Basically, the clouds told a weather story to the
meteorologist.
In 1930's, the National Weather Service standardized
codes for cloud forms and state of the sky according to
the international system of classification. From there,
charts were created describing this state of the sky. |
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Produced in conjunction with NASA, the NWS Cloud Chart
is a picture of the state of the sky. The chart displays
a mixture of individual clouds and combinations of
clouds. This is all to provide a picture of the state of
the atmosphere at the time of observation.
For example, an overcast layer of Stratus (St) clouds
mean the atmosphere is stable implying little change in
the current conditions for the next few hours.
Cirrostratus (Cs) that is increasing in sky coverage and
thickness implies a change from fair weather to possibly
rainy or snowy conditions. |
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Some cloud classifications on the Cloud Chart represents
the same cloud type but in different stages of
development or in the amount of sky cover. For example,
Cumulus (Cu) clouds of little vertical extinct are
classified as a 'Low 1' but are classified as 'Low 2' if
there is moderate vertical growth. A Cumulonimbus (Cb)
without a visible anvil is classified 'Low 3' but with
an anvil it is then classified as a 'Low 9' cloud.
One thing to remember, clouds are identified based upon
your observation point at your elevation. From
sea-level, one might observe Altocumulus clouds over the
top of a mountain. However, if you were on that mountain
top and observing that same cloud you would likely
report Stratocumulus. Therefore, two observers looking
at the same cloud can often report different cloud
classifications. |
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The exception to this is cumulus or
cumulonimbus clouds over a mountain. Even though these
low-level clouds typically have bases under 6,500 feet
(2,000 meters) above ground level, if you were to
observe them over a mountain top, their base might be
12,000 feet (3,600 meters) or higher relative to your
location. However, over the mountain the base of those
clouds would be well within 6,500 feet (2,000 meters)
above ground at their location so they could still be
called Cumulus or Cumulonimbus clouds seen from your
location.
The Cloud Chart shows examples of each of these 27
categories. Today, the 27 different cloud symbols for
the 'state of the sky' are not plotted on surface maps
in the United States. As weather observations became
computerized in the 1990's, the automated observing
systems can only detect cloud height; not cloud type.
However, weather observations by humans continue in most
of the world and include the 'state of the sky' using
these symbols. |
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 National
Weather Service: NWS Cloud Chart |
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