Type 1 (cirrus in the
form of filaments, strands, or hooks):
Cirrus clouds form very high in the atmosphere, usually between
15,000 and 30,000 feet above ground, and occur year-round in the
Keys. Type 1 cirrus are very thin and whispy, and are usually
in the shape of strands and hooks, or looking like feathers or
horse tails. They do not give any significant indication
of approaching weather.
Type 2 (combination of
dense cirrus, cirrus with sproutings, and cirrus in tufts):
Type 2 cirrus are thicker, and often appear as white puffs or
sheaves, sometimes with wispy edges. They indicate that
there is deep moisture high in the atmosphere.
Type
3 (dense cirrus remaining from a CB anvil):
Type 3 cirrus clouds are usually very thick, and result when CB
clouds produce anvil-shaped tops, then dissipate, leaving the
anvils behind. For a while these cirrus will maintain their
shape, then they will gradually spread out and become more diffuse.
Type 3 cirrus clouds mean that (low cloud) type 9 CB were previously
present, and generally indicate deep moist and unstable conditions.
Type 4 (cirrus in hooks
or filaments, progressively invading the sky and growing denser):
The only difference between type 4 cirrus and type 1 cirrus are
that type 4 cirrus are progressively invading the sky. As
discussed on the middle clouds page, progressively invading high
or middle clouds can sometimes indicate an impending change in
weather, such as an approaching cold front or storm system.
However, many times they simply indicate a pocket of moist air
passing high overhead, or some other unremarkable phenomenon.
Type 5 (cirrus and/or
cirrostratus [CS] progressively invading the sky, veil extends
less than 45 degrees above the horizon):
Cirrostratus (CS) clouds vary in thickness, cover large areas,
and are generally shapeless and diffuse except at their edges. Sometimes
they are thick enough to hide the sun or moon, and other times
are so thin that they are only visible because they change the
sky color (as in the second photo below type 7 CS). They
indicate a large area of moist air at high levels, and when thick
are sometimes are associated with spreading from the tops of large
storm complexes. Type 5 cirrus/CS are progressively invading
the sky, but are still relatively distant because they extend
less than 45 degrees above the horizon.
Type 6 (cirrus and/or
CS progressively invading the sky, veil extends more than 45 degrees
above the horizon):
Type 6 cirrus/CS are identical to type 5 except that they extend
more than 45 degrees above the horizon, and can sometimes cover
almost all of the sky. However, they cannot cover the entire
sky, otherwise they are classified as type 7 CS.
Type 7 (CS covering the
entire sky):
Type 7 CS are similar to type 5 and type 6, except that they cover
the entire sky. Because of this, they can no longer be identified
as "progressively invading".
Type 8 (CS not invading
or covering the entire sky):
Type 8 CS do not cover the entire sky, but are also not progressively
invading the sky. In other words, they are usually either
moving by without getting closer, moving away, or dissipating.
Type 9 (cirrocumulus as
the predominant high cloud present):
Cirrocumulus clouds often appear as thin white areas of ripples
or small puffs. They can resemble AC clouds, except that
the individual cloud elements (puffs) are much smaller.
The pictures below both show type 9 cirrocumulus (smaller elements)
above (mid-cloud) type 7 AC (larger elements). Cirrocumulus
indicate an unstable layer high up in the atmosphere, but do not
necessarily portend the approach of any significant weather.
Low Clouds
Middle Clouds