Some Synoptic Code Symbol Descriptions

0 : No or very few clouds
1 : 1/8 coverage (1 octa)
2 : 2/8 coverage (2 octas)
3 : 3/8 coverage (3 octas)
4 : 4/8 coverage (4 octas)
5 : 5/8 coverage (5 octas)
6 : 6/8 coverage (6 octas)
7 : 7/8 coverage (7 octas)
8 : Overcast, perhaps with small breaks
9 : sky obscured - no discernible ceiling

C_L - Low Clouds

0 : No low clouds (no symbol).
1 : Cumulus with little vertical extent or flattened (humilis, aka fair weather Cumulus), ragged Cumulus not of bad weather, or both.
2 : Cumulus with quite significant vertical development generally of domes (congestus) or towers. Other Cumulus and/or Stratocumulus with similar bases may be present.
3 : Cumulonimbus with summits which at least partially lack sharp outlines, but are distinctly neither cirriform nor anvil-shaped. Cumulus, Stratocumulus, and/or Stratus may be present.
4 : Stratocumulus formed from spreading out of Cumulus. Cumulus often present also.
5 : Stratocumulus not formed from spreading out of Cumulus.
6 : Stratus and/or Fractostratus not of bad weather.
7 : Stratus and/or Fracto{stratus/cumulus} of bad weather (scud).
8 : Cumulus and Stratocumulus with different bases, not because of spreading out of Cumulus.
9 : Cumulonimbus with a clearly fibrous (cirriform) top, often anvil-shaped - aka thunderheads. Other low clouds may be present.
/ : Low clouds invisible because of darkness, fog, blowing snow, dust or sand, or other similar obscurations.

Notes : The description for type 1, and comparison of those for types 6 & 7 illustrate well what "of bad weather" implies. 1 are puffy Cumulus during a nice, generally sunny day. A low Stratus cloud layer with fog and drizzle is 6. Drizzle is precipitation, but not like that associated with strong storms with which type 7 are common. 7 is actually seldom reported because they often occur beneath 3 and especially 9. It is not clear to me if a low stratus deck with continuous rain or snow is intended to be 6 or 7. I believe the official classification does not even include stratus in type 7 - yet they do occur with bad weather, and that seems to me to be the relevant distinction among 6 & 7. Though the word tower is mentioned in 2, these are more what I consider Cumulus Congestus. 3 are more what I consider Towering Cumulus (or very tall and narrow 2's), though often with sharp cloud edges.

C_M - Middle Clouds

0 : No middle clouds (no symbol).
1 : Thin Altostratus, mostly semi-transparent. The sun or moon may be visible.
2 : Altostratus, mostly opaque enough for hiding the sun or moon; or Nimbostratus.
3 : Thin Altocumulus, mostly semi-transparent; cloud elements not changing much and at one basic height.
4 : Thin, patchy Altocumulus (often almond, or fish-shaped), mostly semi-transparent; cloud elements greatly changing and/or occurring at more than one height.
5 : Thin Altocumulus, in bands or a layer gradually overspreading sky and generally thickening.
6 : Altocumulus formed from spreading out of Cumulus or Cumulonimbus.
7 : Altocumulus in a thick or multiple layer(s), not increasing; or Altocumulus with Altostratus and/or Nimbostratus.
8 : Altocumulus (castellanus) with cumuliform tufts or turrets.
9 : Altocumulus of a chaotic sky, usually at several levels. Patches of dense cirrus usually present.
/ : Middle clouds invisible because of darkness, fog, blowing snow, dust or sand, or other similar obscurations; or more often blocked because of a continuous layer of lower clouds.

Notes : No exclusive category exists for Nimbostratus, which are among the most common rain clouds ! 2 can either be those or thick Altostratus, but a huge difference exists. Nimbostratus are actually low clouds, which can cause several inches of rain or a couple feet of snow during a day. A desirable aspect of classifying these as middle clouds (perhaps the original intention) is that a fractus layer of low type 7 can also be reported.

C_H - High Clouds

0 : No high clouds (no symbol).
1 : Cirrus filaments (mare's tails); scattered and not increasing.
2 : Dense Cirrus patches or twisted sheaves, usually not increasing; sometimes like Cumulonimbus remnants, towers, or tufts.
3 : Dense Cirrus, often anvil-shaped; formed from or associated with Cumulonimbus.
4 : Cirrus, often hook-shaped, gradually overspreading sky and usually thickening.
5 : Cirrus and Cirrostratus, often in converging bands; or Cirrostratus (alone) generally overspreading sky and thickening, the continuous layer in lower part of sky (below 45° altitude).
6 : Cirrus and Cirrostratus, often in converging bands; or Cirrostratus (alone) generally overspreading sky and thickening, the continuous layer in upper part of sky (above 45° altitude).
7 : Cirrostratus veil covering entire sky.
8 : Cirrostratus not increasing and not covering entire sky.
9 : Cirrocumulus, or predominately Cirrocumulus with other high clouds.
/ : High clouds invisible because of darkness, fog, blowing snow, dust or sand, or other similar obscurations; or more often blocked because of a continuous layer of lower clouds.

Notes : Though many types of Cirrocumulus occur (e.g., Mackerel sky), 9 is its only mention. Persistent aircraft condensation trails and clouds formed from them are reported, but not those rapidly dissipating (which can also be middle clouds).

W - Past Weather

0 : Clear or few clouds (no symbol)
1 : Partly cloudy or variable sky (no symbol)
2 : Generally cloudy (may be broken) or overcast (no symbol)
3 : Drifting or blowing snow; or sand or dust storm
4 : Fog, ice fog; and/or thick haze or smoke
5 : Drizzle
6 : Continuous Rain
7 : Continuous Snow
8 : Showers
9 : Thunderstorm (with precipitation or not)

Note : Highest numbered type is chosen first if occurring.

a - Pressure Tendency

0 : Increasing, then decreasing - total change positive or 0
1 : Increasing, then steady or more slowly increasing
2 : Increasing (steadily or unsteadily, but with no specific trend)
3 : Decreasing or steady, then increasing; or increasing then more quickly increasing - total change positive or 0
4 : Steady - no significant change
5 : Decreasing, then increasing - total change negative or 0
6 : Decreasing, then steady or more slowly decreasing
7 : Decreasing (steadily or unsteadily, but with no specific trend)
8 : Increasing or steady, then decreasing; or decreasing then more quickly decreasing - total change negative or 0

Notes : Note that 0-3 are opposite of 5-8, the former for total increases, the latter for total decreases. 4 is generally reported if initial and final pressures do not differ with a tenth (.1) mb measurement resolution and with similarly small changes between.

Text and images are copyright of Joseph Bartlo, though may be used with proper crediting.