1. Low Clouds
   1. Cumulus [Cu]
      1. Species
         1. Cumulus fractus [Cu fra]
         2. Cumulus humilis [Cu hum]
         3. Cumulus mediocris [Cu med]
         4. Cumulus congestus [Cu con]
      2. Varieties
         1. Cumulus radiatus [Cu ra]
   2. Stratocumulus [Sc]
      1. Species
         1. Stratocumulus castellanus [Sc cas]
         2. Stratocumulus lenticularis [St len]
         3. Stratocumulus stratiformis [St str]
      2. Varieties
         1. Stratocumulus duplicatus [Sc du]
         2. Stratocumulus lacunosus [Sc la]
         3. Stratocumulus opacus [Sc op]
         4. Stratocumulus perlucidus [Sc pe]
         5. Stratocumulus radiatus [Sc ra]
         6. Stratocumulus translucidus [Sc tr]
         7. Stratocumulus undulatus [Sc un]
   3. Stratus [St]
      1. Species
         1. Stratus fractus [St fra]
         2. Stratus nebulosus [St neb]
      2. Varieties
         1. Stratus opacus [St op]
         2. Stratus translucidus [St tr]
         3. Stratus undulatus [St un]
2. Middle Clouds
   1. Altocumulus [Ac]
      1. Species
         1. Altocumulus castellanus [Ac cas]
         2. Altocumulus floccus [Ac flo]
         3. Altocumulus lenticularis [Ac len]
         4. Altocumulus stratiformis [Ac str]
      2. Varieties
         1. Altocumulus duplicatus [Ac du]
         2. Altocumulus lacunosus [Ac la
         3. Altocumulus opacus [Ac op]
         4. Altocumulus perlucidus [Ac pe]
         5. Altocumulus radiatus [Ac ra]
         6. Altocumulus translucidus [Ac tr]
         7. Altocumulus undulatus [Ac un]
   2. Altostratus [As]
      1. Varieties
         1. Altostratus duplicatus [As du]
         2. Altostratus opacus [As op]
         3. Altostratus radiatus [As ra]
         4. Altostratus translucidus [As tr]
         5. Altostratus undulatus [As un]
   3. Nimbostratus [Ns]
3. High Clouds
   1. Cirrocumulus [Cc]
      1. Species
      2. Varieties
   2. Cirrostratus [Cs]
      1. Species
      2. Varieties
   3. Cirrus [Ci]
      1. Species
      2. Varieties
4. Multiple Levels
   1. Cumulonimbus [Cb]
      1. Species
      2. Varieties