continuously forms, while the trailing or leeward edge continuously  dissipates. These  clouds  usually  form  downwind  from  a mountain range or over a mountain in a small portion of the sky at one level, but may form in different levels and appear stacked on top of the lower level cloud elements. They may also form in a layer downwind from a mountain  range  with  individual  cloud  elements  well separated  from  each  other.  Lenticularis  clouds  need  not form in conjunction with other orographic clouds, the rotor  clouds,  and  cap  clouds. The  process  that  forms  altocumulus  lenticularis clouds also on occasion forms the same clouds closer to the ground in the low-etage, and frequently forms similar clouds in the high-etage. The difference in the classification   of   stratocumulus   lenticularis, altocumulus lenticularis, and cirrocumulus lenticularis stems solely from the height at which they form, and should not be based on the apparent size of the cloud, which, in this case, may be very misleading. Studies have  shown  that  although  lenticularis  clouds  usually form with bases in the mid-etage range, they can form downstream of larger mountain ranges, with the base of the  lowest  lenticularis  in  the  20,000-  to  30,000-foot range.  Bases  of  the  higher  elements  of  stacked lenticularis may be as high as 35,000 feet, with cloud tops near 40,000 feet. Q16. Q17. Ql8. Q19. Q20. Q21. Q22. Q23. REVIEW  QUESTIONS Describe   two   potentially   dangerous   wind phenomena  associated  with  an  outflow boundary. A wall cloud will usually form in what location of a CB cell? Stratus   fractus   clouds   generally   form   in conjunction with what other type of cloud? When must altostratus clouds be reclassified as nimbostratus? What  might  the  formation  of  altocumulus castellanus or altocumulus floccus indicate? What are cirrus clouds composed of? Explain the formation of a rotor cloud. Explain the formation of a cap cloud. CLOUD  AMOUNTS The  amount  of  cloud  cover  is  the  second determination you must make in observing the overall condition of the sky.  Clouds of the types we have just discussed may form at various levels in the atmosphere. It is not uncommon to have different layers of low-etage clouds, along with several layers of mid- and high-etage clouds. Estimation by the observer is the primary method used to determine cloud amounts.   However, automatic weather systems can measure the amount of clouds at each  level,  and  these  inputs  may  be  used  as  a supplemental tool by observers for cloud layer coverage and total sky cover. Generally, there are two different types of cloud amount measurements necessary for an observation. The  more  difficult  measurement  is  cloud  layer coverage. The easier measurement is the total sky cover. In cloud layer coverage, the amount of cloud in each layer must be estimated. Both are estimated in eighths (oktas) of the celestial dome (the total area of sky or the dome of the sky).  Layer coverage is used to determine the cloud ceiling, which is the lowest layer or layers that block 5/8 or more of the celestial dome from being  seen. Layer  Coverage A  cloud  layer  is  defined  as  "clouds  and/or obscuring  phenomena  aloft,  either  continuous  or composed of detached elements, that have bases at approximately the same level." Both continuous and detached elements may combine to form a layer, and all layers and obscuring phenomena are to be considered opaque. The essential requirement is that bases be at approximately the same level. The upper portions of cumulonimbus clouds are often spread horizontally by the wind, and form a layer of cirrus spissatus, dense altostratus, or dense altocumulus clouds, Velum may also be present. These horizontal extensions are regarded  as  separate  layers  if  their  bases  appear horizontal, and they cover 1/8 of the sky or more. A layer  may  be  a  combination  of  cloud  types  or  a combination of obscuring phenomena as long as the bases are all at approximately the same level. For example, cumulus mediocris and cumulus congestus may be considered as the same layer if their bases are the same height. When  observing  layer  coverage,  you  must  not  only estimate the amount of clouds in each layer, but also consider phenomena that hide 1/8 or more of the sky as a layer. (A partial obscuration hiding less than 1/8 of the sky is ignored.) Obscurations may be surface-based or aloft and include phenomena such as rain, snow, fog, smoke, or haze. However, liquid or frozen water 1-25