stations  are  limited  to  three  reportable  layers  unless  CB or TCU are present; in which case, a fourth layer may be reported. Total Sky Coverage To  determine  total  sky  coverage,  simply  add together the amount of clouds and/or obscurations in each layer. Total sky cover cannot exceed 8/8. Summation  Sky  Coverage Summation sky coverage is a concept used to determine cloud ceiling. Summation sky cover is determined for each layer of clouds by adding the coverage in the cloud layer to the coverage of all layers below it. In the example that follows, this has been done. LAYER LAYER SUM OF SKY SUMMATION NUMBER AMOUNT COVER LAYER COVER 1 1/8 1/8 FEW 2 3/8 4/8 SCATTERED 3 3/8 7/8 BROKEN 4 1/8 8/8 OVERCAST Summation coverage for each layer is converted into plain language terms. They are clear, for no clouds present; few, for greater than 1/8 to 2/8; scattered, for 3/8 to 4/8 clouds; broken, for 5/8 to 7/8 clouds; and overcast, for 8/8 clouds, as shown in table 1-2. The term thin is not used in METAR/SPECI observations. Table 1-2.—Cloud Ceiling Summation Coverage and Terms Contraction Meaning T    e    x    t T  e  x  t SKC Sky  clear 0/8 FEW trace >0/8-2/8 SCT Scattered 3/8-4/8 BKN Broken 5/8-7/8 OVC Overcast 8/8 VV Vertical 8/8 Visibility CEILING  DETERMINATION The terms ceiling and cloud ceiling are defined as the height-above-ground level of the lowest broken or overcast layer. If the sky is totally obscured, the height of the vertical visibility (VV) is used as the ceiling height. Table 1-2 relates the measurement of sky cover as observed in eighths, to the terms used to discuss sky cover and ceilings. In the example we just covered, the ceiling would be at layer No. 3, the broken layer. In addition to determining layer amounts, total sky coverage,  summation  coverage,  and  the  ceiling  based on  the  summation  coverage,  you  must  also  identify cloud  layer  heights  and  ceiling  height. CLOUD  LAYER  HEIGHTS/CEILING HEIGHT The height must be determined for the base of all layers of clouds. The height for the lowest broken or overcast layer is used as the ceiling height. A surface- based  obscuration  also  constitutes  a  ceiling;  vertical visibility into the obscuration is used as ceiling height. Cloud layer height and ceiling height may be determined by several methods. Estimation is the most frequently  used  method  for  cloud  layer  height.  It  is acceptable  for  determining  heights  of  low  scattered cloud layers or higher cloud layers. When the cloud layer forms a ceiling, especially if the layer height is below 3,000 feet, one or more of the more accurate methods should be used. The following list of cloud layer  height  determination  methods  is  generally  ordered from the most accurate to the least accurate: Measurement by comparison to known heights of structures or landmarks within 1 1/2 miles of the runway Measurement  by  ASOS  or  SMOOS  cloud- height  sensor Measurement by rotating beam ceilometer for heights  less  than  10  times  the  baseline Measurement  by  ceiling  light  and  clinometer Estimation  by  rotating  beam  ceilometer  for heights  more  than  10  times  the  baseline Estimation  by  pilot  during  ascent  or  descent Estimation  by  ceiling  balloon Estimation  by  comparison  to  terrain  or  structures more than 1 1/2 miles from the runway Estimation  by  convective-cloud-height  diagram Estimation  using  the  Skew-T,  Log  P  diagram Estimations from conventional weather/Doppler radar Estimation using other station reports in the vicinity Estimation  by  observational  experience 1-27