Page "Cloud" Paragraph 66

The international synoptic code ( or SYNOP ) provides for reporting the three basic altitude ranges for tropospheric clouds, but makes no special provision for multi-level clouds that can occupy more than one altitude range at a particular time.

Consequently, cloud genera with significant vertical development are coded as low when they form in the low or lower-middle altitude range of the troposphere and achieve vertical extent by growing upward into the middle or high altitude range, as is the case with cumulus and cumulonimbus.

Conversely, nimbostratus is coded as middle because it usually initially forms at mid-altitudes of the troposphere and becomes vertically developed by growing downward into the low altitude range.

Because of the structure of the SYNOP code, a maximum of three cloud symbols can be plotted for each reporting station that appears on the weather map ; one symbol each for a low ( or upward growing vertical ) cloud type, a middle ( or downward growing vertical ) type, and one for a high cloud type.