Total Cloud shows less % than High Cloud

Shimon Avitan

How does it make sense ? why in Clouds (general item) i see lower % than when i choose High clouds ?

stitch

I did some investigation and trialled various cloud amounts for the available cloud groups (High/Medium/Low) and compared them to the Total Cloud %.

This is what I found:
High 99%, Med 0%, Low 0% => Clouds (Total) 29%
High 0% Med 95% Low 1% => Clouds (Total) 49%
High 0% Med 0% Low 99% => Clouds (Total) 99%

The % value associated with Total is dependent on which cloud layer grouping. Maybe think of it like this - how opaque is the cloud cover to objects like the sun/moon. Overcast high cloud like Cirrus can still allow some definition of the sun for surface observers. Whereas, an overcast sky of stratus or stratocumulus will have a greater level of opaqueness, or is more likely to make the sun indistiguishable to a surface observer.

The official definition from ECMWF didn't clear it up for me:
Total cloud cover is a single level field calculated from the cloud occurring at different model levels through the atmosphere. This parameter is the proportion of a grid box covered by cloud.

High cloud is a single level field calculated from cloud occurring on model levels with a pressure less than 0.45 times the surface pressure. The proportion of a grid box covered by cloud occurring in the high levels of the troposphere.