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Cu base

The altitude at which cumulus forms.

Cu base over southern Germany on a strong summer day, as SoarScope shows itCu base over southern Germany on a strong summer day, as SoarScope shows it

Cu base is the height of the cumulus cloud base, above sea level.

How to read it

Section titled “How to read it”

Where there is no cumulus, nothing is drawn: that is a blue day, not a low base. A high base gives a deep working height; a low base keeps you close to the ground.

The value is above sea level, as your altimeter reads it with QNH set.

What goes into it

Section titled “What goes into it”

Cloud base is where rising air from the ground cools to the point where its moisture condenses. Warm, dry air gives a high base; humid air a low one. See Working height and cloud base.