Working height and cloud base

Cloud base

As air rises it cools. The moisture it carries condenses once it has cooled enough, and that is where the cumulus base forms. Warm, dry air has to rise far before that happens, which gives a high base; humid air condenses sooner, which gives a low one.

Cumulus only forms if the thermals actually rise as high as that level. If they stop below it, the day is blue: thermals without clouds to mark them.

Soarable height

The soarable height is the top of the usable lift. On a blue day it is where the thermals weaken too much to climb in. On a cumulus day it is the cloud base, because a glider does not climb into cloud.

Both are worked out against the height of the ground under each place, so a valley and the hills beside it can have quite different working heights under the same cloud base.

Why it matters

Working height decides a lot about a cross-country day. More height means longer glides between thermals, more choice of which thermal to take, and more margin. A low working height forces you to take the first thermal you find and keeps you close to the fields. See Cross-country speed and distance.