The most obvious signs in looking for thermals are cumulus clouds. However, though they may offer important information, they might tell us nothing – or they could actually mislead us. Don’t forget that clouds only tell us what’s happening ( or not happening) where they are. The challenge for us is to interpret them in relation to the air where we are. A pilot at 2,000 feet AGL who chases cumuli with bases at 6,000 might find a quick route down! Moreover, since a cumulus sometimes remains visible long after the lift that caused it is gone, any cloud chosen randomly and without regard to genuine signs of convective action may prove to have no tactical value.
In assessing the myriad information available through your canopy, view the world from aloft as divided into two arenas, sky and ground, distinct from each other yet highly interactive. Before deciding whether or not to search beneath a particular cloud for thermal lift, estimate the height of cloud base (AGL) and divide by two. Below that median altitude, dismiss the clouds (except as producers of shade and indicators of wind aloft) and look to features on the ground for ideas. Above the halfway height, consider clouds as primary indicators and terrain as secondary.