The secret is:

Never stop looking for stronger lift. If you find yourself circling in steady 360s then you aren't trying hard enough. There is no benefit to making flat turns if they don't keep you in the lift. Bank as hard as you need to to stay in the strongest lift. Often when you encounter lift the first bit you encounter is not the core. The core is usually less broken than the weaker stuff around it (most thermals are multi-cored).

Low down thermals are usually elongated along the direction of drift - if you lose the core try going up or downwind. High up cores tend to be attached to the clouds - where depends on the day - if you find the core going into the upwind end of the cloud on the first climb the same is likely to be true for the rest of the thermals in that airmass. If you get halfway to cloudbase and lose the core this may be because you have hit an inversion: look around, if you have hit an inversion you should see a layer of smog at your altitude (usually obvious even when the inversion is weak). If you are at an inversion keep circling and stay with lift, then extend your search pattern because thermals often break through the inversion somewhere other than where the original core hit. The place where the thermal broke through the inversion tends to be in the same place relative to the original core in successive thermals on the same day - so if it was south east of the core in the first climb try looking south east in the second. Only the strongest thermals break through (in the UK) so it is worth climbing through and trying to stay above the inversion where the lift is stronger (but thermals are further apart).

In flatlands above halfway to cloudbase I find thermals by going to building clouds (bright white, flat bottoms, visibly growing), however Hugh Miller says he never goes to clouds - instead he finds thermals by positioning himself above the downwind edge of a cloud-shadow - apparently the shadow triggers thermals as it moves onto the sunny ground. (I haven't tested this but can imagine that it works by putting you above ground that is in an area where clouds are formed (i.e. not a blue hole) and where the sun has been on the ground for longest).

On blue days, when low, the classic sources work - hills, forest edges, farmers working fields, isolated buildings, tarmac, roads, factories, all of the above together. Also, lake edges and rivers work. Ripples on ponds point to thermals on light wind days. Graham Steele and I gained a hundred feet or so over a garden bonfire yesterday. Stubble fires and factory chimneys work.

Almost all thermals have an element of rotation in them - if you find the centre of rotation you have found the core (and you can feel it twist your wing). Dustdevils are usually the bottom of thermals and I have found lift 3000 feet above dustdevils. Lower down they work but can be a bit hectic.

Cheers,
Adrian

Adrian Thomas 
adrian.thomas@zoology.oxford.ac.uk