During the spring and summer, the perfect soaring day is characterised by a blue sky with the appearance of white, puffy clouds that start to appear at about 11am. The more solid and well defined the clouds are, the better the day. It is a day when an approaching cold front follows warm, dry, sunny periods. In essence, it's all about convection.
Heat stored in the ground is released when the ambient air temperature decreases (eg. by an approaching cold front). The cooler air triggers the release of stored heat in the ground, which rises up as thermals. As the heat rises it carries moist air with it that condenses when the temperature of the thermal equalises with the higher ambient air, releasing the moisture to form clouds. The benefit is that clouds mark where the thermic activity is as they're sitting on the drier, thermic air below and are being fed by it.
Wave flying is similar to ridge lift in that the wind is deflected upward by the faces of mountains, but as the air-mass continues down-wind, oscillations or waves are created in the atmosphere which can continue for hundreds of kilometres as the air tries to reclaim it's equilibrium. The amplitude of the waves can be thousands of feet high, so high altitude flights are quite possible. 'Wave' is often characterised by long cigar shaped clouds called lenticulars, distinct by their smooth appearance . Glider pilots fly or 'surf' the upward part of the wave to climb and gain height. Flights of 20,000 feet are quite possible in these conditions, though you'll need oxygen and it's very cold. Wave flying is lovely as the air is very smooth though getting into it can be a bit of a bumpy ride as you may have to break through quite a bit of turbulence first - but its well worth the effort. It's worth mentioning that in the UK, gliding flights of nearly 40,000 feet have been acheived in mountain wave.