Like other atmospheric effects, streets may be small or enormous, weak or tremendous. Typically, we might expect to see a chain of ordinary, evenly spaced cumulus, but the frequency and size of clusters may vary over time or distance. Lines of individual cumulus may coalesce into huge banks of solid cloud miles wide and continuous over the horizon, or they might consist of merely the faintest irregular pattern of wisps. (There may also be good lift between clouds as well.) Or perhaps ‘blue’ streets lie waiting for only the pilot perceptive enough to detect them – or predict where they will be.
With streets, size isn’t everything; a long, broad boulevard is delightful if it’s handy, but sometimes a narrow alley one block long is the best short cut to where you’re going. The prime characteristic, whether reading terrain or telltale clouds, is linearity. Even if there is not enough lift to offer a climb, a linear pattern of rising air may be beneficial in gaining ground on course. Linearity also organizes lines of sink, such as downwind of (or between) ridges and lines of cloud.
In tactical terms, think of thermal streets simply as elongated thermals. Expect the best lift on the windward end or sunny side, and sink near the lee end or shady edge. The lower you are, the less clouds will tell you about the air down where you are – except as relates to shaded areas. When a large ‘shade street’ obstructs your intended course, climb wherever you can and look for a detour upwind of the shade. Thermals are sometimes triggered when sunny ground is suddenly shaded, but that is not a benefit you can count on when low. Rather, study patterns of cloud development and avoid areas that were recently, or may soon be shaded.