Pilots also keep a look out for newly forming cumulus clouds, or even large birds soaring without flapping their wings, which can also be signs of thermal activity. These areas, such as asphalt parking lots, dark plowed fields and rocky terrain, are a great way to find thermal columns. Pilots keep an eye out for terrain that absorbs the morning sun more rapidly than surrounding areas. As the air near the ground is heated by the sun, it expands and rises. Thermals are columns of rising air created by the heating of the Earth's surface. For comparison, a commercial jetliner might have glide ratios somewhere around 17:1. This means they can glide for 60 miles if they start at an altitude of one mile. Modern gliders can have glide ratios better than 60:1. This ratio tells you how much horizontal distance a glider can travel compared to the altitude it has to drop. The way you measure the performance of a glider is by its glide ratio. Angling the glider downward, trading altitude for speed, allows the glider to fly fast enough to generate the lift needed to support its weight. Since there's no engine on a glider to produce thrust, the glider has to generate speed in some other way. But, the wings and the body of the glider also produce drag, and they produce more drag the faster the glider flies. If the glider flies fast enough the wings will produce enough lift to keep it in the air. The faster the glider goes the more lift the wings make. The wings on a glider have to produce enough lift to balance the weight of the glider. The string on the windshield tells the pilot if the plane is yawing
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