A glider is a type of radio-controlled airplane that normally does not have any form of propulsion. They are able to sustain continuous flight by exploiting the lift produced by slopes and thermals. Slope soaring uses the lift produced by wind blowing up the face of a steep slope on hills, mountains, and cliffs. Dynamic soaring, utilizing the leeward or "backside" of a hill, has recently become very popular. Thermal soaring uses columns of warm, rising air called thermals to provide lift for a glider. They are normally launched with a bungee cord catapult, a winch, or towed by a powered plane. This is often combined with slope soaring. Thermals from elsewhere can drift in over the hill to combine with the hill lift or they can be formed by the hill itself, if the slope is angled to the sun causing the slope to heat up faster than in the surrounding areas. The resulting warm air will then flow upwards pulling in air from the valley below, causing a wind up the slope. The lift is thus a combination of ridge lift and thermal. This has produced new term, "slermal", to describe the mixture of both slope lift and thermal activity coming up the hill face.  Powered gliders use electric motors, engines or even jet turbines to provide propulsion for a glider to get in the air. They are normally used to get thermal soarers in the air and are powered down once the model is flying on its own.