Ridge lift (or slope lift) is created when a wind strikes an obstacle, usually a mountain ridge or cliff, that is large and steep enough to deflect the wind upward. If the wind is strong enough, the ridge lift provides enough upward force for gliders, hang gliders, paragliders and birds to stay airborne for long periods or travel great distances by 'slope soaring'. Although unpowered aircraft are usually descending through the air, they will climb if the surrounding air is rising faster than their sink rates. Model glider enthusiasts refer to this technique as "slope gliding" or "sloping".