A gale is a very strong wind. The Beaufort scale is a way to measure weather conditions. It is based on observations. The first level of the scale that uses the word gale is 7 beaufort (of a total of 13 steps). There are other definitions as well. The U.S. Government's National Weather Service defines a gale as 34 to 47 knots (63 km/h to 87 km/h or 39 miles per hour to 54 miles per hour) of sustained surface winds. Forecasters usually issue gale warnings when they think there will be winds of this strength.
Other sources use speeds as low as 28 knots (52 km/h) and as high as 90 knots (170 km/h). The 90-knot (170 km/h) definition is rare. A common alternative definition of the maximum is 55 knots (102 km/h).