From Glossary of Meteorology
(Or escape speed.) The minimum speed that an object directed radially outward from an astronomical body must have in order to escape its gravitational attraction.
The escape velocity for an object at a distance R from the center of a spherically symmetric body is
where g is the corresponding acceleration due to gravity. For R equal to the radius of the earth and g its surface acceleration due to gravity, the escape velocity is about 11 200 m s-1. The earth retains its atmosphere only because this escape velocity is appreciably larger than the mean speed of air molecules.