Final answer:
The surface wind's velocity usually differs from the gradient wind by 10-30% due to surface friction, which slows down the wind and alters its direction.
Step-by-step explanation:
The velocity of surface wind typically differs from the gradient wind mainly due to the effect of surface friction. This friction acts as a drag force on the wind as it contacts the Earth's surface, causing the surface wind to be slower and change direction as compared to the gradient wind that is not influenced by the surface.
In meteorology, the magnitude of this difference is subject to several factors, including the roughness of the surface and the stability of the atmosphere. Surface winds are generally slower and can differ by about 10-30% from the gradient wind speeds aloft, although this can vary widely.