From AMS Glossary
(Also called feather.) A means of representing wind speed in the plotting of a synoptic chart; it is a short straight line drawn obliquely toward lower pressure from the end of a wind-direction shaft.
In the commonly used five-and-ten system, one barb represents 10 knots and a half-barb represents 5 knots. Previously, a full barb was used to represent a Beaufort force of 2, with a half- barb to represent a Beaufort force of 1. Customarily, the barb is often drawn obliquely or perpendicular to the wind direction.