Wind is the movement of air in the atmosphere. Understanding the direction of this airflow is important for interpreting weather maps, as wind influences the movement of air masses, fronts, and overall weather patterns. Surface analysis charts use symbols to communicate the precise direction from which the wind is blowing at specific locations. Because the wind’s direction is always defined by its point of origin, a wind blowing from the west is called a “west wind.” Learning to decode these specialized symbols provides a localized understanding of air movement.
Understanding Wind Direction Symbols
The most common symbol for displaying wind information on a weather map is the wind barb, which is part of a station plot indicating conditions at a specific point. A wind barb consists of a central circle or dot representing the observation point, and a line or shaft extending from it. The orientation of this shaft immediately tells you the wind direction.
The shaft of the wind barb points into the direction the air is moving, but the wind direction itself is reported as the direction from which the wind originates. For example, if the shaft extends directly to the east, the wind is blowing toward the east, meaning it is a west wind. Translating the barb’s angle into a compass direction is straightforward using the cardinal and intercardinal points.
A wind barb oriented straight upward indicates a North wind, as the air is coming from the North and blowing toward the South. Conversely, a barb pointing straight down represents a South wind. Intercardinal directions, such as a wind coming from the Southwest, would be shown by a barb pointing toward the Northeast.
Calculating Wind Speed from Barbs
While the shaft’s orientation dictates direction, the wind speed is communicated by small lines or flags attached to the end of the wind shaft. These attachments are known as barbs or feathers and follow a standardized tallying system, with speeds reported in knots. One full-length line, or full barb, represents a wind speed of 10 knots.
A half-length line, or half barb, indicates 5 knots, and it is always added at the outermost end of the shaft. To calculate the total speed, you simply add the values of all the barbs and flags present. For example, a single full barb and a single half barb combined signify a 15-knot wind.
For higher wind speeds, a triangular flag or pennant is used, which represents 50 knots. This flag is drawn as a filled-in triangle on the shaft. A wind speed of 65 knots would be represented by one triangular flag, one full barb (10 knots), and one half barb (5 knots). If the wind is calm (2 knots or less), the station plot will display a circle around the central dot with no shaft or barbs.
Interpreting Wind Flow Using Isobars
In addition to specific station plots, the general wind flow across a large area can be interpreted by looking at isobars, which are lines on a map connecting points of equal atmospheric pressure. These lines are fundamental to understanding large-scale air movement, as wind is driven by the pressure gradient force, which pushes air from higher pressure to lower pressure.
The initial impulse for air is to flow directly across the isobars, perpendicular to them, from a high-pressure center toward a low-pressure center. However, the Earth’s rotation introduces the Coriolis effect, which deflects moving air. This deflection causes the wind to flow roughly parallel to the isobars, rather than crossing them directly, particularly higher in the atmosphere.
Near the surface, the force of friction with the ground slows the wind down, reducing the Coriolis effect’s influence. This results in the surface wind crossing the isobars at a slight angle, typically about 15 to 30 degrees, moving inward toward the lower pressure area. In the Northern Hemisphere, surface winds circulate clockwise and outward around high-pressure centers, and counterclockwise and inward around low-pressure centers. Observing the spacing of the isobars provides a further clue: closely packed lines indicate a steep pressure gradient and stronger winds, while widely spaced lines suggest lighter winds.