Two basic wind measurements, direction and speed, are particularly significant to weather observers. One simple device for determining both measurements, a wind sock, is a common sight near airport runways and helipads (▼A). The cone-shaped bag is open at both ends and is free to change position with shifts in wind direction. The degree to which the sock is inflated is an indication of wind speed.

Winds are always labeled by the direction from which they blow. A north wind blows from the north toward the south, an east wind from the east toward the west. The instrument most commonly used to determine wind direction is the wind vane (▲B). This instrument, a common sight on many buildings, always points into the wind. The wind direction is often shown on a dial that is connected to the wind vane. The dial indicates wind direction, either by points of the compass (N, NE, E, SE, etc.) or by a scale of 0 to 360 degrees, where 0 degrees and 360 degrees are both north, 90 degrees is east, 180 degrees is south, and 270 degrees is west.

Wind speed is commonly measured using a cup anemometer (refer to ▲B). The wind speed is read from a dial much like the speedometer of an automobile. Places where winds are steady and speeds are relatively high are potential sites for tapping wind energy, like the mountain passes of southern California shown in this chapter’s opening image.

When the wind consistently blows more often from one direction than from any other, it is called a prevailing wind. ▼A, for example, demonstrates the prevailing wind in the northeastern United States is from the northwest (NW). You may be familiar with the prevailing westerlies that dominate the circulation in the midlatitudes. In the United States, for example, these winds consistently move the “weather” from west to east across the continent. Embedded within this general eastward flow are cells of high and low pressure, with their characteristic clockwise and counterclockwise flow. As a result, the winds associated with the westerlies, as measured at the surface, often vary considerably from day to day and from place to place. By contrast, the direction of airflow associated with the belt of trade winds is much more consistent, as can be seen in ▼B.

By knowing the locations of cyclones and anticyclones in relation to where you are, you can predict the changes in wind direction that will occur as a pressure center moves past. Because changes in wind direction often bring changes in temperature and moisture conditions, the ability to predict the winds can be very useful. In the Midwest, for example, a north wind may bring cool, dry air from Canada, whereas a south wind may bring warm, humid air from the Gulf of America (Gulf of Mexico). Sir Francis Bacon summed it up nicely when he wrote, “Every wind has its weather.”

• The two basic wind measurements are direction and speed. Winds are always labeled by the direction from which they blow. Wind direction is measured with a wind vane, and wind speed is measured using a cup anemometer.

• cup anemometer: An instrument used to determine wind speed.

• prevailing wind: A wind that predominately blows from one direction more than from another.

• wind vane: An instrument used to determine wind direction; the wind vane points into the wind.