Pressure Gradient Force
The speed and direction of the wind is governed by three forces; the pressure gradient force (PGF), the Coriolis Force and friction. PGF is the force produced by differences in barometric pressure between two locations and is responsible for the flow of air from an area of high pressure to an area of low pressure.
The diagram above shows an idealized surface weather map containing a 1030+ mb high pressure system and a 1002+ mb low pressure system. In the absence of the Coriolis Force and friction, the wind flows directly from the center of the high to the center of the low. The speed of this flow is dictated by the magnitude of the change in barometric pressure and the distance between the centers of the high and the low.
The diagrams below demonstrate an environment characterized by weak (left) and strong (right) pressure gradient force. In both examples, the surface high is observed at 1012 mb and the surface low is at 1000 mb resulting in a 12 mb difference. The curved lines between the high and low represent a portion of the 1008 mb and 1000 mb isobars, or contours of constant barometric pressure. In the example of weak PGF (left), the high and low are 100 km apart resulting in a PGF of .12 mb/km. In the example of strong PGF, the high and low are only 20 km apart which produces a PGF of .60 mb/km — a 500% increase over the weak PGF example.
Stronger winds are generally expected when significant pressure differences occur over relatively short distances. On most surface weather charts produced by NOAA agencies, isobars are plotted at 4 mb intervals. It is difficult to accurately determine wind speeds by looking at a surface weather chart, but it is possible to identify areas where the wind is relatively stronger or weaker by examining the spacing of the isobars.
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For example, the surface analysis from 12Z (8 am) on June 21, 2011 (above, left) shows a low pressure system dominating the central Plains and a high pressure center located over Virginia. The isobars surrounding this low, particularly on the northwest side, are much closer together than those near the high in Virginia. These closely packed isobars indicate pressure is changing quickly over a much shorter distance thereby suggesting a much higher PGF. As a result, one would expect the wind speeds to be higher in Nebraska and South Dakota than in the Virginia area, an assessment that is confirmed by the chart of surface wind speeds at the same time (above, right). The chart indicates that wind speeds on the northest side of the low were 16.5 knots and a mere 1.5 knots near the center of the high.
The Coriolis Force
A quick look at the flow of the wind (streamline analysis) valid at 12Z on June 21, 2011 (below) will dispel any notion that the wind flows directly from the center of a high to the center of a low. In fact, it is difficult to find a straight line among the many twisting and turning arrows covering the United States. And it is very evident that there isn’t a wind arrow originating near the high in Virginia and ending in the low in eastern Nebraska.
The flow represented on the streamline analysis above, is the result of the combined influence of the Coriolis Force and friction on the pressure gradient force. Due to the rotation of the Earth, the Coriolis Force causes the wind to curve to the right in the Northern Hemisphere (and left in the Southern Hemisphere). The magnitude of the Coriolis Force is not constant and its influence is enhanced with increases in latitude and wind speed. As such, the influence of the Coriolis Force is essentially zero at the Equator and over very short distances and reaches its maximum near the North and South Pole. In the absence of friction, the Coriolis Force tends to offset the pressure gradient force to produce a wind that flows parallel to isobars. Of course, wind always encounters friction at or near the surface, so let’s turn to the explanation of this last force.
Friction
The surface of the Earth is not smooth, and as the wind blows it is subject to friction as it encounters surface features such as mountains, hills, buildings, trees, etc. Frictional influence on the wind is essentially restricted to the atmospheric layer below 5,000 feet (1,500 meters) and varies considerably based upon the roughness of the terrain. Friction is at a minimum over water and strongest over mountainous areas. Regardless of the magnitude, friction acts to slow wind speeds and reduce the impact of the Coriolis Force.
When the pressure gradient force, the Coriolis Force and friction are combined, the result is a wind (in the Northern Hemisphere) that flows clockwise and outward around an area of high pressure and counter-clockwise and inward towards an area of low pressure as shown on the idealized image above. Note how the dark arrows representing the wind cross the isobars surrounding the high and low at a slight angle. The angle at which the wind crosses isobars ranges from 10° over a smooth surface such as water and and as much as 40° over mountainous areas.
Because the atmosphere doesn’t form perfectly round and neatly arranged highs and lows, applying the wind’s controlling forces to the interpretation of an actual surface chart (below, left) can be a little challenging. However, identifying the low pressure system in eastern Nebraska on the streamline analysis valid at 12Z on June 21, 2011 (below, right) is reasonably straightforward. The large number of arrows converging in a counter-clockwise manner point the way to the low’s center. Once the low has been identified on the two charts, the high can be located by following the streamlines backwards to their origin. For example, notice the arrows that begin in and near Virginia (near the high) and then flow north towards Ohio before curving west and joining the circulation of the low in Nebraska.
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In summary, the wind is controlled by the pressure gradient force (differences in barometric pressure), the Coriolis Force and friction. Wind speed is primarily dictated by the pressure gradient force, while all three controllers combine to guide the wind’s direction.
Mesoscale Winds
The preceding section described the forces that control the wind on the synoptic scale (large scale), dictated by the relationship and transitions in areas of high and low pressure. Wind can also be generated by much smaller-scale dynamics such as the strong winds associated with a thunderstorm downdraft (an account of a downdraft impacting a marina can be found here). When the synoptic scale pressure gradient is weak, opportunities arise for the development of small, or mesoscale, winds that form in response to very subtle differences in barometric pressure over relatively short distances.
In the Great Lakes region, lake and land breezes are the king of mesoscale winds. The thermal properties of the land and water are dramatically different. Land heats up quickly during a sunny summer day and cools off just as quickly as the sun sets. In contrast, the surface temperature of a large body of water such as Lake Erie changes very little over the course of a single day. When the synoptic wind is weak, heating of the land during the day promotes the development of a zone of low pressure along the shore. As shown on the schematic of a lake breeze (below, left), the barometric pressure of the relatively cooler air over the water is higher, therefore a pressure gradient exists between the region of cooler air over the water and land near the shore. And since pressure gradient produces wind, a gentle breeze flows towards the land from the lake.
As the sun sets, the cycle reverses itself as the land quickly cools.
This cooling along the shore promotes the development of a zone of relatively higher pressure (above, right). The pressure gradient reverses itself as the zone of higher pressure is now over the shore and a region of lower pressure is above the warmer water. The land breeze, in the form of air flowing from the land towards the water, is created. Lake and land breezes can only form in situations when the overall synoptic pressure gradient is weak because strong winds disrupt the formation of the small zones of low and high pressure and the requisite pressure gradient.
While lake and land breeze explanations typically refer to relationship between the shoreline and the lake, modest sized islands, such as the Bass Islands and Pelee Island are large enough to produce island scale wind circulations, particularly when the land and lake temperatures are quite different. The many islands in Lake Erie’s western basin, combined with the irregularly shaped shoreline, also promote the development of very small scale wind patterns.
Conclusion
The forces controlling the wind can act on a large or small scale. For sailors, the most interesting, aggravating and difficult to predict winds are those that form when the large scale dynamics are relatively weak. Marine forecasts are essentially useless as the spatial and temporal resolution of computer-generated wind forecasts are simply too coarse to capture such short-lived and small-scale events. However, a sailor that possesses a basic understanding of the wind’s controlling forces can quickly identify and take advantage of the fickle winds produced by weak dynamics.