An Interesting Local Wind
Introduction
Sailors are never entirely satisfied with the wind. Either there's not enough wind and they are forced to rely upon a noisy engine, or it's too strong and they are forced to seek refuge in a protected harbor. A favorable wind speed from the wrong direction is no better, as tacking back and forth significantly lengthens a journey. Like Goldilocks, sailors are always seeking a wind that is "just right". And sometimes, when you least expect it, Mother Nature provides the surprising gift of a perfect wind.
Special Note: The analysis presented on the remainder of this page requires an understanding of the atmospheric dynamics that control the wind. If you are unfamiliar with these dynamics, or need a refresher, please review this instructional page. |
An Interesting Wind
The plan for the third day of our vacation was to sail from Leamington, Ontario to the mooring field at South Bass Island (map). At 8 am the wind at the CMAN station on South Bass Island was reporting a northwest wind (310°) at 22 knots, gusting to 25.1. The buoy in Lake Erie's western basin had similar observations with a northwest wind (300°) at 17.5 knots gusting to 21.4. Wave heights were approximately one meter. Leamington does not host an official weather station, but our masthead anemometer (at a height of 58') was reading from 20 to 25 knots from the northwest.
The brisk winds on Lake Erie were associated with a strong barometric pressure gradient on the southwest side of a low pressure system located north of Lake Ontario (above, left). The west to northwest flow over the lower Great Lakes is readily apparent on the streamline analysis (above, right). The marine forecast issued by the National Weather Service, as well as forecast model output, expected the winds to diminish as the day wore on, so we decided to delay our departure until after lunch.
NATIONAL WEATHER SERVICE CLEVELAND OH
405 AM EDT WED JUN 29 2011
FOR WATERS BEYOND FIVE NAUTICAL MILES OF SHORE ON LAKE ERIE
.TODAY...NORTHWEST WINDS 10 TO 20 KNOTS DIMINISHING TO 5 TO
15 KNOTS. WAVES 3 TO 5 FEET SUBSIDING TO 1 TO 3 FEET.
.TONIGHT...NORTHWEST WINDS 10 KNOTS OR LESS BECOMING VARIABLE.
WAVES 2 FEET OR LESS
We departed Leamington at 2 pm local time (18Z) for the four-hour trip to South Bass Island. A sampling of the weather stations in and around Lake Erie's western basin indicated that wind speeds had declined significantly from 8 am and were generally from the west to northwest in the range of 8 to 12 knots. The observation from our anemometer as we left the harbor was 12 knots from the northwest (310°).
Not long after our departure, the wind began to steadily decline. By 2:30 pm (1830Z) the combination of leftover waves and a lack of wind (6 knots from the northwest) prompted us to furl the headsail and start the engine. Approximately 30 minutes later (3:00 pm/19Z), the wind abruptly switched nearly 180° at our location to southeast (150°) at approximately 10 knots. In light of the low pressure system to the northeast and the persistent west to northwest winds, this change was very surprising, and difficult to explain. Wave heights of nearly one foot from the southeast suggested this new and unanticipated wind direction had persisted for at least an hour.
A subsequent check of the wind observations at nearby weather stations at 19Z (above) indicated that west to northwest winds continued to dominate the western basin. The exceptions were a northeast (40°) reading of 5.8 knots at the NCDC buoy 45005 and at our location to the southwest of Leamington, ON. Based upon these observations, it appears that the wind shift was affecting a relatively small area of Lake Erie.
We continued to experience southeast winds in the 10 to 13 knot range for the next forty-five minutes before it slowly began subsiding. At 4 pm (20Z), we observed 6 knots from the southeast (140°) as we were now well within the lee of Pelee Island. The wind at buoy 45005 was 7.8 knots from the east (90°). Elsewhere across the western basin (below) wind speeds had diminished, but they remained firmly from a westerly or northerly direction. Buoy 45005 and the lighthouse on South Bass Island are a mere 20 miles apart, and while the winds were similar in speed, they had a directional difference of approximately 130° (320° v. 90°).
Over the next hour, our wind continued to diminish, and began fluctuating between an easterly and a westerly direction. By 5 pm (21Z), a light (5 kts) wind had filled in from the southwest (240°). The observations from elsewhere in the western basin (below) show a consistency in speed but a wide difference in direction from northwest (310°) at Gross Isle, in the mouth of the Detroit River, to east southeast (110°) at buoy 45005. The north-northeast (10°) observation at Harrow, Ontario, combined with the observation at buoy 45005, raise the possibility that a light southeasterly wind persisted near the north end of Pelee Island. However, the absence of a weather station on or near the island prevent confirming this suggestion. We continued to experience a light west to southwest breeze for the remaining 10 miles to South Bass Island.
The Explanation
The shift from northwest to southeast was a welcome surprise. While it was relatively short-lived, the wind offered an opportunity for some great sailing. I enjoyed the sail, but was perplexed by the suddenness and magnitude of the wind shift. The solution to the puzzle was found in an analysis of sea level pressure and observations from weather stations located in or near Lake Erie's western basin.
The Storm Prediction Center (SPC) publishes a variety of analyses of weather parameters on a hourly basis. Below are cropped versions of the analyses of sea level pressure, in millibars (mb), from 17Z (1 pm), 18Z (2 pm) and 19Z (3 pm) on June 29, 2011. Isobars are plotted at 2 mb intervals on SPC charts.
At 17Z (above, left), the 1018 mb isobar was bisecting the eastern portion of Lake Erie while a narrow area of higher pressure (1020 mb) was centered over Lake Michigan. The 1018.9 mb observed at South Bass Island at 17Z is consistent with this analysis. At this time, the wind at South Bass was 19 knots from the northwest which was also consistent with the orientation of the isobars. Over the next two hours (17Z to 19Z), the orientation of the isobars to east remained essentially unchanged, however the 1020 mb high over Lake Michigan steadily shrunk in size. During the same period, the pressure at South Bass fell .6 mb to 1018.3 mb. The pressure gradient force also relaxed across the region and, in response, wind speeds decreased (South Bass fell from 19 knots to 11.1 knots during the two hour period).
An interesting transition in sea level pressure occurred between 19Z (3 pm) and 20Z (4 pm) which set the stage for our puzzling wind shift. The disappearance of the 1020 mb isobar on the 19Z sea level analysis (above, left) indicated that the pressure over Lake Michigan had declined below 1020 mb. On this same analysis, the 1018 mb isobar which had been bisecting the eastern end of the lake was replaced with a closed 1018 mb high over the central portion of the lake. The existence of this small 1018 mb high over Lake Erie and the 1018 mb in eastern Michigan indicates that the area between these 1018 mb isobars (including the western basin) had a relatively lower pressure. Observations confirm that sea level pressure in this area was in the range of 1017.5 and 1017.9 mb.
The decline in sea level pressure to the west of Lake Erie led to a further slackening of the pressure gradient force and steadily decreasing winds. At 20Z (4 pm), the wind at South Bass was 7 knots and 7.8 knots at NCDC buoy 45005. Over the next two hours, the sea level pressure field over the western basin changed modestly, which promoted gently falling sea level pressure, pressure gradient force, and wind speeds.
The steadily decreasing pressure gradient force across the western basin presented a supportive environment where small-scale winds were able to develop in response to minor differences in barometric pressure. We'll examine this environment by analyzing the observations from four weather stations (below) -- Lambert Field, MI (KDUH), South Bass Island, OH (SBI), the NCDC buoy 45005, OH (Buoy 45005) and Detroit, MI (KDTZ).
The two charts below offer a comparison of barometric pressure readings (in millibars) between KDUH and SBI and KDTW and Buoy 45005. The pressure gradient force (PGF) is the difference in pressure divided by the distance (in nautical miles) between the weather stations. The strongest PGF (.0297 millibars per nautical mile) was observed between KDUH and SBI at 17Z.
A review of the observations at KDUH and SBI (below, left) shows that with the exception of 21Z, the pressure at KDUH was higher than SBI. Since air flows from higher pressure to lower pressure, this pressure difference promoted westerly winds in the southern portion of the western basin. This conclusion is supported by the consistent west to northwest winds observed at SBI during the afternoon of June 29th. The decreasing difference in barometric pressure (and PGF) accounts for the steadily falling wind speeds from 16Z to 22Z.
The chart of observations at KDTW and Buoy 45005 (above, right) tells a slightly different story. From 16Z to 22Z, the pressured dropped at both KDTW and Buoy 45005. Although the pressure was initially higher at KDTW, the pressure fell faster than at the buoy, allowing the pressure to equalize at the two locations not long after 18Z. This trend continued after 19Z resulting in higher pressure at the buoy than at KDTW. These observations support the conclusion that the pressure gradient force over the northern portion of the western basin decreased faster than the southern portion which supported a decrease in the northwest winds. Ultimately, a reversal in the pressure gradient (higher pressure at the buoy) promoted the development of light east and southeasterly winds in an area extending from the buoy northwest towards KDTW.
When we first encountered the southeasterly winds north of Pelee Island, the speed was in the range of 10 knots. Almost immediately, the speed increased to 12 to 13 knots and persisted for the next hour. While the pressure gradient was clearly supportive of the direction, the modest difference in barometric pressure over the region probably wasn't adequate to produce wind speeds in this range. It is likely that the wind passing through the gap between Pelee Island and Pelee Point (above) acted to increase wind speeds due to the Venturi effect. The absence of weather stations don't permit this suggestion to be proven, but this area has a reputation among local forecasters for producing this effect.
Conclusion
In conclusion, the surprising southeasterly wind was the result of two small-scale effects. First was the transition in pressure gradient over the region. Second, the flow of wind that developed in response to this pressure gradient was enhanced by geographic influences.