Bell's Beer Bayview Mackinac Race Climatology
Published May 2015
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Introduction
Reviewing the long-term average wind and wave conditions on Lake Huron for the middle of July is a great way to set the stage for your Mac weather forecast, particularly for those who are participating in the Race for the first time.
Wind and Wave Observations
The National Data Buoy Center (NDBC) maintains two floating discus bouys in Lake Huron. Buoy 45003 is located in the northern basin, while buoy 45008 is located in southern Lake Huron near the entrance to Saginaw Bay (click here for a map of Lake Huron).
During the summer, large areas of high barometric pressure are a regular feature over the Great Lakes. In response to the light pressure gradient that accompanies these highs, the long-term average wind speed at both buoys reaches an annual minimum of approximately eight knots during June and July (figures 1 and 2, below). The average wind gusts aren't much higher, with Buoy 45008 at nine knots (view graph) and buoy 45003 (view graph) slightly higher at ten knots.
The wind roses (figures 3 and 4, below) show the distribution of observed wind direction and wind speed at the buoys for the middle of July (July 10 to 20) from 1995 to 2014. The predominant winds at buoy 45008 are southerly, with southeastely to west-southwesterly winds occurring nearly 44% of the time. Although the buoys are only 67 nautical miles apart, the wind rose for buoy 45003 (figure 4) shows a dramatically different wind pattern from its southerly neighbor. While the southerly quadrants of the two wind roses share a common pattern, buoy 45003 has a significant (and surprising) northwesterly component, with northwesterly winds occurring nearly 29% of the time.
The prominent northwesterly component at Buoy 45003 is likely related to its location east of the Straights of Mackinac. Several Old Goats have commented that 9 out of 10 years the final push to the Island is dead upwind. This observation is supported by the wind rose (figure 5) from NDBC MACM4 (Mackinac City, MI) which shows predominantly westerly winds at the station.
Figures 6 and 7 (below) show the distribution of observed wind speeds at the buoys for the same period covered by the wind roses. The winds at buoy 45008 were 10 knots or less 63.0% of the time. Winds from 10 to 15 knots were observed 24.2% of the time. The winds at buoy 45003 were slightly stronger than those at buoy 45008, with winds less than 10 knots comprising 58.9% of the observations. Winds from 10 to 15 knots occurred more frequently at buoy 45003 than 45008 -- 28.2% versus 24.2%. Winds over 15 knots occurred relatively infrequently at both locations -- 13.8% at buoy 45008 and 12.8% at 45003. Based solely upon the long-term averages, the Bayview Mac would be a light wind race.
It should be no surprise that the wind speed minimum in July is accompanied by an annual minimum in wave heights. The average wave height at both buoys in July (figures 8 and 9, below) is approximately .7 feet.
Water Temperatures
The long-term surface water temperature of Lake Huron is shown in figure 10. Although early season water temperatures have been colder in the last couple of years compared to the preceding ten years, they are quite close to the long-term average. As of May 26, 2015, the average surface temperature of Lake Huron was only two degrees colder than the long-term average.
Below normal water temperatures make for uncomfortable night watches and increase the potential for the formation of dense and expansive fog banks. But of primary importance to Bayview Mac sailors, colder than normal lake temperatures are likely to have an impact on the wind. A cold lake often creates a layer of cold air several hundred feet thick just above the surface, resulting in a low-level temperature inversion. A temperature inversion is an atmospheric condition where the temperature in an air column warms instead of cooling with increasing height. Low-level temperature inversions can have a significant effect the wind, most often by suppressing wind gusts.
Using data from NDBC station HRBM4 and buoy 45008, let's look at an example from southern Lake Huron on June 13, 2014 to illustrate the impact that a low-level temperature inversion can have on the wind. HRBM4 is located on the western shore of Lake Huron near Harbor Beach, Michigan approximately 28 nautical miles from buoy 45008.
Both stations were located in an area of building high pressure following the passage of a cold front. The sustained winds at HRBM4 (figure 11) ranged from 10 to 15 knots, while gusts peaked slightly above 20 knots. During the same period, the sustained winds at 45008 (figure 12) reached only 11 knots and gusts peaked at 13 knots. A comparison of the wind gust observations (click here) shows they were subdued at the offshore station.
Both stations were located in a similar surface pressure gradient, therefore the difference in wind observations was due to the nature of the temperature profile of the air column above each station. Figure 13 shows the temperature profile of the air column above HRBM4, with the red line representing the temperature and the green line showing the dew point. At HRBM4, the warmest temperature was located at the surface with steadily decreasing temperatures aloft, represented by the steady shift of the temperature trace (in red) to the left. A low-level temperature inversion was not present above HRBM4.
The surface air temperature was much colder at 45008 due to the cooling influence of Lake Huron. In contrast to the air column above HRBM4, the warmest temperature in the column above 45008 (figure 14) was not at the surface, but nearly a thousand feet above. The path of the temperature trace to the right (annotated profile) is the telltale signature of the low-level temperature. This cold layer of air near the surface suppressed wind gusts by preventing momentum from stronger winds aloft from reaching the surface.
A comparison of the change in temperature in the air columns above HRBM4 and 45008 is shown in figure 15. The dramatic warming -- instead of cooling -- with height above 45008 is easy to recognize. Above the inversion (at nearly 1,000 feet), the temperature profiles at both locations were nearly identical.
The strength of the low-level temperature inversion created by the cold lake isn't consistent across the entire lake. It is typically strongest near the center of the lake where the water is coldest and diminishes near the shore where water temperatures are much warmer. The wind direction also plays a role. In a brisk southwesterly or westerly wind, the creation of a low-level temperature inversion in the near-shore waters would be inhibited by the transport of warm air from the land out over the lake. The bottom line is that competitors on the Cove Island Course, or Shore Course sailors favoring an offshore route to Mackinac, will be more affected by low-level temperature inversions than those staying closer to shore.
Lake And Land Breezes
Since lake and land breezes form in response to temperature differences between the air over the land and the lake, colder than average water temperatures have an impact on their formation and strength. If the large-scale weather pattern is supportive of the development of subtle wind patterns, colder water temperatures should increase the speed of the daytime lake breeze. In contrast, the typically weaker nighttime land breeze will be further weakened by the below-normal off-shore water temperatures.
Because the subtle thermally-driven winds develop within the context of the larger-scale wind, potentially stronger lake breezes and weaker land breezes aren't inherently good or bad. It all depends upon the direction of the large-scale wind. For example, if the large-scale winds are offshore, stronger lake breeze dynamics may result in weaker winds in the near shore waters. In contrast, if the large-scale wind is onshore, better lake breeze dynamics will result in stronger onshore winds.
Marine Weather Forecasts
While a climatological analysis provides competitors with an awareness of the average conditions on Lake Huron during July, it is weather -- not climatology -- that governs a long-distance sailboat race. For the latest marine weather forecasts and links to a wide range of marine forecasting resources, please visit the LakeErieWX Bayview Mackinac Race Weather Page or my Lake Huron Marine Weather Dashboard.