Chicago Yacht Club Race To Mackinac--Climatology
(Published May 2015)
Reviewing the long-term average wind and wave conditions on Lake Michigan for the middle of July is 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 Michigan. Buoy 45002 is located in the northern basin north of the Manitou islands, while buoy 45007 is located in southern Lake Michigan approximately 43 nautical miles southeast of Milwaukee, Wisconsion (click here for a map of Lake Michigan).
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 both locations having a long-term average of approximately 10 knots (view 45007 graph) (view 45002 graph).
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 winds at buoy 45007 are predominantly southerly, with the direction distributed from southeasterly to southwesterly. In addition, northerly winds occur slightly more frequently at buoy 45007. Although the buoys are only 162 nautical miles apart, the wind rose for buoy 45002 (figure 4) shows a noticeably different wind pattern than its southerly neighbor (buoy 45007). The predominant winds at buoy 45002 are southerly, with south to west-southwesterly winds occurring nearly 46% of the time. In addition, northerly winds occur less frequently at buoy 45002.
Figures 5 and 6 (below) show the distribution of observed wind speeds at the buoys for the same period covered by the wind roses. The winds at buoy 45007 were 10 knots or less 61.0% of the time. Winds from 10 to 15 knots were observed 27.3% of the time. The winds at buoy 45002 were slightly stronger than those at buoy 45007, with winds less than 10 knots occurring less often and comprising 51.8% of the observations. Winds from 10 to 15 knots occurred more frequently at buoy 45007 than 45002 -- 29.1% versus 27.3%. Winds over 15 knots occurred more frequently at the northern buoy -- 19.1% at buoy 45002 and 11.6% at 45007. Based solely upon the long-term averages, the Chicago Mac is a light to moderate air 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 7 and 8, below) is approximately .5 feet.
The long-term surface water temperature of Lake Michigan 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 27, 2015, the average surface temperature of Lake Michigan was only four 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 Chicago 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.
Let's look at an example from southern Lake Huron on June 13, 2014 using data from NDBC station HRBM4 and buoy 45008 to illustrate the impact that a low-level temperature inversion can have on the wind on one of the Great Lakes. 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 10) ranged from 10 to 15 knots, while gusts peaked slightly above 20 knots. During the same period, the sustained winds at 45008 (figure 11) 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 12 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 13) 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 14. 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 along the western shore of Lake Michigan would be inhibited by the transport of warm air from the land out over the lake. Similarly, a strong easterly or northeasterly wind would produce a weaker low-level inversion near the eastern shore of the lake. The bottom line is that competitors favoring an offshore route to Mackinac will be more affected by low-level temperature inversions than those staying closer to either the western or eastern 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 Michigan during July, it is weather -- not climatology -- that governs a long-distance sailboat race.
Additional Chicago Mac weather forecasting resources:
- Managing Your Mac Forecast, an article on how to use a variety of weather forecasting resources. (click here)
- Chicago Yacht Club's Race To Mackinac Weather Resources (click here)
- Lake Michigan Marine Weather Dashboard (click here)