Winning Weather Predictions for the Chicago-Mac

With the 2008 Chicago Yacht Club Race to Mackinac starting on July 19, /SW/ contributor John Gallagher explains how gradient winds interact with the region's shoreline breezes. "First Beat" from our July 2, 2008, /SW eNewsletter/

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Hot air above the streets of Chicago tends to intensify onshore thermal breezes during the day and reduce offshore thermals at night.Lucille O'neill

Every other year during the 330-mile Chicago Yacht Club Race to Mackinac it seems the fleets spend at least half a day becalmed and drifting under the center of a slow moving high pressure system. If such conditions lie in store for this year's Chicago-Mac, skippers who understand the interaction of gradient winds with shoreline thermal breezes will have a competitive advantage. While satellite weather equipment can forecast the size and movement of a high, racers still need to decide where they want to be with respect to the shoreline when the system arrives.

Shoreline thermal breezes develop as the result of shoreline land heating up above lake temperature during the day and dropping below lake temperature at night. Sea breezes are directed onshore and land breezes, offshore. Both thermals initially develop perpendicular to the shoreline.

Gradient wind is wind that develops from the atmospheric pressure gradient around a high pressure center, often called cyclonic air flow, and around a low pressure center, often called an anti-cyclonic air flow. Wind flows clockwise around the center of a high and counter-clockwise around the center of a low. Air movement is strongest near the center of a low and weakest near the center of a high--that's why a forecast of light and variable winds frequently accompanies the arrival of a high pressure system. While lows typically follow the path of the jet stream, passing fronts can dramatically change the direction of the wind.

If there is no gradient wind, the center of the lake may be becalmed. If a gradient wind is present, it will combine with the shoreline breeze at a magnitude and direction equal to the resultant vector sum.

Many subtle factors affecting the strength and direction of sea and land breezes. For an afternoon onshore sea breeze to fully develop, you must have a sunny day over land along the shoreline. Absence of cloud cover over the shoreline allows the temperature of the land to rise rapidly, increasing by as many as 20 degrees above the temperature of the lake water. As the air over the shoreline heats up and rises it pulls in cooler air from over the water. This onshore sea breeze starts in the late morning, building to full-strength (usually around 10 knots) by mid afternoon. The hotter the ambient air on shore, the stronger the onshore sea breeze. It typically veers (clocks) about 10 degrees per hour starting mid-afternoon.

For an overnight, offshore land breeze to fully develop, there must also be an absence of cloud cover, allowing the shoreline land to cool down rapidly to as many as 10 degrees below lake temperature. The relatively warmer air over water will rise, pulling cooler air out from over the land. An offshore land breeze is always much weaker that an onshore sea breeze and usually peaks just before dawn.

Onshore and offshore breezes develop as a result of the high heat capacity of water as compared to that of the land. Lake Michigan acts like a giant heat reservoir: it takes a long time to heat up, and a long time to cool down. Compared to land, water must absorb more heat for its temperature to rise, and it must lose more heat for its temperature to fall. For practical purposes over the few days of a sailboat race, we can consider lake water temperature at any particular latitude to be relatively constant, unaffected by the sun or ambient air temperature.

Land, on the other hand, has relatively low heat capacity. When the sun goes down, land will lose its heat rapidly and the temperature will drop. The temperature of the land along the shoreline can swing as much as 20 degrees above the lake water temperature during the day, and drop as much as 10 degrees below the lake water temperature at night.

Cities along Lake Michigan, such as Milwaukee on the western shore with its high volume of asphalt, concrete, and stone, will absorb considerably more heat from the sun's radiation than the forested shoreline, causing much longer and stronger afternoon sea breezes, but greatly diminished overnight offshore land breezes.

Because sand also absorbs a lot of heat, you would expect the Sleeping Bear sand dunes along the eastern shore of Lake Michigan to have an effect similar to a large city, with stronger and longer onshore sea breezes during the afternoon, along with very weak offshore land thermals overnight. But the elevation of the dunes comes into play, causing afternoon onshore breezes to be deflected upwardly, disrupting the circulation loop of the lower level thermals so as to so as to introduce an unstable behavior. As you sail north along the 35 miles of the Sleeping Bear, the elevation of the sand dunes increases until reaching 400 feet near the Manitou islands, with disruption of onshore thermals increasing with shoreline elevation.

As racers proceed north on Lake Michigan in July, the lake water temperature drops from about 75 F near Chicago to about 65 F near the Manitou islands, 200 miles to the north. The land-water temperature differential during the day typically will increase with increasing latitude, increasing the strength of an afternoon onshore sea breeze. As you proceed north, the temperature differential will be less during overnight hours, decreasing the strength and duration of overnight offshore land breezes.

As humidity increases, shoreline air, warmed by land during the day and by lake water at night, becomes heavier (and thus less buoyant) due to its increased moisture content. This slow-rising air prevents the shoreline thermals from fully developing. Very high humidity, anything above 90%, as evidenced by poor visibility or even fog, creates such a dense layer of air that the heat from land during the day and water at night is not enough to cause the air to rise, thereby preventing development of thermal winds.

The afternoon sky over the shoreline contains clues as to probable strength of onshore shoreline breezes. A clear blue sky will allow an onshore sea breeze to fully develop in the afternoon, and if the sky remains clear overnight, the offshore land breeze will fully develop. A sky with ragged, broken cumulous clouds, having little vertical height, indicates humidity in the air that will slightly reduce the strength of the thermals. Dark cumulonimbus clouds along the shoreline, early morning haze, and fog that "burns off" all indicate high humidity and reduced-strength thermals. A heavily overcast sky during the day will prevent the sun from heating the land, resulting in weak shoreline winds. Overnight, an overcast sky will hold heat in the air, likely resulting in a total shutdown of the offshore land breeze.

Wind Near the Shore

If there is no gradient wind, as often happens in the center of a high, sea breezes and land breezes will develop along all shores, leaving the center of the lake becalmed.

If you're along the Wisconsin shore with a gradient wind moving in from the west (across the land), the effect on the thermal depends on what time of day the system arrives. In the afternoon, the gradient wind will cancel or reduce the onshore sea breeze, leaving holes in the transition zone. If the gradient wind is light, say 4-8 knots, it may be best to stay offshore at least 10 miles to clear the transition zone. If the gradient winds arrive overnight, they will add to the offshore land breeze, extending the enhanced air zone out to 15 to 20 miles offshore. The strongest air will be within 10 miles.

If, along the Wisconsin shore, the gradient arrives from the east in the afternoon, the wind will add to the onshore sea breeze, extending the enhanced air zone out to about 12-15 miles. By late afternoon the onshore sea breeze will slowly die out, followed by a few hours of gradient wind, until offshore land breeze starts up an hour or two after sundown. Work out to 10 to 12 miles offshore before sundown. Monitor water temperature if you're near the cold water zone centered near Sheboygan, where the magnitude of the onshore sea breeze will substantially increase due to the increased temperature differential. Arriving at night, a gradient wind from the east will be reduced or cancelled by offshore land breeze, with the transition zone full of holes. Stay out at least 10 miles to avoid the transition zone.

The water temperature in the lower one-third of Lake Michigan averages near 70 F in July. After a few days of a strong westerly winds, however, a 40-mile-long, banana-shaped band of colder, 55-degree shoreline water develops along the Wisconsin shore, centered near Sheboygan and extending out about 10 miles from shore. This cold band of water parallel to the shoreline is being pulled up from the deepest part of Lake Michigan, the result of strong westerly winds driving surface water toward the eastern shore. This cold band of water will tend to increase the onshore sea breeze in the afternoon and prevent an offshore land breeze developing overnight.