Published: Wednesday, August 29, 2012 7:00 am
By: Ken Quant

The Great Lakes are known for highly variable wind conditions. It’s not uncommon to see everything from dead calm, to strong winds, all in the same day. No matter what the strength or direction, all winds on the Great Lakes are created from two predominate sources: Lake Breezes and System Breezes. By understanding the dynamics of these two types of winds, you will have a better chance of enjoying your sailing day on the water.

Three Basic Weather Factors You Should Know:

• Warm air rises and cold air sinks.
• High pressure air moves towards low pressure air.
• In the northern hemisphere, high pressure systems spins clockwise and low pressure spins counterclockwise.

Lake Breezes
One of the most common type of wind on the Great Lakes is a lake breeze. They are generated mostly on sunny days because the sun creates a temperature difference between the water and the land.

During the day in the summer, and even on some cloudy days, the land heats up much quicker than the water. This sun warmed air over the land rises and creates a weak low pressure area over the land near the shore. Cooler dense air centered over the lake water then effectively becomes a small scale high pressure system over the lake. This cooler denser higher pressure air then heads toward the weaker land based low pressure air to fill in the void caused by the raising land warmed air. This creates a lake breeze. Lake breezes become less intense as the surface water temp increases.

Lake Breeze Direction
The direction of a lake breeze is determined by your location along the shore in relation to the center of the cool lake water. Because the water cooled air over the center of the lake forms a weak high pressure system, lake breeze winds spin clockwise. This is why Lake Breezes are always from an onshore directions.

Why Does the Lake Breeze Die in the Evening
Lake breezes generally kick in around late morning because it takes a while for the sun to heat the land enough to cause the raising air. Conversely, as the sun sets, the land cools so the temperature difference abates and the lake breeze dies.

System Winds
Generated by the pressure differences between large scale high and low pressure systems. The tighter the pressure gradient between the high and the low is, the stronger the wind.

Two Factors that Determine Pressure Gradient and System Wind Strength
First is the strength of the pressure at the center of the high and low pressure system itself. Generally the higher or lower a systems center pressure is, the tighter the pressure gradient between systems and the greater the wind.

The second factor to system wind strength is the physical proximity of the highs and lows to each other. Generally, the closer they are together, the stronger the pressure gradient between the two, so the stronger the wind.

In some parts of the Great Lakes, the lake breezes are stronger then in other parts, so you actually need a strong system of overtake the lake breeze. In favored Lake Breeze locations, if the pressure gradient is tight enough to generate a system wind direction greater than 15 knots offshore, it will generally overcome the usual lake breeze creating offshore sailing conditions near shore. The stronger the wind, the more likely it is to kill any potential for lake breeze development.

What Causes the Different System Wind Directions
System wind directions are determined by where the pressure systems pass your location.

System Passing to your North
Generally, if the center of the low passes to the north of your location, you will get southeast winds as it approaches, southwest winds when it’s near and west to northwest winds once it passes.

This type of setup creates classic “clocking” wind directions during the day, so it can be anticipated that the wind will gradually shift to the right as the system passes.

System Passing to your South
Generally, if the center of the low passes to your south, you get east to northeast winds as it approaches and north to northwest winds once it passes.

This type of setup often creates a “backing” wind as the wind direction moves towards the left as the system passes to your south.

System Enhancement of Lake Side Breezes
Because there are no buildings or trees to slow surface winds over the lake, onshore breezes are often stronger at the lake than inland. This lack of surface friction is also the reason why system winds off the lake seem to blow so hard.

Why Does the Wind Always Seem to Die in the Evening, Even on an Offshore Day?

There are almost always upper level winds blowing above the surface. During the day, the uneven heating of the land by the sun creates raising pockets of air that mix with the upper level winds. This mixing of the air pulls some of the upper level winds down during the day creating windier conditions at the surface. In the evening, as the sun sets, this mixing subsides and the upper level winds return to a more stable position above the water surface.

Watch for the Dreaded Transition Zone
A transition zone is the area where the lake breeze meets the system breeze. This convergence of wind directions often creates an area of little or no wind. This moves around during the day as the lake breeze builds and subsides. You often see a near shore transition zone in the late morning and early evening as the lake breeze forms or dies.