Thermals and Chicago Kiteboarding
Air moves from one point (point A) to another (point B) through the atmosphere due to a pressure differential between these two points, A and B. If point B is lower pressure than point A, wind will blow from A to B. Pressure is created by heat and the expansion of the air molecules in a “column” of atmosphere.
Let us assume that point A is a location over the middle of Lake Michigan and point B is the heart of the city of Chicago. In the initial state, the atmospheric column of air molecules at point A and point B are both at equal temperatures (consider this to be early morning before the sun comes up). At this state, there is no thermal wind component, so whatever prevailing winds are present, will be unaffected. There is no pressure differential between point A and point B.
Now, as the sun comes up and warms the city at a much faster rate than the lake (due to blacktop and concrete, etc. all absorbing heat much greater than the lake, which tends to stay relatively cool), the column of air above the city expands due to temperature and exhibits a higher pressure. During the spring and summer, with the lake and city temperatures quite different, a large pressure differential will result. This pressure differential creates a new wind component that will affect the prevailing winds.
At the lower level, the amount of pressure remains the same, but the column expands above the upper level line, which creates more atmosphere at the upper level. As we extrapolate this to the Chicago vs. Lake Michigan, since Lake Michigan is cool, the city’s increasing high-pressure “dome” expands outward through the day, creating a “dome” of high pressure that expands outward to the shoreline of Lake Michigan where it meets the cooler air. As cooler air moves in from over the lake, or from over land, it meets this wall of higher pressure air and cannot pass. The wind component is then forced upward, and over, the city area. As the wind comes in over the lake and nears the city, this upward component of wind takes effect near the shoreline.
So at the shoreline, with the city heating up, the winds can be cancelled by an offshore flow working opposite to the winds. The city’s pressure expands at the surface level to the cooler shore-line waters as the airflow further out on the lake experiences an upward slope. This off-shore component working in the opposite direction pretty much cancel the wind at ground level and kitesurfing at the city beaches is very difficult as a result. However, 200 yards out, sailboats are experiencing winds off-shore since the wind hasn’t begun to rise at that point.
Coastal and low-level winds tend to follow the terrain of the land. The city of Chicago rises above the land and water with great volume. The buildings are large, tall, and the amount of construction in and around the city is dense. Just because of this geometry, the wind cannot go through the buildings, so it must go around, or pass over, them.
This second factor works against winds and along with ozone creation, the dome gets very thick and dense. During the summer, the city of Chicago contains stagnant, hot air that lasts for weeks at a time. Even after the city has cooled somewhat, it still retains ozone well into the next morning which compounds the effect of heat and pressure, strengthening this dome, which forces the air over and around the city.
This is why the kitesurfer should not be opposed to driving out of the city area for summer kitesurfing sessions.