NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Great Lakes Currents
From: Frank Reed CT
Date: 2006 Jun 7, 19:25 EDT
From: Frank Reed CT
Date: 2006 Jun 7, 19:25 EDT
Bill, you asked: "Given the rate of change, could it be some sort of set up, and small seiche(s) at work?" Although Lake Michigan has natural frequencies of oscillation like any lake, the oscillations you're seeing right now are primarily lake effect weather. This time of year, when the weather is mild, there is a strong tendency for the wind to pick up out of the west in the morning and blow from Illinois across towards Michigan. The lake follows the wind. Later in the day, as the wind dies, there is often a cool "lake effect" breeze that blows back towards the west from the lake. Late this afternoon in Chicago, there was thick fog within one mile of the lake and it was about five degrees cooler. You can watch the water flow back into the lagoons over the course of a few hours, raising the local water level by two inches or so. Then the whole cycle will repeat tomorrow. In this case, the lake is a passive partner. The cycle is in the local atmosphere. That said, yes, there are seiches in Lake Michigan. The primary oscillation is a north-south motion. There is some confusion over the use of the word "seiche" in the Great Lakes. A seiche properly refers to a tide-like oscillation at a natural frequency, e.g. the 14 hour periodic pseudo-tide on Lake Erie. This is usually intiated or "driven" by weather systems. Some people apply the term "seiche" to something like a tidal bore that results when a major difference in lake elevation has been created by a strong storm or a strong pressure differential. These "bores", which can be deadly, are often followed by periodic seiches lasting for several days. Incidentally, Lake Michigan does have real tides --tides driven by the combined gravity of the Sun and Moon, that is. They have a maximum range of about 2cm, which is just barely detectable in lake level statistics. Approximate tidal constants for the southern tip of Lake Michigan are: M2: 0.750cm 37.4deg S2: 0.310cm 48.3deg In small bodies of water (Lake Michigan is small by the standards of the gravitational tides), the tide basically tracks the Moon's position in the sky with a small inertial lag, which is why the phase angles are low. The luni-solar tides in the other great lakes are quite a bit smaller. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars
