The currents of Lakes Michigan and Huron / John C. Ayers.

THE "NORMAL" CURRENTS

Lake Michigan
Charts 3 and 4 show the surface currents of Lake Michigan on 28 and 29
June 1955. On each of these days the lake was completely covered by the simultaneous cruises of eight vessels -- giving an overall picture of conditions
in the whole lake on each day. Chart 19 shows the courses of the vessels and
the sampling stations they visited.
Winds prior to these coverages had been from quarters more or less normal for that time of year. In the author's opinion, the wind conditions (and
hence the currents) in the southern half of the lake were somewhat more typical on 29 June than on 28 June, while in the northern half the wind directions
at Escanaba and Sault Ste. Marie were more nearly from the prevailing directions on 28 June than on the 29th. It is believed that the normal current pattern is most accurately represented if the currents of 29 June for the region
south of latitude 440 (Ludington - Manitowoc) are combined with those of 28
June for the region north of 44~. The "normal" current pattern described
later will be from this combination of days.
The southern half of the lake is sufficiently wide to exert a modifying
effect on winds crossing it. Summer winds coming to the lower lake from
southerly or westerly quarters are warm from their passage over the inland
states. Upon moving out onto the lake, which is cooler than the air, the lower
levels of the moving air are cooled by the lake and contract. The contraction
of the lower air starts a descending motion in the moving air and barometric
pressure over the lake will be somewhat higher than that over the land upwind.
Cooling by the lake, setting up a descending motion in the moving air and
causing over the lake a "high" of greater or less strength, should produce some
tendency for winds to radiate around the "high" in a clockwise direction. The
condition of clockwise effective wind directions around the southern half of the
lake (arrows marked "W" in the charts) was somewhat better realized on 29
June 1955 than on the preceding day. The currents are driven by the wind but,
because of the rotation of the earth, they move in a direction about 45~ to the
right of the wind direction (arrows marked "T" in the charts). The somewhat
more typical arrangement of wind directions on 29 June argues that the current pattern of that day will be somewhat nearer the "normal" than will the
one of the 28th. Table 1 gives the winds of the periods of the Lake Michigan
surveys.
The southern half of Lake Michigan (in Chart 4) contains two dominant current features. A large flattened clockwise eddy commonly lies close to the
Michigan shore and extends from off Grand Haven to Michigan City. Associated with this eddy are two smaller ones which are counterclockwise and which,
in the lee of the shore from Little Sable Point to Benton Harbor and from
Michigan City to Chicago, have currents contrary to the wind. The second
major feature of the current pattern is the outflow current which rises from
the Wisconsin shore at about Rawley Point, then flows southward (just off
shore) to about the offing of Waukegan where it turns offshore and crosses to
the central and east-central part of the lake; here it becomes associated with
the west side of the large east-shore eddy and moves northward to come to
the Michigan shore at Big Sable Point. Small clockwise eddies lying off
Waukegan, off Kenosha, and off Milwaukee are associated with the outflow