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

6

THE UNUSUAL CURRENTS
Lake Michigan
On 9 and 10 August 1955 multiple-boat single-day coverages of Lake Michigan were carried out. Winds at Chicago and Milwaukee were from easterly
quarters on the 7th, 8th, and 9th; at Grand Rapids and Sault Ste. Marie they
were from easterly quarters on the 8th and 9th; Escanaba had north winds on
the 7th and south winds on the 8th and 9th; on the 10th the winds at all these
stations were from westerly quarters, except at Chicago which had east winds.
In other words, the surveys on the 9th and 10th came after winds which were
from unusual quarters. The current patterns in the lake on these two days may
be taken as representing the currents to be expected during the second day of
a two-day easterly blow. Table 1 gives the winds that preceded all four of the
Lake Michigan surveys.
Currents in the lake do not respond immediately to wind changes, in fact
the evidence indicates that the currents present on any given day reflect the
wind directions plus rotation of the earth ("T" arrows around Lake Michigan
charts) of the days prior to the one in question, but not of the day in question.
If you are sailing today, you are encountering the currents set up by the winds
of yesterday, the day before, the day before that, etc., but the effects of today's
winds will not be showing until tomorrow. The effectiveness of previous winds
in today's current pattern falls rapidly as the days become farther in the past.
Yesterday's winds would be ranked, for example, as 1; those of the day before
as 1/2; those of the day before that as 1/4, etc. For practical purposes the
winds of the preceding four or five days are usually all that need be considered,
but to be strictly accurate about ten days need to pass before the effects of a
day's winds are completely gone from the current pattern. The "W" arrows
on the Lake Michigan charts are the effective wind direction of the preceding
ten days.
Today's winds spend their energies changing the current pattern left by
yesterday; the changed pattern will be operative tomorrow at the soonest. If
today's winds are from the same direction as yesterday's, their effects will
show in increased current speeds tomorrow. If today's winds are opposite to
those of yesterday, they will show as slowed currents, stopped currents, or
reversed currents tomorrow according to whether today's winds are weaker,
equal to, or stronger than yesterday's. Wind shifts amounting to less than
actual reversal of direction will cause changes in both the directions and speeds
of the currents, as a moving billiard ball when struck by the cue ball takes up
a new direction and a new speed that combine those it originally had with those
it received from the cue ball.
Chart 11 gives the currents in Lake Michigan on the second day of an easterly blow (third day at Chicago). The dominant feature in the current pattern
on this day, 9 August, was a strong and wide southward current along the east
shore from the Straits to Chicago. The large eddy found against the Michigan
shore under "normal" winds was displaced to the center of the lake. The outflow current also was crowded away from the Michigan shore. It ran almost
up the middle of the lake as far as the offing of Frankfort where it crossed
to the mouth of Green Bay and then went eastward along the shore of the Upper Peninsula of Michigan. In the western part of the Straits of Mackinac the
outflow current was denied escape into Lake Huron by the water transport
direction ("T" arrow) accompanying the winds in the Straits.