Lake Michigan’s Spring Turnover

Right now, there is a thin layer of water on the surface of Lake Michigan that is colder than 39F (4C) and therefore less dense. This thin layer of water floats on top of the lake and under the ice throughout the winter. We’ve been swimming in this less dense layer of water since November.

However, were we to dive down past 30 feet, we would find the lake temperatures increasing to 39F and remaining at that temperature all the way to the bottom of the lake. This layer is known as the hypolimnion, which is a constant in cold, deep-water lakes. So over the past few months, we’ve had this scenario:

With the arrival of spring, the ice melts off of Lake Michigan and the top layer of water on the lake gets warmed by the increasing levels of sunlight. Once the surface temperature reaches 39F, the surface waters sink until the water temperature at all depths reaches approximately 39F. This sinking, combined with spring winds mixing up the water, causes a phenomenon known as the “spring turnover” which is beginning to take place right now:

As spring transitions into summer, warmer water collects on the surface and the lake eventually stabilizes into three layers:

As the summer wanes into fall, the surface temperatures cool and the process reverses itself. We get a similar turnover in the fall, and then we’re back in winter mode.

The key point to note here is that the majority of Lake Michigan never really goes above or below 39F. In addition, due to its enormous variability in surface area and depth, Lake Michigan is never really that far from lapsing back into “turnover mode” on both ends of the summer months.

So always be sure to bring a variety of gear with you out to Ladder #1, because you never know what you might get – even in the middle of the summer!

4 Comments

  1. diver dave
    March 26, 2009

    CHRIS AND I SWAM IN 74 DEGREE WATER AND RETURNED ONE DAY LATE AND WE WERE SURE WE WERE IN A COLD SPOT TO START THE SWIM. WELL IT NEVER WARMED UP AND AT THE END OF THE ONE MILE SWIM I TOOK THE TEMPERATURE AND THE LAKE WAS 59 DEGREES!!!! A TRUE 15 DEGREE DROP IN 24 HOURS…… I SEEN 5 TO 10 DEGREE DROPS BUT THIS WAS A FIRST IN THE 34 YEARS OF SWIMMING OAK STREET.

  2. srhernan
    March 26, 2009

    I actually harvested some of this info from the Tribune article you gave me last week. The rest I looked up online. Pretty interesting stuff, and it explains why the lake can still be nice in mid June and then turn cold the next day!

  3. diver dave
    March 25, 2009

    ? DID YOU TAKE A COLLEGE CLASS ON LAKE MICHIGAN ? NOW THIS IS VERY INTERESTING IMFORMATION….. GOOD WORK STEVE


Mentions Of This Post

  1. […] I took the official lake temperature, and I was elated to see a reading of 50F. ┬áThis means that Lake Michigan’s Spring Turnover is taking place right now – and the days of sub-40F water are behind us until […]

Leave a reply to » Swim Report – April 15, 2010 – The Spring Turnover has arrived at Oak Street Beach!

Click here to cancel reply.