Have you ever wondered why the water at Duluth's beaches can be freezing one day and warm the next day, in the middle of summer?
Perhaps you are a Duluthian, who took some house guests to the beach. Once there, your uncle Al tried to get at least part of his body into water — it's a beach after all. If it was a typical summer day in Duluth he got in up to his ankles or knees before saying "forget this."
You may have told him sometimes the water is warm. You told him it has something to do with the wind pushing the warm water into the shore.
At this uncle Al laughed.
"You just got done telling me how Lake Superior is so big that the temperature doesn't change much," he said.
At this you may have not known what to say.
It turns out these big differences in Lake Superior's water temperatures off Duluth's beaches really are caused by the wind.
"If we have a period of northeast winds, we will have much warmer water temperatures, at least surface temperatures," said NOAA Duluth meteorologist Carol Christenson. "So, that layer of water near the surface gets blown toward the shore. So the good time to go swimming is when you have those offshore winds. We will have warmer water temperatures."
So, you can show this quote to your, uncle who laughed. He was thinking of the large heat capacity of Lake Superior as evidence by things like the "by the lake" phenomenon.
While this stubbornness of Lake Superior to change temperatures is true, this is the overall temperature. It can vary from point to point and form layers. Depending on conditions, like how warm it was the day before, and how much the water has been churned up, Lake Superior can form a shallow layer of warmness at the top. What would happen if you could collect just this warm water and swim in it?
That is basically what happens in downwelling. The wind collects all this warm water over miles of lake and puts it in the area along the beach. Downwelling looks something like this:
As you can see the warm surface water is pushed into the shore replacing the water already there. It doesn't leave as quick as it arrives and pools in the shallow area along the beach.
It works out for surfers that the warmer water will be present at the same time the big waves are out, when the wind is blowing strong from the lake toward the shore. A long fetch also helps. A "fetch" is the distance the wind is over the water. A fetch can fetch warm surface area to put on the beach, fetch big waves to also put on the beach, and fetch the cold air off the lake and bring it to the beach.
That's right, you may see colder air temperatures, at the beach, on the days when the water is warmest at the beach. The wind giveth and the wind taketh away.
I personally witnessed the effects of downwelling earlier this year. It was May 7th. I waded in the water near the Park Point boat launch. Ok, I had motor problems and had to jump in the shallow water to keep the boat from hitting the shore.
I found the water was quite warm, for this early in Duluth. I came back a few days later and out of curiosity checked the temperature. It was quite cold this time.
I had just witness a strong demonstration of downwelling.
The day before was the day that the news reported that Two Harbors was the hottest place in America. That day, it was very hot in Duluth, as well.
The day before, apparently the temperatures had warmed up the surface of the water. The next day, when I was there, a strong northwest wind was blowing big waves into that cove. It was also pushing that warm surface water into the shallow cove, making it very warm for the beginning of May.
The wind can also make the water very cold, when it blows toward the lake. This is called upwelling.
This upwelling looks something like this:
As you see the surface water is pushed away from the beach and the cold water, being part of nature, fills that vacuum of water that it abhors so much. The water isn't literally freezing, since the densest water is 39.16 degrees Fahrenheit and it likes to hang out on the bottom of the lake. But, 39 degrees is still pretty cold.
So look for a day with a strong northeast wind, for a nice swim on Park Point. You can go to the website for Park Point for the current water temperatures. Then, tell your visitors that the beaches in Duluth really can be warm enough to swim in. This warmness however can come and go with the wind.