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Duluth: Colder/Warmer By the Lake Examined

An old drawing of Duluth and Superior and Lake Superior

Duluth (1893) by Olin Dunba Wheeler. Public domain from Wikimedia Commons

The expressions "colder by the lake" and "warmer by the lake" describe two curious phenomena of Duluth weather and Duluth's Temperatures. In Duluth, a five minutes drive can mean a jump or drop of 20 degrees, or more.

One person who has studied Duluth weather including the "by the lake" effects is NOAA meteorologist Carol Christensen. Carol knows it is all about heat storage.

“Right now [early summer], the lake is still very cold because the heat capacity of water is very high. So, it takes a lot of energy to heat up water,” said Carol.

Heat storage is crucial to life on earth, more than even your smartphone's storage. This storage is not measured in gigabytes but in units like calories, BTUs, and joules.

Heat is just the random movement of molecules. The faster the movement the more thermal energy (heat) an object holds. Some materials just hold more heat than others, at the same temperature. For example, it takes over four times as much energy to raise a gram of water by one degree as it does a gram of aluminum.

You've probably always wanted to know more about heat storage and heat flow, so here you go. I spent way too much time making a series of animations, which hopefully make these concepts clearer. Let's start with a single tank.

A tank holding liquid representing heat storage

Imagine that heat is a liquid in all objects and it can flow. Heat is NOT a liquid, but in science we are always using some analogy to compare something hard to understand with something easier understand, and easier to picture. This is fine, as long as you know it is just an analogy.

Actually, scientists once believed heat was the result of a real fluid called "caloric". So, this isn't that crazy.

The amount of liquid in our "heat tank" is proportional to the amount of heat an object has and

Two tanks holding liquid representing heat storage

the height of the liquid represents the temperature.

Three tanks holding liquid of different heights representing different heat capacities

Now, the volume in each tank represents the heat capacity. As we see, three objects can be the same height (temperature) but hold vastly different amounts of liquid heat.

Heat is sort of a nomad and doesn't like to stay in one place long. When it moves it is called "heat flow". This requires that two or more objects are in some sort of contact. The better the contact the better the flow.

Two tanks with liquids representing heat storage connected to illustrate heat flow.

Liquid will move from a higher level tank to a lower level tank until they reach the same level (temperature).

The only thing that determines if liquid (heat) flows is a difference in tank levels (temperature). Once two objects are at the same temperature the flow stops. If your can of pop is at 40 degrees and you put it in a bucket of 40 degree water, nothing happens. If you put it in a bathtub of 40 degree water or even an ocean of 40 degree water there is still no effect.

However, heat capacity (tank volume) will determine how much heat flows.

Four tanks with liquids representing heat storage connected to illustrate heat flow.

Here we see hotter objects (tanks on left) will lose more heat reaching equilibrium with an object with a large heat capacity than with a low capacity tank. Their temperatures will also drop more.

The reverse of this happens if the tanks on the right start out higher (hotter) then the left tanks. Then imagine that each left tank represents your hand.

If you drip a drop of hot wax on you, it will hurt, but not that much. The drop of wax quickly transfers its little bit of stored energy to you and doesn't do that much damage. On the other hand, if you pick up a brick at the same temperature as the wax it will transfer a lot of heat to you and hurt a lot. Heat capacity does make a difference after all.

Two tanks holding liquid representing heat storage

Here is where we apply these cutting edge animations to Lake Superior and the surrounding land. Just like before, remember the liquid in the lake tank is not water but represents the heat in the water. I made the liquid red to help remind you.

As we said before, water has an enormous ability to store heat, or a large heat capacity. This heat capacity can be expressed as per object, per mass, and per volume.

In this situation, it is really helpful to think about heat capacity per surface area. The two tanks each roughly represent the heat stored by a square mile of land near Lake Superior and a square mile of the lake itself. I based heights on some publicly available archived data on average surface temperatures on Lake Superior and land in St. Louis county, in the same year.

Having the same surface area, these tanks each receive about the same amount of heat as sunlight. Also roughly proportional to their surface area, they lose heat as radiation and gain and lose heat from passing air masses, among other things.

Both levels (temperatures) go up and down, but the land tank gets filled and emptied much quicker than the water tank. Temperatures vary little in the spring and fall and widely in the early summer and early winter.

Among other complications, in the winter ice can dampen the "by the lake" effect because the ice acts as insulation.

So the time of the year is crucial in determining the degree of by-the-lakeness. The other big factor is the wind. It depends on if the wind is blowing onshore or offshore and also the fetch

"It depends on the fetch of the wind. For Duluth, A northeast wind has a much longer fetch. So, the winds will be across the lake a much longer time and will bring colder temperatures," said Carol.

Just think of "the fetch" as the winds ability to fetch coldness. If it is blowing toward Duluth you might want to fetch a coat or fetch wood for a nice fire in your backyard. These fetches also have a big effect on Duluth waves and beach water temperatures.

We can classify the fetches as "long fetches" or "short fetches". The following animation illustrates a long fetch.

Many tanks holding liquid representing heat storage of Lake Superior and the land and the effect of a long fetch

Here we see our old friends lake tank and land tank again. We have put together four to eight of these square mile tanks to represent all the lake and all the land near the lake.

We introduce the new character in this drama, namely "air tank." The air tank is a piece of air which is skinny since it can gain and lose heat very quickly.

During a long fetch, the air tank dumps a lot of heat into the lake and is still pretty cold passing through the shore areas. Then there is a short fetch.

Many tanks holding liquid representing heat storage of Lake Superior and the land and the effect of a short fetch

During a short fetch the wind blows at an angle where it isn't over the lake long. It dumps some heat and cools the shore areas some, but not by as much.

An example of a long fetch is a northeast wind. A common short fetch is straight north toward Duluth, which is about 10 miles across water. An example of a "no fetch" is a northwest wind. Expect no heat relief from the lake during a northwest wind, except when the lake surprises us again.

"Sometimes there are days where it is pretty obvious that the lake breeze won't really have a chance. But then again, even if everywhere the wind is northwest we can still have a good lake breeze kick in," said Carol

This complication for Duluth weather forecasting is again caused by the large heat capacity of the lake, as seen in this final illustration.

Many tanks holding liquid representing heat storage of Lake Superior and the land and the Lake Breeze

Here the sun pours about the same amount of heat into each tank (square mile) on the land or lake. The land heats up quicker and pushes up air quicker. Wind from the lake comes in to replace this pushed-up air.

This wind can bring coldness to Canal Park and increased sales of hoodies. Visitors will remember the expression "colder by the lake."