Last winter I got my hands on historic water temperature data (20 years worth) of Lake Siljan from a researcher at the University of Uppsala. Between that information, and a variety of anecdotal remarks from my Swedish friends and their friends (who regularly spend a fair amount of time at and on Lake Siljan), I am expecting the lake temperature at the surface to be somewhere between 53 and 69 degrees F (11.5 and 20.5 degrees C), and most likely, I think, will be a temperature of 63 degrees F (17.5 degrees C) or warmer. The historic data and the prevailing opinions of these folks hold that the warmest water at the surface will be between July 24 and August 7. They (the people and the data) would say that after the first week of August, due to colder nights and diminishing insolation, the lake is experiencing daily net losses in heat, and that summer is slipping away.
The water in a lake is very much stratified based upon the temperature of the water. See this picture (model) of typical summer water temperature dynamics in a lake. This picture is illustrating many things, but for starters, see that a lake is divided into three categories along the water column. The epilimnion is the warm layer at the top of the lake, which grows warmer and thicker as summer carries on. The coldest water is at the bottom of the lake, in the layer called the hypolimnion. This water tends towards 39.5 degrees F. (4
degrees C.), which is the temperature at which water is heaviest (Yes, as the temperature of water drops below 39.5 degrees, it gets lighter!) The middle layer is called the metalimnion, and it provides a buffer between the upper and lower layers, and largely prevents mixing of these two layers as summer unfolds.
The metalimnion is also called the thermocline, and this next image shows the relative temperature dynamics of the three layers. The top layer is relatively shallow and has a
fairly uniform temperature; the thermocline evinces a significant dropoff in temperature, and then the hypolimnion shows a slow and steady dropping to 39.5 degrees F (4 degrees C).
All this is really just to say that I am very grateful for the dynamics that create the epilimnion. It grows fatter and warmer throughout the summer, providing a comfortable portion of the lake in which I can swim. An added bonus is that this layer is located at the surface of the lake, and so not only do I get to swim in this (relatively) warm water, but I can just turn may head and breathe the air. Simple to do in the epilimnion, but don’t try it in the thermocline or the hypolimnion.
To make a stab at empirically manifesting these facts, I combined blue food coloring and water, and made some blue ice cubes. Then I carefully laid one of the ice cubes in a plastic cup with water in it that was about 60 degrees F (15.5 degrees C). As you can see in the photo to the left, as the ice melted, the cold blue water streamed down to the bottom of the glass. Then I introduced some hot water (colored yellow), using an eyedropper, into the glass with the melted blue ice cubes in it. The yellow water was about 135 degrees F (57 degrees C), and as I eased it onto the surface of the colder water, most of it stayed right at the surface. The following two images show this.
What I found was that when I first put the warm, yellow water on the surface of the cold, blue water, the warm water bounced around and often dropped into the blue water, turning part of it green. But as I gradually, carefully added the warm water, it developed into a top layer that was cohesive, and was distinctly yellow (or maybe sort of green), and obviously separated from, and above, the colder blue water. Once this top layer was established, I was able to add to it with the eyedropper easily. As it grew, it developed a structural integrity, which made it behave like a distinct entity from the rest of the water in the glass.
Below is the same glass, annotated to show what might be the three distinct layers that represent the water column in a lake.
In an ideal summer, the top layer would accumulate a lot of warmth, and grow to be a couple of meters thick. In a windless world, that layer would keep fattening up. However….a couple of days of substantial wind could very much undo the integrity of that upper layer. Wind across the lake’s surface can start gouging down, and displace the topmost water so that other, colder, deeper water comes up to replace the top water (this is called “upwelling”). The first graphic in this post depicts how wind can confound the idealized layers and cause this upwelling motion, which mixes the metalimnion with the epilimnion. And the graphic below shows how the colder, deeper water, once set in motion, can infiltrate the epilimnion and compromise its integrity, causing it to mix with cooler, lower water and lose its warmth.
Once the top layer is mixed with the middle layer, it can take days or weeks before the top layer recovers the warmth it loses due to wind. So, I am hoping for a couple weeks of high pressure and light winds only, in the lead up to the swim date.
In terms of some water temperatures (from past marathon swims) as points of reference:
- Anacapa Crossing in 2010 (a 7 hour swim) was 61 degrees F (16 C)
- Lake Tahoe in 2011 (a 14 hour swim) was 66 degrees F (19 C) the entire swim
- Catalina Island channel crossing in 2013 (a 12 hour swim) was 67 degrees F (19.5 C) until the last 30 minutes, when it suddenly dropped to 62 degrees F (16.5 C)
I believe that if I encounter 60 degree F (15.5 C) water (or warmer) I will be able to complete the swim.
I believe if I encounter 54.5 degree F (12.5 C) water (or colder), my ability to complete the swim will be in great jeopardy.
I believe that if I encounter sustained water temperatures between 54.5 F and 60 F, I will have my fingers crossed, hoping that I can sustain my core body temperature for the twelve hour long swim.