Large Lakes Observatory

University of Minnesota, Duluth

Duluth, MN 55812



This is certainly one of the most interesting measurements we're making; This instrument, a thermistor string, measures temperature at 10 depths, from about 1m (3ft) to 28m (90ft) deep below the buoy. This shows us the vertical structure of temperature in the water column (of course, this site is actually about 50m deep; for technical reasons, we're measuring a little over half of the water column).

Water likes to stratify- light water sitting on top of heavier water. Everybody is familiar with this phenomenon at some level- diving into a pond on a still, summer day and finding the water a few feet down much colder than that at the surface, or even going upstairs on a really hot day. For fresh water above 4C (about 39F) the warmer it gets, the lighter it is, so warm water will tend to sit on top of cool water.

Temperature at depth can change for a number of reasons: It can recieve heat from (or lose heat to) the atmosphere at the surface (i.e. if warm air blows over the surface, the water can warm), it recieves heat from the sun, it can mix internally, or it can change due to currents in the water. The multiple pathways that might cause water temperature to change makes predicting this structrure very difficult.

One of the most interesting phenomena to observe in the time series shown here is internal mixing; i.e. no heat is gained or lost by the water, just redistributed. If you mixed a gallon of 70F water and a gallon of 50F water, you'd end up with two gallones of 60F water. The same thing happens if the wind blows strongly over the surface of the water; the waves created mix up the water, typically causing surface temperatures to drop and temperatures below to rise. Likewise, on a sunny, still day, heat from sunshine gets deposited in the top few meters of the water column without getting mixed in; often you'll see temperature differences of ~5C (10F) over the top few meters of the water column!

The top graph shows temperature as a function of time and depth; The little arrows on the left show the actual depths of the measurements. The plot below show the raw data from each of the thermistors. A rule of thumb is thatt the water is stratified so that the top curve is the top thermistor, and the bottom curve is the bottom thermistor, at about 28m depth (about 90 feet). The bottom plot is nice because you can very distinctly see mixing events, where several thermistors in a row register exactly the same temperature.



By the way, the thermistors look like this:



In this picture, the yellow wire is the strong cable that keeps the buoy in place; the black wire carries the signals from the thermistors to the surface buoy. The black object strapped to the yellow wire is the thermistor itself. There are ten of these.

In the background is a swivel (on the left) and a thermistor that logs internally. We'll get data from this one when we recover the mooring.
























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