This morning, the green lake looked different yet again.

If you remember yesterday’s pictures, we ended the evening with the lake being a fairly well mixed green color (picture on the right).

Now imagine my surprise when I came back in the morning and it looked like this:

The right side of the lake is still green, but the direct connection between inflow and outflow is an even brighter green! And the green inflow detaches once more at the tip of that little island (which it only did during the first observation yesterday, and two hours later the mixing had progressed around the tip).

There are only two ways I can think of how that could have happened:

a) During the night, there was a lot of un-dyed water added to the lake. Maybe through rainfall? But the effect would have been that the green color in the lake would have gotten diluted and, when the rain stopped, the inflowing water appears a lot greener than the surrounding lake water. Possible, even though I didn’t notice any rain during the night.

The other option is this:

b) Someone added more dye to the leaking pipes. This is the more probable explanation to me. The effect would be the same as above: A more intense inflow into a less intense lake.

In any case, the plume we are seeing now can only have been flowing with that intense a coloring into less green water for a couple of hours, otherwise the whole lake would have been mixed through and through.

I guess the easiest way to know which explanation is right would be (well, in addition to asking them directly) to have an objective measure of how green the water is, so that we would know if that changed over night or if the plume is really more intense now than yesterday. But with light that is always changing that is really not possible to say.

But this new green inflow is definitely beautiful: Look at the instabilities where it meets the stagnant lake water!

And more instabilities on the other side.

So those pictures were taken at around 7 in the morning. When I came back in the afternoon, the lake looked like this (sorry about the confusing lighting with the shadows and directly lit spots, can you ignore those and imagine what the color would look like under better light?):

Completely mixed and very very green! Interesting, isn’t it? So apparently the inflow stayed as intensely green as in the morning and, over the course of the day, mixed the whole thing.

P.S.: The company that puts the dye tracer in said on my Instagram @fascinocean_kiel that they are using uranine as dye, and that it’s completely safe for the environment. And, interestingly, that’s what we use in tank experiments under the name fluorescin, and that means that it is a fluorescent dye! I really need your UV light, Uta!! :-)

You know I love dye tracers (remember the beautiful fluorescent green we used in the 2017 experiments in Grenoble, when we got to play with the 13m diameter rotating tank?) but today I found some “in the wild” again — on the way back from my morning run & swim no less, in Kiel’s Forstbaumschule.

I’ve seen a dye tracer here several times before, and it’s basically just an indicator for a leak in the district heating (and everybody claims that it isn’t harmful to the environment despite its color).

Dye as a flow tracer

Spotting leaks would be very difficult if you just had normal water running into places where there is other normal water. Last winter you could clearly see that the dyed water was quite a lot warmer than the rest because it melted ice away where it went, but at temperatures like to day you might be able to see a thermal signature with thermal imaging equipment, but it is nowhere near as obvious as during winter.

But today my timing was lucky: The pipes can’t have been leaking for very long yet, because there were clear boundaries visible between the “old” lake water that wasn’t dyed yet, and the plume of dyed water entering into the lake and leaving it on the other side.

Dye as age tracer

So in a way the dye also acts as age tracer (since there are currently no other inflows into that lake. It would obviously be different if there were): the “old” water is still dye-free, whereas the “young” water is bright green. And then there are the regions where older and younger water mix and the color isn’t quite as intense.

Dye to visualize mixing

On the boundaries between the dyed water and the old lake water you see mixing in form of tiny eddies, and I’m pretty sure that when I go back this afternoon, the whole lake will be this awesome fluorescent color. And I am curious to see if there will still be flow structures visible or if it’ll all just be bright green :-)

Update: 2 hours and 11 hours later

And I went back. Twice.

Below you see how the coloring changes at the inflow mixes more and more with the lake water: left the picture taken at 7:15 am, then 9:15 am, then 7 pm. Fascinating! :)