Do you know how, on sea walls, you can see what a typical water level is by the kind of gunk that is growing on the wall in horizontal lines?
Well, then what do you think of this?
Pretty fascinating, ey? Are the waves always be leaping up in those indents or what is going on there?
I’ve been thinking about an old friends recently. I met Anton on a RV Knorr-cruise I was on in 2011, and we spent many fun night watches running CTDs. Together with Sindre, this was our favorite toy: The echo sounder. Many CTD packages have an altimeter on them so they don’t accidentally hit the bottom, and if the package is deep enough, funny pictures develop.
In the picture below, for example, you see the CTD at its deepest at the very left of the screen. The x-axis is time – as new time steps are added on the right, old ones eventually vanish to the left. The y-axis is time in milli seconds, which is more or less proportional to the distance of the pinger to the receiver on board the ship.
So on the left you see those two lines very close together – the top line being the CTD that is sending the chirps, the bottom line is the sea floor. As the CTD comes up again, its own signal rises to the top of the screen (because it’s coming closer to the receiver and hence the run times get shorter) and the sea floor sinks away (because the signal now has to travel down further and further before it is reflected up again).
Then what you see at the right of the plot are more parallel lines emerging. And those occur because the run length of the sound signal is longer than the time between two chirps, so the receiver registers both the actual reflection from the bottom as well as the reflection from a previous chirp.
This is fine when the CTD is going up, but when it’s going down you have to pay close attention because you might have to cross what looks like a reflection from the sea floor a couple of times before you come close the actual sea floor. Unnerving if you aren’t as nerdy as Sindre and me and enjoy playing with this equipment!
In the plot below you can see more funny features: The left half of the screen shows the ship’s echo sounder (so the sea surface and sea floor staying in more or less the same place over time), and on the right we played with different scalings and zoomed in. Obviously, the smaller the units on the scale, the larger the magnification.
So yeah. It was fun. And thanks for all the good times, Anton.
Lets go back and talk about one of my favorite non-oceanographic topics: Rainbows!
When I had my rainbow phase about a year ago, my mom sent me the movie below, which shows what you see when you look directly into the prism that paints these kinds of rainbows all over my parents’ living room:
Rainbow from glass prism
When you look directly into the prism, you don’t see a rainbow like the one projected on the wall, but you see one color at a time. Only as the prism moves you experience all the different colors of the rainbow. And that is interesting because in a rainbow you see all colors at once, yet here you don’t. This is going to go into the next version of my rainbow movie, but for now check out my mom’s:
My renewed interest in rainbows was sparked one Saturday where I saw one on my way to the swimming pool in the morning.
And then a double rainbow on an evening walk with a friend.
And then another friend, F., sent me the picture below which he had taken at Heathrow and which he kindly allowed me to use for educational purposes on my blog.
Are you as exited as I am that we are finally getting back into rainbows? :-)
I’m sure I’ve talked about stream lines and streak lines and all the other kinds of lines that you learn about in hydrodynamics at some point. The other day I saw a very nice (and slightly disgusting) example of trajectories.
Trajectories of ducks
The ducks paddling in muddy waters left distinct trajectories of where they had been. Additionally there was some flow, so the ducks as well as their trajectories were advected with the flow. Very interesting to watch!
My sister and I did a little sight-seeing tour in Hamburg the other day, and one of the most fascinating things I saw was — a diving duck. Now, that is not a reflection of how exciting the rest of Hamburg is, but if you don’t see it after watching the movie below, when you read the upcoming blog post on Friday, you’ll understand why this was so exciting to watch :-)
[vimeo 131966138]I’ll give you a hint:
Diving duck :-)
Another one of those days where I kinda wish I had taken at least some meteorology at some point (only “kind of” because I wouldn’t want to miss any of the stuff I actually took…). But on my way to work I saw the clouds below:
The internet says they might be cirrocumulus undulatus clouds.
In any case, the wavy clouds started to disintegrate into cirrocumulus-like clouds.
But whatever they were, they were very pretty!
Meteorologists out there (Torge! :-)) – what kind of clouds were they and why did they form?
Relating the phases of the moon to one side of the moon being lit by the sun and the other side being in the dark sometimes appears a bit unintuitive. One thing that books and “the internet” always recommend is holding up a sphere in the direction of the moon and pointing out how the same side of the sphere and the moon are lit.
I’ve tried this before using apples or other fruit that I had on me when I happened to see the moon in the sky, but it is not really satisfying. Fruit in the sun always look like fruit in the sun, plus it is really hard to photograph (or can you spot the moon above the nectarine in the picture below?).
See the tiny moon right above the nectarine and how its light and dark sides are the same as those of the nectarine?
So I was really happy when I managed to take the pictures below:
In both pictures: Model of the moon between my fingers on the left, and moon in the background on the right. See how the lit and dark sides of both spheres are in the same position?
This is the first time I felt like it actually worked for me.
Another early morning crossing this bridge.
And the current and the sun glint were perfect for this kind of photos:
They almost look like schlieren photography images in those super old papers, don’t they?
And I find it extremely fascinating how you can see the boundary layer between the flow and the stagnant water, and how wind waves don’t manage to cross that boundary.
See the tiny capillary waves on the right side of the boundary? Those are locally generated because the larger waves on the top left just don’t make it over the strong shear.
You want to watch a movie? Sure!
And another thing I love on those early morning trips? Being completely alone in a pretty park, with dew on the grass and flowers in the sun :-)
The other day I was waiting for my friend and her daughter and noticed a weird stripe-y pattern in the distribution of algae. As I kept watching, the pattern started to change.
At first I thought that maybe the algae were collecting in nodes of standing waves that were reflected from the sea wall (ok, lake wall) I was sitting on, but this really does not fit with how the pattern developed later, and I have no clue what was going on.
Watch the movie and tell me what you think, please?
Seriously, though. What is going on? I don’t think the pattern is formed by advective processes – you see bubbles and the occasional leave and they don’t move a lot. I noticed that whenever the wind changed, the pattern in the algae also changed, but I didn’t notice a clear rule. And the wave theory only works for the waves coming in in parallel to the wall, I think. Any ideas?
A common problem in hydrodynamics is to distinguish between all the different kinds of lines that characterize a flow field: Stream lines, streak lines, path lines, time lines, and probably more that I can’t think of right now.
A common way to think of streak lines is that they are similar to hairs caught in the flow of a blow dryer. So when I saw these long grassy things caught in a flow recently, I thought they would be a nice visualization of streak lines.
Algae showing streak lines in the water
But when you look at them moving, you realize that they are not actually showing streak lines. Streak lines would be visualized if, at the root of each of those blades of grass (or whatever they are, I’m not a biologist), dye was dispersed. The dye streak would be exactly showing the streak line. But looking at the grass move, you see that it is sometimes being jerked one way or another, when the direction of the flow changed and the blade is pulled in the new direction of the flow, even though the downstream end might still be caught up in some old flow.
So yes, there are points in time when a streak line is visualized by hairs in the air or grass in the river, but there are also times when they are not. Right?