I witnessesed a very powerful storm on a lake yesterday and I was again reminded of a question I have had for a very long time. When it rains on a lake, these streaks appear on the surface. By streaks I mean long arms of slightly different colored/textured water. The only other time I’ve seen this is a boat or pwc will leave a trail on the water that has similar characteristics. I have 2 WAGs, neither of which I believe are true.
Tempurature Change. The water in the rain is cooler than the lake water so it pools together. Or the lakewater is pooling. After typing that, this seems like a pretty silly theory.
Aeration. In the process of being splashed, the water picks up some additional air and is either differently colored/textured by the air or its new density. Of my theories, this is the one I like the most given that water craft cause the same effect. The disruption of the water causes it briefly (5-10 minutes) contain more air than the surrounding water. The only reason I don’t like this is that water is falling on the water in pretty much a uniform manner and the whole lake would be a different color, not just skinny persistant streaks.
I thought of another, water cohesision is b. . . no never mind, that’s stupid too.
I give up.
If it rains today I’ll get a buddy of mine to take a picture and post it on the web, for those of you who don’t know what the hell I’m talking about.
Are you sure it’s just water? When it rains hard around here, the lake gets streaks kinda like you’re describing but if you get closer, you’ll see that it’s really debris washed into the water. It tends to form streaks depending on the water currents. We also occasionally see streaks caused by loose dirt being washed into the lake, usually near a construction site.
Are the streaks foam? Could they be the result of Langmuir circulation? (small corkscrew circulation cells in the lake running parallel to the direction of the wind, causing foam to remain at the surface where water converges and sinks.) There’s a pic on the linked page.
No, I think I know what the OP’s talking about, and it’s not foam. Imagine irregular several-foot-wide stripes and peninsulas of slightly darker blue-grey in with the regular blue-grey. It’s very subtle, and you probably wouldn’t notice it unless you were looking at the water for a while, or were looking for this particular phenomenon.
I’ve seen this too; the rainwater is usually going to be a different temperature from the lake; if the rain is warmer, the phenomenon might be just pools of relatively unmixed rain sitting on top; if the rain is colder, it might be that we’re seeing the top of convection cells.
If I know what the OPs talking about, it’s also visible on a clear day if there’s a good bit of wind. Some of the water gets ripples from the wind, other parts don’t.
I think that it could also be caused by anerobic bacterial action. There are a number of bacterium that respirate through metal reduction. They are all over, but can be found in streams, mud, etc. When it rains, the stream beds and muds are flushed downstream and will end up in the lake.
Yeah, that’s it! The last picture is really representative of what I’m talking about, especially on the left in the bay. And you’re right, the streaks are usually smoother than the surrounding water.
If I understand Bruce Daddys question correctly. It seems that there are streaks or trails in the water that seem to be getting more surface disturbance from the raindrops than others.
My theory is that there are thermoclines in the water near the surface that affect the surface tension.This would appear to make the surface seem “slicker” in streaks than the surrounding areas.
It is the pattern effect of the surface of the water being disturbed by the wind in combination with the rainfall which is also non-uniform in its distribution. You can easily see this on pavement during rainfall as the rain drops seem to fall in bands of greater and lesser concentration.
I knew exactly what the OP was describing, lived on the Delaware all my life and pondered the same question just as long. Bruce_Daddy’s picture has it to a Tee.
The photo just above is obviously something different from anything that happens on pavement, but there’s an explanation for the bands or sheets in heavy rain - not clear to me which thing the OP is about.
The bands or sheets of rain are the result of turbulence in the air the rain is falling violently through. Every cell of air that rotates drives rain toward its periphery by centrifugation, and so rain concentrates at the boundaries of the cells. This is especially pronounced on a size scale of 2 to 5 meters. Imagine a very coarse foam rubber in which the bubbles are 3 meters across, and imagine it’s falling and passing through the ground surface. You see bands of rain wherever the solid part of the rubber is passing into the ground surface at that moment.
Somebody mentioned Delaware - this was only figured out perhaps a decade ago by a mechanical engineering professor at the University of Delaware.
Well it looks like an interaction between wind and the water surface.
Perhaps in some patches the water forms a smooth surface due to a laminar flow of air above the water surface. In other parts the water surface is roughened up, the air moves turbulently and this feeds back into maintaining a rough surface. After a while the water may fall back into a smooth state with laminar air flow (dark patches).
Perhaps there is a chaotic fluctuation between the two states - sort of like at the boundary between laminar and turbulent flow in a pipe.
It would be interesting to see time lapse video of this phenomenon.
I’m liking mangetout’s theroy of water temperature. Not so silly after all, Bruce. Is this lake spring fed? Could it have anything to do with the temperature and/or purity of the spring water in relation to the rest of the lake?
