Lake-surface patterns during rain

When there is a vigorous rain, but no wind, the surface of the Lake becomes patterned with both brighter and darker regions (pictures, below). Why does it do this? What causes it to look this way?

The answer is not obvious. Indeed, as I have been able to find no mention of these easily seen and reasonably common patterns in books or on the web, most people probably consider the questions irrelevant. Nevertheless, I press on with a discussion below the pictures.

I can think of two different mechanisms that might give rise to these patterns: one involving surfactants; the other involving upwelling. My suspicion is that the second is the important one locally.

The patterns are always seen when it is raining and the wind is either light or absent. The raindrops hitting the Lake’s surface produce ripples which then flow out from the spot of impact. In the absence of the larger wind–produced waves, the myriad ripples produce a textured surface, one with an almost granular look to it.

But, why is this granular texture greatly diminished in some regions? After all, the rain is falling everywhere and so ripples are being produced equally everywhere.

Some backgound about waves and ripples:
Ripples are somewhat unlike the waves produced by the wind or a boat. For those big waves, the restoring force is gravity. For the ripples (also known as capillary waves), the restoring force is (the water’s) surface tension. Not only do the scales of ripples and (gravity-driven) waves differ, but their behaviours differ. The dividing line is a wavelength of about 1.7 centimetres, which corresponds to the waves which move slowest. For waves longer than this, the longer the wavelength, the faster it moves. Curiously, for waves shorter than this, the shorter the wavelength the faster it moves.

Some background about currents in a lake:
The water in a lake does not merely flow horizontally.  There are regions where the water gently rises to the surface (upwelling) and spreads out (divergence). There are regions where the surface water flows together (convergence) and sinks (downwelling). In nice weather, a boat trip out onto the Lake often reveals regions of convergence where surface debris has collected. Such a region is frequented by ducks which feed on the biota (leaves and insects) collected there.

The discussion that follows assumes that only ripples produced from the impact of raindrops are relevant. In the absence of wind and boats, the (gravity-driven) waves are absent. Further, there is moderate to heavy rainfall (not light or drizzly) with a sufficient number of large raindrops (say, a radius of a millimetre or so) to excite the slowest moving ripples (wavelengths between about 1.5 and 2 centimetres).

First possibility:
Surfactants suppress ripples in regions of surface convergence.
A surfactant is something on the water—usually small quantities of natural organic material from plants or algae—that alters surface tension. A surfactant reduces surface tension and so inhibits the formation of the ripples that gives the rained–on water the granular appearance. So, in regions containing a surfactant, the water will look calmer. Of course, oil or gas from boat traffic will do this also, but that source of surfactant is probably not relevant to most of the patterns we see on Kootenay Lake when it rains. So, places where surfactants collect may produce the smoother surface.

Second possibility:
Outflow impedes ripples from entering regions of surface divergence.
For the patterns seen in the rain, the divergent flow of upwelling regions is more interesting. Ripples which form in that region can flow out into the surrounding region, but ripples which form outside the region are impeded from flowing in to it by the mere fact that the ripples must travel against the current of the spreading water. So, by this mechanism, one would expect the regions of upwelling to be smoother. The fairly sharp boundary is the place where the outflow velocity matches the ripple velocity.

Now, it may be that each of these two mechanisms operates. I just don’t know. One would expect ripple suppression by surfactants to dominate regions of surface convergence, while ripple blocking by flow to dominate regions of surface divergence. All that I can be confident of at the moment is that the smoother (darker–looking) regions are places where their ripples are either suppressed or partially blocked.

(In time, this discussion will be moved to the Kootenay-Lake Website, but as yet there is no particular place for it.)

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7 Responses to Lake-surface patterns during rain

  1. declan says:

    I think upwelling is the answer. The temperature delta of fallen water versus the standing water in the lake is an important factor. Cold water sinking due to the rain cooling the surface produces upwelling. A tomography of the lake bed compared yo the patterns should reveal a corrolation.
    Additional this phenomena is. Patterns at sea in calm fog or evening with no wind.
    great article.

    • Alistair says:

      declan, you are the first person to comment on this in the eleven months it has been up. I suspect that you are correct: the drop splash patterns mirror the upwelling and downwelling. It probably has nothing to do with variations in the lake bottom, though, as the patterns shift around a fair bit. An interesting question remains as to the mechanism that transforms variations in the upwelling into the variations of brightness.

  2. Jim says:

    Do you know if anyone has taken a time lapse video of the pattern phenomenon. I stared at my lake for about 5 minutes today, but I could not discern any changes in the pattern – even in the smallest patterns. I imagine that speeding up a video of the lake surface would show some interesting changes in the patterns. This may give some additional insight into the phenomena. Our lake has many underwater springs, but the patterns do not seem to suggest that they are centered around the bigger springs I am familiar with. The pattern appears very random. Union lake in Michigan.

    • Alistair says:

      Jim, you are right that taking a time-lapse movie is next easiest way to gain some insight into the origin and behaviour of these patterns. I have not seen any such movies.

  3. Sam Sciacca says:

    The thing that puzzles me is that these patterns (at least on my lake) seem to take on the shape of long flowing streams/currents. If it were upwelling, I would expect to see areas that were more uniform and circular in nature. By the way….in my lake, these occur with no apparent correlation to lake bottom features.

    • Alistair says:

      Sam, on water with a fair amount of boat traffic, the clear areas follow the trail of the boat, have roughly parallel sides and are almost certainly the result of surfactants either from the boat or from vertical mixing of water caused by the boat. However, convergence and divergence patterns seem to be able to be long and sinuous also. Consider the wind driven Langmuir Circulation where there are parallel lines of convergence and divergence resulting from helical rolls in the water.

  4. ps says:

    I observed this phenomena in my swimming pool from my 7th floor apartment and suspected I would find nothing on the net. I was wrong and pleased to find other people wondering about the same thing, went googling ” water patterns during rain” etc etc and wahla. this is the most informative site I have found so far.
    a few things I would like to contribute-
    1) depth doesnt seem to be a factor because my swimming pool is contant depth everywhere.
    2) I observed edges of the swimming pool were calm without ripples near the edges of the pool
    3) Could the shape of the cloud formation producing the rain be a factor since that can vary the speed/size of falling rain?
    4) water temperature should be quite constant throughout the swimming pool so dont think temperature differences contribute to the patterns.

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