The Breakup Of Bags Brings About Sea Spray

Sea spray is almost certainly a phenomenon that the reader will have heard of before. Despite it being one of the most common effects to occur on the boundary between sea and air the effect of sea spray on storms and hurricanes that develop over the ocean is still very uncertain. Some of the estimates performed on the number and sizes of droplets thrown into the atmosphere from the sea have massive error margins on either side of the predicted value. The difficulty in performing experiments, in the field, in extreme wind means that even the process of the spray forming itself is poorly understood.

Because going to the ocean to perform these experiments is still incredibly impractical, scientists resort to using high speed wind and wave flumes that can recreate similar conditions for observation. Of course to actually collect the observations high speed cameras that can record up to 10000 frames per second are used to catch the spray being produced. A number of distinct mechanism were recorded that all contributed to generating spray.

  1. At the crest of breaking waves, the artificial wind worked to form projections off the crests. These protrusions themselves break into a few droplets with diameters of about one or two millimetres.
  2. Bubbles can also form at the crests of breaking waves which burst, distributing droplets, when they reach the surface of the water. This is often named as the producer of sea spray but the observations here showed that in fact only 5% of bubbles under the surface actually progress to produce droplets.
  3. And finally, bag break up. To picture this imagine a small protrusion once again forms from the wave. The wind catches the middle of the protrusion and pushes it out and flattens it making the protrusion look more like a sail. This is filled with air until it bursts releasing a multitude of droplets. I recommend this video by an Ilia Roisman on YouTube which demonstrates the effect on a single drop.

It was found that this final method was really responsible the majority of droplets projected by the water in high wind conditions. It is believed that their is still at least one more mechanism or at least minor process left to be found but this work has made a massive step towards understanding the border between sea and sky.

Paper links: Bag-breakup fragmentation as the dominant mechanism of sea-spray production in high winds


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