Wind, waves and currents: Coastal convergence

, 3 min, 420 words

Tags: how-stuff-works kayak-adventures

Our top priority as guides is always keeping our clients safe. One key way we do that is by understanding tides, weather patterns, wind, and waves. After slowly putting some of these things together, today we had a culminating lecture discussing wind, waves, and currents in our area. There's way too much to put in one post, but my favorite discussion had to do with coastal convergence.

Wind basics

Coastal convergence is a wind effect that occurs when winds blow along a coast. Some key things to know about wind over water and land:

  1. Over water, there's not much to slow wind down, so it can really get going compared to when it's over land.
  2. As a result, when wind goes from blowing over water to blowing over land, it slows down. Conversely, it speeds up when it blows from land to water.
  3. Because of the Coriolis effect, this slowdown/speedup leads to a slight change in wind direction. In particular, because we're in the northern hemisphere, wind blowing onto land slows down and thus turns to its left. Wind blowing onto water will speed up and turn to its right.

Coastal convergence

Putting all of this together, if the wind is blowing along a shore, some of that wind will blow from sea to land, and thus turn left. If the wind is at my back and the shore is on my right, the wind turning to its left will blow back out onto the water. The net effect? Around a 25% increase in wind speed, and one unhappy kayaker.

On the flip side, in coastal divergence, wind speeds along a coast on the wind's left are decreased by around 25%, because the wind that hits the land gets diverted left, away from the water.

Why do we care?

This has consequences when, for example, you're navigating an island in windy conditions. Let's say the wind is blowing from the south, and this island is basically just a circle, for simplicity. On the east coast of the island, the shore is on the wind's left, which means wind speed will be decreased by coastal divergence. The west coast has stronger winds due to coastal convergence. If you're in a kayak, this means you have a preference for navigating this island on the east side.

Note: I had trouble finding online resources about coastal convergence/divergence, so it's possible that we use different names than usual here at KAW. I'll update here if I learn more.