Despite the unseasonably warm temperatures at the start of the week, the water temperature in coastal waters has started to drop. As someone who likes to get in the water as much as possible, this is apparent even on the warmest days. Where the surface waters were once much warmer and the deeper waters contrastingly cooler, this layering is in the process of shifting. And at some point, it will be completely reversed such that the surface water will be cooler than that at the bottom.

This is all interesting and readily obvious if you feel the water directly. But there are other ways to observe this shift — especially as the weather cools and getting in the water becomes less appealing. The first is that the surface water often can become particularly crystalline in its clarity. As water cools, the growth and reproduction of many of the plant and animal plankton slows down. Also, in a dry year like this one, there has been less runoff from the land, meaning that there are fewer suspended particles clouding up the water. One other fun fact about colder water is that it can absorb more oxygen. Perhaps that’s why cold water feels particularly refreshing.

The angle of light in the fall also contributes to its translucency. Because the sun’s light hits the water at a lower angle, rather than one that is more direct like in the summer, the light penetrates the surface layer of water more thoroughly. That angle of light started to decrease after the last day of summer on Sept. 22. Because Maine is at a relatively high latitude (44 degrees north), that angle really gets low by the time we approach the winter solstice in late December.

One consequence of that autumnally lower angle of light is also that the reflections on the water’s surface can become particularly crisp. When the leaves start to change colors, seeing these colors reflected in the coastal waters is something pretty incredible. Combine that with a late-autumn sunset, and there is somewhat of an illusion of evening warmth even when the temperatures are no longer such.

These are all things happening on the surface. Underneath the water, all kinds of shifts are occurring. It’s easy to see the flocks of migrating birds — the geese being the biggest and the loudest. But as the surface water gets cooler, the animals that once stayed in its warm, productive layer start to migrate either to deeper waters or to places farther away where winter is less mild. In the intertidal, many of the typical tidepool creatures also begin to retreat ahead of winter. Everything is at this point of transition as we more fully enter the fall season.

For now, however, there is a lot of activity and much to observe. The changes in the light, the clarity of the water, the fish and birds that are on the move — these are all happening right along our coast and are not something to be missed in this sliver of a transitional season.

Susan Olcott is the director of strategic partnerships at Maine Coast Fishermen’s Association.