At this time of year when the water temperature is still cool and the air temperature is finally getting warm, the difference between the two drives much of our coastal weather. For most of June, that meant fog. When warm air meets cool water, you get evaporation, and then it rises and condenses, sometimes into dense banks of fog – those thick purply blankets that stretch across the sky and hide the shoreline. This often presents challenges to mariners who have to find their way without the usual landmarks to guide them. But, it is also an invitation to enjoy the quiet calm that accompanies this type of foggy weather. I have often been surprised while out paddling by the call of a loon from the mysterious waterscape around me; or, by the slap of a pogy on the surface. This is not only because they are hard to see, but also because of the lack of wind on these days that allows small sounds to echo loudly.  

But, then comes the wind. It is like a tug of war between land and sea as the wind attempts to balance out the differences in their temperature. Think of the welcome sea breeze at the end of the day when there is a welcome rush of cool air off the water coming towards the shore. This is an onshore breeze. I’m not a meteorologist, so my understanding of these shifts has been more from observation than expertise. But, a bit of research helped me to understand that the reason this happens has to do with pressure – and temperature determines pressure. Cold air is denser than warm air and so there is a kind of vacuum effect of the dense being sucked toward the less dense to fill it up. Add to that the fact that warm air rises so that the cool air that rushes in rushes toward the open space left below. At the end of a hot day, the land is pretty toasty, providing welcome relief on a hot evening. The opposite can occur in the early morning when the temperature on land is cooler than the water temperature. The average water temperature in July is somewhere around 60 F, so when the air temperature dips below that, you can get an offshore breeze.  

Here’s the part I was particularly interested in, though. Why does the wind seem to change direction when the tide turns? This is perplexing because the timing of the tides changes each day, so it doesn’t happen regularly in the morning and at night. It turns out that temperature is yet again the culprit. In Maine where the tides can carry the water more than ten feet higher up the shore, they can conversely leave ten feet of height uncovered at when the tide ebbs. This can result in a massive amount of intertidal area being exposed to the hot sun. It is a temporary boundary between land and sea and it experiences the same tug of war between temperatures. When the tide is out, the sea floor heats up, pulling cool air towards it. Then, when the water comes floods in again, things reverse.  

In between both of these shifts, you sometimes find the glassy calm reminiscent of June’s fog. Things are at a temporary equilibrium and there is a sweet period of the middle of the day or at slack tide when you can hear the fish slap and the loons call. But, don’t be lulled by this temporary calm, as the wind is likely to shift again. It is yet another reason why the coast is so interesting and there is so much to be learned from it.