Our old friend the sea breeze is the local wind of near-shore sailors, and a feature of July and August for a good many waters of the Northern Hemisphere. This wind is an important component of our sailing, but, surprisingly, it's not at all well understood.
A lot of articles about weather are presented in simple, black and white terms. The problem with being a nuts and bolts meteorologist, as myself, is that in this science, no matter what's said, there will always be the exceptions - the grays, if you will. However, let's talk about the majority of cases of sea breeze, and hit a few of the exceptions later.
First, the temperature differential between the warm land and the cool adjacent water is NOT the driving factor. The force that produces an onshore breeze is vertical thermal activity. The sun heats the land and bubbles of warmed air rise, just like balloons. If there's moisture present, the rising bubble will create a fair-weather cumulus cloud. But whether the cloud is present or not, the thermal will exist. As this bubble rises, a lowered pressure is induced below it. To replace it, the cooler, denser air from seaward is drawn over land. This explains an onshore breeze even if the land is cooler than the water. Cooler, yes, but the thermal of air that's warmer than the ambient will still be there. So, it's a pressure differential, not temperature.
Another misconception is that there is a rotary flow with a sea breeze, or that as the air moves onto land at the surface, there is a counter flow aloft that moves back out over the water. This is true if the existing pressure gradient happens to be going that way, but otherwise, the winds just off the ground will remain pretty much what they would be without the sea breeze. This is why an onshore flow that relies on thermals alone will peak out in the late-morning to early-afternoon period. Watch the cumulus clouds, if present. They represent the top of each thermal bubble. With time, these bubbles will move farther inland as the cooler sea air inhibits further thermals. As the draw for the onshore breeze moves away, the breeze starts to tail off. So, always keep an eye on the cumulus clouds; they are the telltales needed to evaluate the thermal activity over land.
But if the winds aloft are going offshore anyway, the cumulus clouds will move seaward. The net result is the same. Eventually the cumulus over land stop forming as the thermals are suppressed, and they clear out. These cumulus are associated with changeable surface breeze and carry the bubble with them to sea as they move over the water, till it gradually decays. But that's a whole other discussion.
The sea breeze is different when it occurs with higher terrain inland, hills or mountains. The rising thermals meet a more durable origin, fixed landscape. Unlike the thermal bubbles, these projections don't move. As a result, the thermals will continue to rise over them. In that case, the onshore breeze tends to hang around longer. It will blow toward the high terrain, the resulting wind direction.
Knowing the inland terrain helps to ascertain what the wind direction will be. Then, for example, the stronger the resulting onshore flow, whether from thermal bubbles or terrain induced, the stronger the lift. If this is home territory, you'll likely know what these typical directions will be. If it's new territory for you, then a topographical map is a vital tool to evaluate the resultant breeze. If the air is vertically unstable to high altitudes, the thermals will continue to rise until they are no longer warmer than the ambient temperatures, and therefore have no more positive buoyancy. This results in large or towering cumulus, or perhaps to the ultimate cumulus, the cumulonimbus or thunderstorm. Small or large, the mechanism is the same. The more air that rises, the stronger the breeze necessary to fill in the space. So, again, the key is keeping an eye on the clouds, not always on the water. The inclination is to only watch other boats and look at the water surface. I have had to repeat many a time to an afterguard: Look up, evaluate the clouds, they're other telltales of the wind. A wind change associated with a cloud can literally just drop onto the water surface, and not be at all visible on the water.
Sometime try this trick. Think of yourself as an air particle. Look at the terrain and figure out what would be the primary influence. Where would so much air be rising that you would have to go along with the airflow. Big cumulus clouds, mountains, up-slope valleys, straits, etc., you can bet that's where you'd be heading. And that attraction can shift throughout the day as one factor comes into play over another. The America's Cup course off Auckland has all sorts of these affects, most of which turn out to be entirely logical. I could go into this in detail but (if I did, I'd have to kill you, as this is hard-earned knowledge) in keeping with all things America's Cup, it is highly secret. As the keels are shrouded, so too are the wind secrets.
As usual, I'm just touching on the subject. If there's further interest in more detail, let me know and we'll cover it point by point in the future. One follow-up topic, for example, might discuss these questions: When is the gradient or ambient wind too strong to allow a sea breeze; what causes shifts during an onshore period?
I'd like to take a moment to announce that I've gone into semi-retirement. I'll be doing the weather for Team New Zealand's defense of the Cup, and whatever one-off special project that I might be considered for, including this column. Hard decision to make, but the time eventually comes for everyone. And I guess this is mine. My thanks to everyone.
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