Thursday, March 5, 2009

Which Way the Wind Blows

With the Wind Festival right around the corner, I thought it was a fine time to talk about the science behind how and why the fair city in which I live is so darn windy. Most of the cities in our region have a token produce item that they celebrate annually. But, what makes our city unique? It is the wind, no doubt. I love that we have a festival dedicated to that oceanographic feature that so defines our fair city.

That’s right, I called the wind an oceanographic feature, and it is not just that I am a marine scientist that makes me view the wind in this way.

Wind is formed by a pretty basic principle - hot air rises. When hot air rises, the cooler air rushes in to take its place, and viola, you’ve got wind. That basic principle explains global wind patterns, as the air is warmer at the equator than at the poles, and local wind patterns, such as the off-shore winds that surfers use to predict how good the surf will be. The ocean is a strong factor in determining which way the wind blows, as ocean temperatures drive the giant conveyor belt of air that winds its way around the globe, interrupted by the land masses that form a mere 30% of the earth’s surface.

Spring, Wind Festival time, is probably the consistently windiest time of year here. This is because the ocean off our coast is at its coolest temperature, thanks to spring upwelling. Upwelling is the movement of cool deep waters up to the ocean surface. They are brought to the surface because of global winds, winds blowing towards the equator. Air at the equator is hot and rising, cooler polar air masses are moving in to replace it. As these winds blow down past our coast, the rotation of the earth causes the surface water to be pulled by the winds the to west, or out to sea. As the surface water is pulled away from our coast, the cool deeper water rises up to take its place. Thusly, as any surfer knows, the water off our coast is the coldest in the spring.

Cooler ocean temperatures mean cooler air temperatures sitting on top of that water. As the inland air gets warmer and rises, the cool coastal air is sucked inland to replace it. This temperature differential is probably the greatest in the spring, and particularly in the afternoon in the spring. That means lots of wind.

Land heats and cools faster than the ocean. The ocean actually changes temperature by only a few degrees. This means we typically get offshore winds when the inland areas are cool, like in winter (off shore winds are good for surf, along with big swells brought by winter storms generated far away). We get on-shore winds when the inland areas are warm, such as during the upwelling periods described above. We might get on-shore and offshore winds in the same day depending on the temperature change inland. In our city, we know this well, as we spend our summers watching the fog “burn off” in the late morning and get pushed out to sea (as the heat reduces the moisture content in the air), only to get sucked back on land in the late afternoon.

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