Don’t you hate it when forecasters prattle on and on about upper troughs at 18,000 feet blah, blah etc. One, you ask, what the hell is an upper trough! And 2nd. how can anything thousands of feet above you have anything to do with the wind in your kite or sail? Well below is some imagery that well help you out. The first image shows the output from one model of the wind flow at 18K feet today. Notice the blue cooler air and how it takes a southward dip out over the pacific. This type of dip is called an upper trough. Also note the wind flow at 18 K feet goes from NW to SW in the upper trough. Notice how there is WSW to S flow at 18K feet above most of California. For complex reasons an upper trough in this position causes the marine layer to deepen.
It is meteorological gospel that an upper trough, especially with the shortwave seen in the image, will push the marine layer clouds and fog deep inland in the Bay Area and Southern California. So this answers, in simple terms, the first question. But how about the 2nd. question… how do events 3 miles above my sail/kite impact the wind that propels my board? Well, today is a day of heresy because the answer is….they don’t! To see why this heresy is true on this special day take the visual tour below: