As a San Jose native, I was very familiar with the wind patterns. I lived in South San Jose for many years and could plan my rides knowing pretty well how the wind would blow: Go out early and get the tailwind on the way home. Go out later in the afternoon and get the headwind on the way home. Certainly this did depend on the direction of my ride but for the most part I could easily predict the patterns.
So, what’s the deal here? Well, thanks to “A Sierra Club Naturlist’s Guide” by Stephen Whitney I’ve found some answers.
Mountain winds: “Winds and breezes passing over a rugged mountain range such as the Sierra follow tortuous courses over and around ridges, up and down canyons, through gaps in the crest. Eddies set up by the irregular terrain blow here and there in vigorous gusts that rattle trees and shrubs one minute only to abruptly die down the next. In the protection of a large boulder or grove of trees there may be scarcely any wind at all, while just a few yards away, in a more exposed area, it howls furiously.”
Okay, so that makes sense. It’s somewhat analogous to the flow of a river. Rocks = rapids. Eddies are often downstream of those big rocks and flat water can bee seen where there aren’t any big rocks to interrupt the water’s flow.
Let’s dive a little deeper, though. Why is it that I can head up Hwy. 4 (Ebbett’s Pass) with a headwind and then not get that tailwind on the way back? Okay, sometimes I do get the tailwind but it’s not consisent like it was in Silicon Valley. Well, Mr.Whitney has a bit more information in that regard. It’s about mountain breezes, valley breezes and the mountain ranges “intrusion” into the tropopause.
“On a warm summer morning the air next to the ground surface is heated and rises. Cooler air nearby moves in to replace it and rises in turn. This movement is felt during the day as an upslope breeze.” Typically, he writes, this starts about three hours after sunrise and reaches its peak during the hottest hours of the day and then it tapers off again around dusk. This is a valley breeze. “At night, cool air flows downslope, creating the mountain breeze.”
Dust off those cobwebs and cast your mind back (thanks Paul Sherwin) to those high school science days (daze?); here’s some additional data: “Wind speed increases over mountain crests and through canyons and passes because air – like any fluid – accelerates when forced through a narrow passage. As a volume of air moves upslope, it is increasingly squeezed between the mountains and the tropopause, the inversion layer acting as a lid on the lower atmosphere. Since the volume of air remains the same as it squeezes over the mountain crest, it becomes elongated.” Okay so that means it picks up speed. Makes sense. He goes on to write that “air forced to squeeze through a canyon or mountain pass is accelerated in much the same way.”
So, there we have it! Mountain winds, mountain breezes and valley breezes combined with the myriad canyons, crests, passes and that ol’ tropopause challenge those of us who cycle in the California Alps. It’s a rare day when we have no wind and due to the topography you can’t count on consistency. If you ride here then you, grasshopper, like I, need to learn to embrace the wind.
Ride safe out there and let’s kick some passes asses!™ Even the windy ones…