Why is it that the headwind on the way out isn’t a tailwind on the way back? How come those crosswinds turn to headwinds? Why does wind seem to come screaming down one little canyon from the east but on another canyon just up the road it comes from the west?
As a local rider and friend, and member of CA Alps Cycling said to me some time ago (and he’s lived here for decades): “The wind can be a bit vexing here, can’t it?” So, true, Rich. So very true.
I originally published this post in November of 2018. After some recent “howlers” here in the California Alps though — including a ride just last week where the crosswinds were so strong I had to lean in so much I felt like I was riding at a 45 degree angle — I thought it might be time to re-post it. So here ’tis, a little trip down a windy, memory lane.
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 with that moving air here in the Sierra Nevada?
Well, thanks to “A Sierra Club Naturalist’s Guide” by Stephen Whitney, we’ve got 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 be 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 (towards 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 consistent like it was in Silicon Valley.
Well, Mr.Whitney gives us a bit more information: 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
To those high school science days (daze?), that is…
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. Seems reasonable…He goes on to write that “air forced to squeeze through a canyon or mountain pass is accelerated in much the same way.”
In doing a bit more googling today, I came across this article from the Smithsonian. I don’t recall discussing the Bernoulli Principle in Bio 1 (or any other bio or science courses I took in high school or community college), but if I did it’s been too many years or perhaps it’s due to mistakes of a misspent (not all of it, I swear) youth. Nonetheless, it’s on point! The principle, not the misspent youth…
Mountain winds, valley breezes and mountain breezes
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 me, need to learn to embrace the wind.
Ride safe out there and let’s kick some passes asses!™ Even the windy ones…