Why do I have a headwind in both directions?

May 8, 2007 7:50 AM

I'm living the "uphill both ways" joke. I cycle to work, and I've always got a headwind, every day, in both directions. Why?

Friends told me "you're moving at speed, duh," so I dismissed it at an illusion... until I noticed I pass by a flagpole on my route. Every morning for the past week, when I approach by the flagpole, the flag is snapping in the wind, pointing right at me... eastward when I'm travelling west in the morning, and westward when I'm travelling east in the evening. I've noticed other things like dustclouds, bits of trash, and people's clothes blown in the wind, and sometimes knock-me-over crosswinds when I make turns.

I'm somewhat thankful for the extra workout, but it seems really odd that I would get headwinds in the most difficult direction, twice a day, every single day (feels like every day, but I've only been measuring for a week). Why on Earth?

Other details: I live in Montreal, most of my route is along Lachine Canal, although I cross the urban area of Verdun and over Champlain Bridge -- the bridge is perpendicular to the Canal, and yet the headwind is just as strong!

posted by Mozai to science & nature (16 comments total)

I live in Cambridge, MA just off the Charles River, and walking to the subway each morning I walk into wind, and walking home from the subway I walk into wind. I have no idea why. So I'll be interested to hear what others have to say. Good question :)
posted by olinerd at 7:57 AM on May 8, 2007

The Complete Book of Long-Distance Cycling by Edmund R Burke and Ed Pavelka says, "Do you ever get the feeling that the wind is always blowing against you? It actually is, more often than not. According to the laws of physics, only those winds within the trailing 160 degrees of an imaginary circle drawn around you provide assistance. Wind from anywhere in the other 200 degrees works to slow you. In other words, even a pure crosswind raises the drag. It must shift another 10 degrees to the rear before it begins to offer any help (Burke, 46)."

I have no training in physics and I'm not sure if this relates to what you're seeing with the movement of the flag, but thought it interesting nonetheless.
posted by summit at 8:01 AM on May 8, 2007

Alternating winds from moring to evening are a common phenomenon in nature. See Wikipedia on 'Sea Breeze'. Being in the St Lawrence Valley, it wouldn't surprise me if there was indeed a sea breeze effect. See also.
posted by Rumple at 8:12 AM on May 8, 2007

Is there a large body of water nearby? Could this be a "lake effect" wind, caused by the different rates at which water and land heat and cool?
posted by cosmicbandito at 8:15 AM on May 8, 2007

Depending on the large-scale geometry of the area around your path, there may be a small pressure gradient whose direction changes in the heat of the day. here in St. Louis, the winds, as expected, average out to being from the W-WNW or thereabouts. When i had a lot of free time (read=unemployed) I took to riding my bike a lot, and despite that I'm nothing of an early riser, it was easier, on average, to do my route in the morning than in the afternoon. Factoring in wind shielding and hills, this means that I had to have tailwinds in the morningon certain hills, and headwinds in the afternoon.
posted by notsnot at 8:16 AM on May 8, 2007

It's not at all unusual for winds to come from one direction in the morning and the other direction in the evening.

Wind is caused by temperature differences, and because of the regular heating/cooling patterns of night and day, some wind patterns are recurring.

For example the description of differential heating under Wind in Wikipedia:

"Differential heating is the motive force behind land breezes and sea breezes (or, in the case of larger lakes, lake breezes), also known as on- or off-shore winds. Land absorbs and radiates heat faster than water, but water releases heat over a longer period of time. The result is that, in locations where sea and land meet, heat absorbed over the day will be radiated more quickly by the land at night, cooling the air. Over the sea, heat is still being released into the air at night, which rises. This convective motion draws the cool land air in to replace the rising air, resulting in a land breeze in the late night and early morning. During the day, the roles are reversed. Warm air over the land rises, pulling cool air in from the sea to replace it, giving a sea breeze during the afternoon and evening."
posted by justkevin at 8:17 AM on May 8, 2007

The winds measured at the airport in Montreal have been blowing in more or less the same direction morning and evening recently.

Either there is some sort of local wind phenomena along the canal that switches direction or your observations are a bit skewed. I'm pretty surprised that you say there is a headwind crossing the Champlain bridge as there is usually a steady crosswind when crossing the river. Maybe you should stop along your route a few times and try to determine the wind direction while you are stationary.
posted by ssg at 8:25 AM on May 8, 2007

I bike to work in Boston along the Charles, I'm almost always going into the wind both ways - when I do get a break, it's generally in the morning, and on those magical days when (heading eastward) I have a tailwind, I run two gears higher and trim nearly 10 minutes off my commute, so the effects are very real.

Heading home (roughly westward) I can count on one hand the days over the last several years I've had the wind at my back. I would also say (non-scientifically) the wind is generally worse later in the day.

Recent Random Observation: Obviously the wind is rarely blowing perfectly eastward or westward, and I'm not heading perfectly eastward or westward as I follow the streets and the river, but I have noticed over the past several weeks the wind has been hitting me laterally (mostly from the north) more frequently than usual - make of that what you will...
posted by jalexei at 8:27 AM on May 8, 2007

ssg, I'll take that challenge. I understand my perceptions may be skewed, so I didn't say anything until I noticed and observed the flagpole along my route over the past week. There's no such flagpole on Champlain bridge, but can change that...
posted by Mozai at 8:44 AM on May 8, 2007

Mozia, I'm not doubting that the wind is blowing in the direction of the flag you see, I was just a bit surprised to hear about the headwind across the river, so I thought more observations were in order.
posted by ssg at 9:03 AM on May 8, 2007

most of my route is along Lachine Canal

Note: You have possibly the most gorgeous urban bike commute I can imagine. /jealous
posted by desuetude at 9:35 AM on May 8, 2007

Funny, I bike the Lachine Canal almost daily and I almost always have headwind while traveling west and backwind while traveling east.
posted by racingjs at 9:51 AM on May 8, 2007

Mozai, funny, I was going to ask the exact same question. I live in Salt Lake City and often have headwinds during both commutes-i feel your frustration!
posted by neilkod at 10:17 AM on May 8, 2007

Another vote for sea breeze, even if it's only influenced by the river. And anything other than wind measure near you is unlikely to be useful, since Montreal is going to have microclimates galore due to the hills and water.

I live in Michigan over fifty miles from the nearest large body of water, and wind directions here change in the morning and evening pretty frequently. They have to change some time. (And we have less-dramatic microclimates here too; the flat land outside of town gets a lot more wind and rain than the hilly territory in-town.)
posted by ardgedee at 2:07 PM on May 8, 2007

I find the exact same thing here in Toronto!
posted by loiseau at 3:24 PM on May 8, 2007

And on t'other side of the world in Melbourne, Australia, I get the same thing. Afternoon sea breezes are the culprit in my case. I've been commuting by bike for something like 18 years and I reckon I can count on one hand the number of two-tailwind-days. The two-headwind-days are legion. And I'm not talking about the phenomenon summit identifies above, but the honest to god riding into the teeth of the wind type experience. My wife says it is because I live an impure life and god hates me.
posted by tim_in_oz at 3:39 PM on May 8, 2007