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Uphill Melts First

We’ve had a miserable cold winter here, and whereas we haven’t gotten any more snow than average, the snow that we’ve gotten has been a long time leaving. Over the past four or five days the temps have finally been trending up, and as the snow melted on our sidestreets I once again noticed something I’ve seen the last few winters: The uphill lane melts first.

It’s a fascinating business. It’s consistent, and there are a lot of stiff inclines here on the slopes of Cheyenne Mountain. No matter what street I drive on after a snowfall, it’s the uphill lane that melts first. So the citizen scientist in me started chewing on the question: Why?

My first hypothesis was that on the eastern slope of a mountain, roads running east and west have the uphill lane on the north side, meaning that the southerly winter Sun is more likely to fall unshadowed on the north-lying lane. This may be a factor on some streets, but I quickly found hilly streets running north and south and at odd angles. In all cases (I didn’t find even one exception!) the uphill lanes melted first. This was true even on the north sides of small hills where the road surface got little if any sunlight at all.

This left me only a single hypothesis: That car engines have to work harder to move a vehicle uphill, and therefore the undersurface of the car (engine and exhaust system) are hotter going uphill than downhill, when the engine is basically idling. Heat radiating from the undersides of uphill-traveling cars melts more snow than vehicles idling their way downhill. This is a suburban area rather than rural, and there are a lot of houses up here, all on smallish lots. So traffic is significant, especially at rush hours, when conga lines of minivans and four wheelers (necessary on winter roads with 12% grades) commute down and back to Colorado Springs.

I don’t know how true this is, nor how to test it in a controlled fashion. The snow is now gone, but come next week another experiment will be set up, and I’ll have a chance to look again. (I need to keep a camera in the car so I can snap a picture of the effect in action, something I haven’t done yet.)

If you’ve seen something like this happen in your area, let me know.


  1. Andy K says:

    Alternate theory #1:

    Meltwater runs down hill under the snowpack. The total mass is reduced allowing more sunlight to penetrate to the black top creating even more heat.

    1. Andy Kowalczyk says:

      I totally misunderstood this, I thought you meant uphill/downhill in a right/left sort of thing. Like the mountain goat with two short left legs.

      Now that I understand – here is my experience. Over the past couple of months I have been laying out a road rally, which means I have been driving on a lot of snow-packed, hilly gravel roads with a thousandths of a mile reading odometer.

      When I revisit the same portion of dry paved road, my reading are always within a couple of a thousandths as my previous outing. But the gravel roads are problematic. Especially when they are wet. I always read longer than when they were frozen.

      Going up hill you definitely have a lot more wheel spin. Maybe that is more friction heat – but I think it is just the mechanical grinding that exposes the black top.

      I also have a 1/5 mile long driveway that I shovel by hand. If it’s real bad I call a plow service. But usually I just shovel the parking area and the steep parts. If I can get those swept early in the day, even on a cloudy day, there is enough solar boost to have dry pavement by the end of the day.

  2. I would speculate that in addition to engine heat, the tires are hotter pulling the car uphill than merely lubricating its passage downhill. While their heat is small compared to the engine/exhaust system heat, it’s applied directly to the snow, and the snow gets its share of mechanical heating from the process as well.

    Or perhaps it’s simpler – cars slipping going up a hill spin their tires more, generating more friction heating in the snow.

    In any case, my money’s on tire heat more than exhaust system heat. I don’t think the delta in exhaust system heat is fast enough to matter.

    (though the exhaust itself is going to change in real time, and often directed at the road. Hmm.)

    Sounds like a job for a thermographic camera.


  3. Jonathan O'Neal says:

    Not only is the vehicle expending more energy (and emitting more heat) going uphill, but I’d guess that the average speed going uphill is at least somewhat lower than the average downhill speed, meaning that the hotter vehicles are spending more time on that side of the road. I have my doubts about the tires being the dominant factor; if that was the case, I assume you would have posted an observation about the tire tracks melting, not the entire lane.

    I’ve never observed this effect before, possibly due to simple inattentiveness. Next time we get some snow here, I’ll be sure to look for it.

    1. David Stafford says:

      Great theories, above. I’ll add one: Cars going uphill carry more weight on the back tires. Cars going downhill carry more weight on the front tires. The difference in the angle of the car relative to the road depends on the stiffness of the springs in the suspension (and if you want to be picky– the stiffness in the tire carcass and the amount of air inflation in the tires.)

      The exhaust, then, is going to be pointed more directly at the ground as a car goes uphill. The effect becomes more pronounced as the hill becomes steeper.

      There is a secondary effect which may be just as significant if not more so: An engine pulling hard up-hill is pumping out a lot more carbon, too, which will stick in the surface of the ice and help warm it up under sunlight.

  4. Larry N says:

    Uphill salt? The municipality might salt/sand uphill more heavily as slip is more severe in that direction.

    1. Actually, on the side streets (which are what I’m writing about) the city doesn’t do either gravel or salt, which is what makes it an interesting question. Over on my LiveJournal mirror, one of my commenters made a related suggestion: That cars going uphill are going away from the city where the larger streets are (and thus the salt) and then drop salt slush onto the street while the vehicle is going back uphill toward home. I actually think this may be the most likely explanation, but I don’t really know how to test it.

      1. David Stafford says:

        Also, tires coming home will be warmer than tires leaving home.

        Tires leaving home will be close to ambient temps (often below freezing) while tires coming home will be tens of degree warmer.

        1. I thought of that briefly, but couldn’t get a grip on how much warming a 5-8 mile drive would cause in tires. (This is a smallish city, and our neighborhood is about 5 miles from downtown.) It’s true, though: Cars going downhill toward the city have probably driven half a mile or less on side streets before reaching the feed streets. Cars coming back are sometimes coming all the way from Castle Rock (45 miles) or Denver (60+ miles) so if tires do warm significantly, that’s a factor.

          1. David Stafford says:

            Yes, they warm significantly.

            Going from memory here so I may be off: Tread temperature (not internal air temperature) for a typical car or truck travelling at highway speed for twenty minutes will be somewhere around 150 to 180 degrees. If the tire is underinflated or the ambient temperature is unusually hot the tread temperature may be even higher.

            Try touching the tires after a ride and you’ll see how warm they are.

            I’m not ready to declare this mystery solved but a 100+ degree tire sure goes a long way to explaining the effect you’re seeing.

  5. Erbo says:

    I noted the same effect not long ago on a sloped section of East Florida Avenue just east of South Quebec Way. (Coordinates roughly 39.689356, -104.89566 in Google Maps. A bridge over the High Line Canal sits at the top of the slope, on the east.) Since this street is decently-trafficked in both directions (it serves to link South Quebec Street and South Quebec Way with East Parker Road, and provides access to a couple of residential areas to the north and south via South Uinta Court and South Uinta Way, at the top of the hill), that would tend to eliminate the “coming home” effect.

    At the time I noted it, I wasn’t thinking about the causation, though, I was just grateful for the lack of ice on the side of the road I was trying to drive up.

    1. David Stafford says:

      That rules out two promising theories:

      1. Passive salt depositing.

      2. Warmer tires.

      And leaves us with:

      1. Exhaust pointed at road.

      2. Carbon deposited on snow/ice.

      3. Engine heat generated from climbing effort.

      I have another one to offer:

      4. Traction slippage. It breaks up the snow/ice and generates additional heat via friction.

      (Yes, we’ve had a bumper harvest of theories and we’re discounting them to move!)

      1. David Stafford says:

        Oops, failed to notice that Jim Strickland mentioned #4 much earlier. Sorry, Jim. That’s not a new one, then.

      2. Rich, N8UX says:

        Ok, assuming most modern vehicles on the road today are front wheel drive, could it not be explained in part by the vehicles merely collecting up the material and carrying it with them? Look how much snow and ice adheres to your wheel wells when spinning. A slipping wheel will kick this stuff up onto the warmer vehicle parts, either melting immediately, or adhering to a surface and carried to another location where it will eventually melt or fall off.

        My 2c.

        1. Rich, N8UX says:

          And EVERY vehicle going up the hill would contribute to this. And it would occur regardless of the street’s compass heading.


          1. Rich, N8UX says:

            …or tire temperature, or engine/exhaust temp, or direction of the tailpipe(s).

  6. Patrick, S says:

    Not sure if this is related, but where I’m at (Fremont Co, CO), downhill is to town/work, uphill is from town. The snow plow comes from town i.e., uphill first. Traffic has already packed the downhill lane some, so the uphill lane gets scraped closer to the pavement and melts out first.

    BTW, been reading you since Turbo Technix days

    1. Same here: Home is uphill, town is downhill. But the streets I’m talking about are not plowed at all, in either direction. So whatever the explanation really is, plows are not any part of it.

      Scary to think that in another year and change, Turbo Technix will be 25 years in the past. Thanks for staying tuned.

  7. Carol Pruitt says:

    Quoting Jeff: “… cars going uphill are going away from the city where the larger streets are (and thus the salt) and then drop salt slush onto the street while the vehicle is going back uphill toward home. I actually think this may be the most likely explanation, but I don’t really know how to test it.”

    If that’s an appreciable factor, then the effect should diminish noticeably as you get further from the city and the loosest salt has already dropped off the cars. Is this effect more pronounced closer to town, or pretty much the same everywhere?

  8. Rich Rostrom says:

    Applause for noticing this.

    ISTM that the best way to resolve this question would be to seek out the opposite case (where people drive on the left) and see what happens there.

    AFAIK the only temperate-zone countries that drive on the left are Great Britain, Japan, Ireland, and New Zealand. Scotland, Northern Japan, and southern New Zealand might have enough snow accumulation in winter for a test; they also have mountains, which is the other requirement.

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