This Is Why Snow Makes Those Patterns On Your Car's Hood

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Since a lot of the country is about to be pummeled by snow and possibly die from overexposure to milk and bread, I figured this would be a good time to explore an interesting little phenomenon that lives at the intersection of cars and snow: those funny patterns snow makes when it lands on your car hood. What’s happening there?

I’ve seen this happen on many cars, as have many other car-noticing Earthlings. Assuming we’re not in some crop-circle situation, where otherwise intelligent aliens are picking the absolute stupidest way to communicate with us, I’d say whatever is going on is the result of some fairly simple physics.

The picture up top there is of one of the many often-immobile cars in my driveway, right after a light dusting of snow this weekend. The car hadn’t been run in at least two weeks, so whatever is going on, it doesn’t have anything to do with residual heat from the engine, at least not at this point.

Now, I can tell, based on the shapes formed in the snow, that what I’m looking at here is related to the system of support struts on the underside of the hood. What I’m not entirely certain of is why the snow seems to be melting quicker on areas delineated by these supports.

If the car has been sitting outside for a week or two without being run, you’d think everything would be pretty much the same temperature, right? It’s not, though, so something must be going on.

I noticed a likely related, but different pattern on a car that had been driven much more recently:

So, let’s think this through. For the first car, that Passat, if we look at the underside of the hood, we can get some clues about what’s going on:

So, you can easily see how the patterns of the underside hood supports match up with the patterns of snowmelt on the hood. Also, in that inset picture of the Jetta with no outer hood skin, you can see that the hood supports are U-shaped in cross-section.

One theory may be that it’s not the greater mass of metal that is somehow the factor, but rather the airspace inside those U-shaped channels. Air is a very good insulator, so perhaps the outer hood metal skin gets cold, but on the areas over the pockets of air, it retains a bit of heat, since the air pocket is a good insulator.

For the picture of the Ford Fusion that had been run recently, the pattern seems to match the thermal insulation under the car’s hood. Where the regularly-spaced dots are, the insulation is thin, so it can be secured to the hood. The snow is over the areas of the most insulation, which is shielding the snow from residual engine heat.

This ‘air and insulators are doing it’ theory needed to be run by someone smarter than me, so I reached out to Jalopnik’s tame physicist, Dr. Stephen Granade, to see what he made of all this:

You’re absolutely right, Jason: the air trapped in the under-hood support is acting as an insulator. Air that isn’t moving around is a great insulator, because the atoms in air are far apart and heat energy doesn’t move quickly through it. Metals have a high thermal conductivity — they pass heat through very quickly — because their atoms are much more tightly packed. It’s why metal is used for pots and pans. The heat spreads through them quickly so that you don’t get some spots that are hotter than others, potentially scorching your meal.

The insulating air in the under-hood support makes parts of your car hood change temperature more slowly than other parts. The parts of the hood over the supports won’t heat up as fast when the engine is running, and they won’t cool down as fast when the snow begins.

Weirdly, that means that your hood might make different patterns depending on the situation. If you’ve just started driving your car and it begins to snow, then the parts of the hood over the supports will be insulated from the engine heat. Because they’re a bit cooler than the rest of the hood, you’ll get a bigger buildup of snow on those parts than on the bare metal parts of the hood. However, if your car’s been parked and the temperature drops enough for it to begin to snow, or your hood’s absorbed some of the heat from the feeble, feeble winter sun, then the bare metal parts of the hood will cool faster. You’ll get a bit of melting happening over the hood supports. Because of this, you can end up with a hood with more snow over the supports, or with more snow over the bare metal parts of the hood. The two patterns are like Flash and Reverse Flash, only with less confusing time travel continuity.

(I said that this was because air insulates. That’s part of the story. But also, the air will trap some of the heat energy and slowly release it back into the hood. It won’t store a lot of heat energy. It has low thermal mass, the measure of how much heat a substance can soak up before it changes temperature. But it’ll store some. The air will also insulate the bottom part of the support from the cold air outside the hood. The bottom support will release its heat back to the top part of the hood. Modeling heat flow gets tricky, which is why professionals use sophisticated software.)

The results will likely be inverted depending on how warm your engine is: if the car has been sitting and is cold, the melt-lines will be over the air pockets in the struts. If the car has been running, the air pockets or insulated areas will be more likely to have snow, and the melty areas will be the parts with less behind them.

Look at that! Another mystery solved! What a glorious age we live in.



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