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A snow covered Smart Car in New York City, the morning after another storm dumped more than a foot of snow on the city. (TIMOTHY A. CLARY/AFP/Getty Images)
A snow covered Smart Car in New York City, the morning after another storm dumped more than a foot of snow on the city. (TIMOTHY A. CLARY/AFP/Getty Images)

Rob's Garage

An explanation for the yellow snow under your car Add to ...

Hi Rob,

I have an issue that I think you could shed some light on and perhaps save a lot of people out there a great deal of grief.

When I went out this morning to move my car, which I had parked last night in some deep snow, I noticed a number of splotches of fresh neon yellow/green liquid in the snow (almost the colour of some engine coolants or A/C compressor oil). I've have noticed this in the past with some concern as well. Naturally I figured that there must be some fluid leak (although I've never seen any leak on the driveway in the summer, only in the snow). The thing is, the liquid is not oily, and I have no fluids in the car that are that colour (blue engine coolant, orange windshield antifreeze).

More related to this story

Well, a web search showed that I'm not the only one with this issue. In various forums, there seems to be a slew of suggestions about what could be leaking and what to check. However as luck would have it, I did find one website that accurately identified the issue (http://www.toyotaownersclub.com/forums/index.php?showtopic=118604) and seems to have provided a good solution to this mystery.

It seems that for reasons that aren't entirely clear, when snow makes contact with a hot rusty exhaust pipe, the resulting melting liquid turns a bright yellow/green colour. What gets left behind looks distinctly like something is leaking from the engine. I can imagine this causing others a fair bit of grief. It might be handy if you could shed some light on this and put a few peoples' minds at ease that their car is just fine.

Thanks, Randy

As you have mentioned, your question has created quite the consternation across the ether. There are many blogs that try to get to the bottom of the yellow snow phenomenon. Some are quite funny, other answers are stabs at far-fetched possibilities.

To answer this question, I had to go back to my old science books - my high school chemistry teacher would be so proud of me.

What twigged me on to the research direction I took was the common thread to the discussion threads. That is, bright yellow spotting of snow that appeared to follow the pattern of the exhaust system. This instantly ruled out leaks, not to mention, there are very few fluids on the market that are bright yellow.

95 per cent of the exhaust systems built today are made up of iron alloy and galvanized steel. Exhaust pipes are made of either iron or stainless steel. Heat shields are made of galvanized steel. Here's where Mr. Podominocov would be giving me a gold star: galvanized steel uses zinc as a rust inhibiter.

Zinc, when hot and in the presence of iron and a catalyst, creates yellow zinc oxide in the form of crystals. The connection is the catalyst - salt water - or compacted snow with road salt mixed in. If the snow is able to touch the exhaust heat shields and the exhaust pipes at the same time, the chemical reaction between the two dissimilar metals creates the bright yellow crystals.

(see the June 18, 2010 blog for more).

As the hot exhaust pipe(s) melt the snow, the yellow substance drops to the snow covered ground - causing much consternation.

There you have it Randy, don't blame the dog or your neighbour's kid after a late night at the bar...but I still wouldn't eat the yellow snow!

Another winter of discontent

Why let a little thing like blizzard conditions make you 10 minutes late for work?
Andrew Clark: Surprise! The snow, the cold, the treacherous driving conditions have taken us by storm - again

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