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All this snow and ice reminded me of those old pictures where people are driving across the Willamette in their Model T's. How cold would it have to get for me to drive my Civic across the river?
—Isaac
Even if the river did freeze, Isaac, our modern authorities would never let you drive on it. Old-timey man lived in a more freewheeling era, when dentistry involved gunpowder and it was considered hilarious to roll the homeless in hot tar.
The picture you're probably talking about is from the winter of 1924. How cold was it? Unfortunately, the National Weather Service temperature database for Portland only goes back to 1940.
Normally, this wouldn't be a problem—I'd just call some hapless government functionary and let them find the answer for me, while I sat around drinking hair spray and translating the works of W.B. Yeats into dogespeak. ("Such beast. Many slouch. Bethlehem, wow.")
Last week, though, these drones either weren't at work or had bigger fish to fry. But here's what I did find out.
You need near-zero temperatures to freeze a river, but more important is how long it stays cold. The river is moving, so ice crystals have a hard time clumping together at first—the river has to go through a Slurpee-like phase called, delightfully, "frazil slush."
Eventually, the frazil will clump together into larger plates called "pancake ice." From here, a sudden temperature drop can be enough to freeze the whole thing over.
But don't look for this to happen again anytime soon—Portland's winters have been getting steadily milder for 150 years. In the 1870s, our average annual snowfall was 20.4 inches; in the 2000s, it was 4.7. Not to beat the global-warming drum yet again, but a more realistic dream for you might be rafting the Mount Hood Skibowl.
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WWeek 2015