What’s with all the potholes around Boston? Blame it on late winter’s freeze-thaw cycle.

Multiple potholes were visible from an overpass on Rutherford Avenue in Charlestown in 2018.David L. Ryan

I turned onto my street over the weekend and slammed directly into a freshly opened pothole. Fortunately, my tire was okay, but you know what I mean when I say it rattles you.

After the coldest and snowiest meteorological winter in the Boston area and most of Southern New England since 2014-2015, the roads have taken a beating. I’ve received a bunch of emails about local roads being in rough shape, and there’s a pretty simple reason behind it.

So, why are the roads so chewed up? During the day, as temperatures rise, the ice melts and water finds its way into cracks in the pavement. Then it refreezes and expands overnight when temperatures plunge again, destroying the pavement over time.

It’s a cycle that happens over and over again in the cold, which is why most potholes form during the winter season. When water pools below and within the cracks and then expands, it can make the asphalt weak and brittle, crumbling it over time like a cookie.

Potholes form mostly in the winter when water seeps through cracks in the roadways, freezes, and then breaks up the asphalt when the ice expands.Boston Globe

We’re into that point in the winter season when the freeze-thaw cycle occurs on a nearly daily basis. Boston saw 14 days in a row, Feb. 16 to March 1, when the daily high reached above freezing, only for the nights to slide below freezing. Warmer temperatures unlock water from the snowpack and the cold refreezes at night, which can lead to black ice and weak or crumbling roadways.

When temperatures drop below freezing, water starts to behave a little differently. The molecules spread out and arrange themselves into a more crystal-like structure as the liquid turns solid. That’s why ice is actually less dense than liquid water. But here’s the catch: If that expanding water is trapped in a closed space, it can cause some serious damage.

Ever put a full water bottle in the freezer? Not a great idea. The expanding ice can pop the cap right off or crack the plastic bottle open. That’s how the freeze-thaw cycle behaves with concrete and asphalt.

Here's how water can seep into concrete, freeze, melt, and refreeze as temperatures fluctuate above and below freezing.Polytrade

Now think about the number of snowfalls we’ve had this year, and the plows doing their job to clear the streets. The plows are mostly massive trucks that weigh tons and drop steel scrapers that slide against the asphalt. That makes already weak spots more vulnerable to breaking down.

When water freezes, molecules separate to become more stable, expanding to form ice.ScienceFacts

March is typically the time, naturally, that we see significant temperature swings across the region. The higher sun angle will drive the normal high temperature in Boston from the low 40s to near 50 degrees by the end of the month. But it’s usually not until the end of the third week of March that the average nightly low breaks above freezing.

In the Boston area, after a frosty day on Monday, it looks as if our temperatures will jump for the next 7 to 10 days, with our highs and lows potentially staying above freezing. But don’t let that fool you, temperatures to the north and west of I-95 should still see the freeze-thaw cycle continue over the next week or so.

With the warmer temperatures, you can count on snow melt releasing plenty of water that can find its way into the asphalt, especially across Southern New England. Just keep an eye on the walkways and roads.

Around a half inch of water can unlock across Southern New England by Thursday.NOAA

When can we expect the last freeze?

It varies from year to year. But typically, the city’s last frost — a low of 32 degrees — is around April 9.

Last year’s final freeze occurred right on the average date, April 9, but the previous four years were notably earlier.

The earliest last freeze on record? March 17, 1952. Check out the average dates of last freeze near you and across the country.

The dates when New England typically sees its last freeze.Boston Globe

Typically, April brings the final frost for much of Greater Boston, with frost more likely to linger into May across Northern New England.NOAA

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