Weird winter windshield water

November 16, 2023 8:15 PM

What strange physics phenomenon am I witnessing in my driveway in the mornings?

For the past several mornings, the windshield of my car has been covered in ice that I have to scrape away, but the other windows have been covered in liquid water that wipes away easily. I'm no physicist, but I fail to understand how this is possible.

• It's not the direction I'm facing; my neighbor's car is parked facing 180 degrees opposite from mine, and they have the same thing (windshield iced, other windows wet).
• It's not the angle of the glass; across the street, one car has ice on both its windshield and its windows.
• It's not about the sun; I'm observing this well before the sun comes up.
• It shouldn't even be possible in the first place; overnight lows are around 35 degrees.

This has happened multiple nights in a row. Weather here is clear, with little-to-no wind, daytime highs in the mid-40s and overnight lows in the mid-30s. All cars are parked uncovered, at least 10 feet away from any structures.

What's going on here?

posted by Dilligas to Grab Bag (4 answers total)

Best answer: I'm gonna guess that it's related to nocturnal surface cooling:

Radiative cooling is commonly experienced on cloudless nights, when heat is radiated into outer space from Earth's surface, or from the skin of a human observer. The effect is well-known among amateur astronomers.

The effect can be experienced by comparing skin temperature from looking straight up into a cloudless night sky for several seconds, to that after placing a sheet of paper between the face and the sky. Since outer space radiates at about a temperature of 3 K (−270.15 °C; −454.27 °F), and the sheet of paper radiates at about 300 K (27 °C; 80 °F) (around room temperature), the sheet of paper radiates more heat to the face than does the darkened cosmos. The effect is blunted by Earth's surrounding atmosphere, and particularly the water vapor it contains, so the apparent temperature of the sky is far warmer than outer space. The sheet does not block the cold, but instead reflects heat to the face and radiates the heat of the face that it just absorbed.

The same radiative cooling mechanism can cause frost or black ice to form on surfaces exposed to the clear night sky, even when the ambient temperature does not fall below freezing.

posted by heatherlogan at 8:19 PM on November 16, 2023 [9 favorites]

Yes, it is definitely radiative cooling and related to the angle of the windshield, which faces the sky, versus the windows.

Take a look at the car that has windows that are iced as well and I bet you will find that the windows that are iced are facing clear sky and that car windows that aren't iced are likely facing some object or possibly the rising sun. It is also possible that the cars are being driven at different times so have different internal temperatures overnight, so the cooler car requires less radiative cooling to freeze the windows.
posted by ssg at 9:26 PM on November 16, 2023 [2 favorites]

Best answer: Windshield glass is usually a sandwich of glass layers laminated over & under a plastic layer, while side windows are tempered glass. This is why, in a minor impact, the windshield 'spider-webs' but stays intact; but when tempered glass is shattered it fragments into tiny pieces. Your 'weird winter windshield water' may be observed on one and not the other due to these differences.
posted by Rash at 10:33 PM on November 16, 2023 [7 favorites]

Radiative cooling effects can also be experienced in daytime... in places with significant winter snowpack it's very noticeable, once it's been pointed out, that when the spring melt gets underway snow thins and recedes much, much faster on days with even a small amount of thin overcast cloud vs. clear sky days when it has direct, bright sunlight shining on it.

I remember the quick and dirty explanation I got way way back on a chairlift during a basic avalanche safety course made my liberal arts head spin with claims of snow being "a black body" and it mirroring "the surface temperature of the sky".
posted by protorp at 12:14 AM on November 17, 2023 [2 favorites]