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Treebeard's Stumper Answer
19 January 2001

Frosty Windows

We've had beautiful weather here in central California since our heavy rains last week, but the clear nights have been very cold. There's still snow on the high mountains. I have to give myself extra time in the morning to scrape the ice from my car windows before I can drive to school. I've noticed that the sloping front windshield of my car usually has heavy frost, but the side windows only have dew or light frost that I can brush away. Why is there a difference? I'm sure the air temperature is the same all around the car. I should mention that I park outside under the stars in front of my house in the woods. (That's a hint!)

Two early-morning photos of my car with ice on the front windshield, but the side windows are just foggy with condensation
or completely clear. In the picture on the left, the roof is white with frost but the hood isn't as frosty. That's another stumper!
My little Chevy always gets more ice than Julie's Volvo in the back, and my sloping rear window is as frosty as the front.

There's more ice on the sloping front windshield of my car on frosty mornings because that window is colder, even though the air temperature is the same. My car cools down at night by radiating infrared heat energy. It also re-absorbs some of the energy being lost by other nearby objects. The vertical side windows mostly receive energy from the ground, trees, and my house and walls. But the sloping front window mostly receives energy from the open sky above, which is much cooler on clear nights. It would help to park under a sheltering tree or roof.


This same effect can make a big difference when gardening or choosing a campsite. It will be a warmer camp under the trees than in an open meadow, though fog drip is another factor to consider, and so is a great view of the stars and where the sun will rise. (Selecting the perfect campsite is a profound skill!) We still have frost-sensitive fuschias and Pineapple Sage flowering under the eaves of our house despite recent temperatures in the teens. Water can freeze even when the air temperature is above 32 degrees because it's colder out in the open under a clear sky. You can measure the difference with a thermometer.

It's easy to think of heat an invisible fluid that flows from place to place. But heat is not a substance. It's just the measure of the motions of the atoms and molecules that make up everything as they jiggle about. Everything has some energy, above the theoretical limit called absolute zero, about -460 °F, where all motion - and all heat - stops.

Heat energy always moves in a direction to even things out, from warm to cold. The greater the difference, the greater the change. But heat transfer happens in several different ways:

When my car cools down at night, it still has the heat of the day. It conducts some heat to the ground through the tires, and it conducts some heat to the cooling air that rises past my car through convection and brings in a flow of more cool air to carry away even more heat. In time, my car reaches equilibrium with the air temperature. But it also radiates heat as infrared energy, and it simultaneously absorbs infrared energy from everything else around it that is radiating heat. It's this shifting equation between absorbing and radiating that makes all the difference on how my car windows freeze.

Heat radiation follows the Stefan-Boltzmann Law:

What's important is that heat radiation depends on the fourth power of the absolute temperature difference. Small temperature differences have a big effect!

On a cloudy night, the water vapor in the air holds much heat energy and radiates it down to keep things relatively warm. On a clear night, the side windows of my car still receive energy from the ground, trees, and my house and walls. But the roof and sloping windshields only face the sky, and that makes all the difference between freezing and not freezing. There's a complicated trade off here since frost requires moisture in the air, yet that same moisture has a warming effect. I get the most frost on clear nights just after a storm. A few days later, there's no frost at all even though the temperature of the dry air is well below freezing.

Heat radiation is the only way to manage temperature on a spacecraft or satellite. My Dunn Middle School Weird Science after-school group got to visit the Raytheon Remote Sensing division in Goleta last fall. They develop IR sensors for NASA weather satellites, among other things. We saw a passive cooling device that looked like a large silver coffee funnel that points at empty space. That's enough to cool their IR sensors to liquid nitrogen temperatures. Same principle!

A few more related frost/heat stumpers:

Here are some starting links for your own research on heat transfer:

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