EUREKA!

WHY IS THE ICE THICKER ON THE WINDSHIELD THAN ON THE SIDE WINDOWS – UNLESS YOU DRIVE A JEEP WRANGLER?

You may have noticed on a cold, clear, calm morning that the ice is thicker on the windshield than on the side windows of your car – unless you drive a Jeep Wrangler or a Model A Ford.

The answer is radiant heat transfer. Everything warmer than absolute zero (about -460 degrees Fahrenheit/ -273C) radiates infrared (heat). With radiation, it is the balance between an object and its surroundings that determines where the heat goes. If an object is warmer than its surroundings and the air is calm, it will cool off partially due to radiant cooling. When the wind is blowing, convection dominates.

If it is windy or cloudy, you won’t get much, if any, ice on the windshield, unless it is falling out of the sky as rain or snow. The wind transfers heat by convection, a much more effective means of heat transfer when the temperature differences are small.

In the Summer when temperatures are moderate, the wind is low, and the sky is clear, the ground radiates infrared upwards and cools off. Moisture in the air condenses on the horizontal surfaces, particularly grass that is not touching the ground. Of course we call that dew.

If you take an infrared thermometer and point it at various objects, you can tell what their surface temperature is without touching them. An infrared thermometer basically operates like a flashlight, but in reverse. Instead of sending out a beam of visible and infrared light, an infrared thermometer senses the infrared light incoming from a cone with an angle of typically about 20 degrees.

With a solid, it is the surface that matters. The trees might read 30 degrees F, the bare ground might be 35 F, the leaves of grass might be 25 F, your house might be 40 F, but then when you point the thermometer up at a blue sky, it likely reads something like -10 F – unless you point it at the Sun.

If you point it up at a starry night sky, it will read a little colder. The top surface of leaves are facing the sky, so they might read 15 F, while the bottom surface of the same leaf might read 30 F. If it weren’t for our atmosphere, water vapor, and clouds, the sky would read -459.7 F/-273.15 C (except an infrared thermometer can’t measure temperatures that low).

On a cloudy day, low, cumulus clouds will be something like 10-20 F/6-11 C cooler than the air around you as they are higher than you. Those clouds are glowing in the infrared and scattering infrared back down – keeping you warm!

When you point the infrared thermometer up at a starry sky, it is measuring the infrared output, thus the temperature of all the molecules and particles above it in a column all the way out to space. If the air temperature is perhaps 30 F and the starry sky reads -20 F, that means that the average temperature is about what the air temperature is at 10,000′ above you (50,000/5 on a clear night). There will be a tiny contribution from the stars and a little from the minuscule 400 parts per million of CO2, but it is water vapor and invisible droplets that dominate the reading. On a cloudy night it is those visible droplets in the sky (clouds) at perhaps a few thousand feet that totally dominate the temperature of the sky keeping things warmer.

So it is the ANGLE of the windshield facing UP at the cold, clear sky that causes the windshield to cool faster than the side windows that are facing your house, the trees, etc. The windshield on a Jeep Wrangler is more vertical than most cars, so it stays warmer longer and crystallizes less ice. Convection also is a factor as the tilted windshield deflects the moist air and the moisture has a better opportunity to crystallize.

Here you can see the widely varying temperatures AT THE SAME TIME on a pleasant Fall day. This variation is quite normal but generally goes unobserved.

So if you want to prevent ice on your windshield, a carport or tent over your car will do the trick. Meanwhile, notice that CO2 didn’t enter into the above, because it is the Sun, water vapor, and droplets in the clouds that really matter with respect to our climate – not the “dreaded CO2”.

Fergus S. Smith

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