MadSci Network: Physics

Re: Why does your car windshield not frost over when parked under a carport?

Date: Tue Apr 1 21:35:43 2003
Posted By: John Link, Senior Staff Physicist
Area of science: Physics
ID: 1049221456.Ph

The question of why a car parked in a carport collects less frost than one in the open takes us to a large subject field. One of my favorite authors of all time has written a book with a chapter dealing with dew and frost, and I am going to quote his work extensively. The author is Dr. Craig Bohren, and the book is What Light Through Yonder Window Breaks? I have the 1991 printing of the book (which was the first printing), by John Wiley & Sons.

Chapter 9 is called "An Essay on Dew", but it discusses frost, as well, since dew and frost are related. The third section of chapter 9 is called "Nocturnal Radiational Cooling", and is the key section for this discussion of frost on a car. Without further ado, here is a direct quote from the book:

"All objects at temperatures above absolute zero emit radiant energy of all wavelengths, an inexorable and incessant process occurring day and night for eternity. ...I cringe...when I encounter statements conveying the notion that matter is equiped with sensors and switches to turn [radiation] on at dusk. Objects do not begin radiating at night. Indeed, since they are usually warmer during the day, they radiate even more then than at night. And although they are no longer warmed by the sun at night, they still are warmed by radiation from their surroundings, including the sky."

"On clear nights, however, an object may emit more radiation than it receives from the sky, thereby cooling (in the absence of other mechanisms for energy transfer, a point to which I shall return). This was recognized as long ago as 1814 by William Charles Wells, who stated in his 'Essay on Dew' that '[w]hatever diminishes the view of the sky, as seen from the exposed body, occasions the quantity of dew which is formed upon it to be less than would have occurred if the exposure to the sky had been complete.' "

"The key words here are view of the sky. Imagine a flat plate with a spider on it. What the spider sees depends on the orientation of the plate. We may divide her field of view into two parts, that above her horizon and that below. When the plate is horizontal and she is on its upper surface, her field of view lies entirely above the horizon. When the plate is vertical, half her field of view lies above the horizon, half below. When the plate is horizontal and she is crawling on its underside, her field of view lies entirely below the horizon."

"The region below the horizon is usually dominated by the ground, buildings, and vegetation, whereas that above the horizon may be dominated by the sky, especially in open areas. Often, the ground is warmer than the sky. I must be careful to distinguish the radiative temperature of the sky from the air temperature: the two are not the same. Imagine the sky to be replaced by a blackbody, the temperature of which is such that it emits the same amount of radiation as the sky. This temperature is its radiative temperature."

"A good rule of thumb is that the clear sky has a radiative temperature of about 250oK. If we confine our attention to nights with air temperatures near or above freezing, the temperature of the ground will be greater than 250oK. Under these conditions, more radiation is emitted by the ground than by the sky. So if our spider wants to keep as warm as possible, she'll stay on the underside of a horizontal plate."

"The consequences of unequal radiative temperatures of ground and sky--and different views of both, depending on orientation--to the formation of dew and frost are evident in Figure 9.2, which shows a dark post that has acquired overnight a non-uniform coating of frost. The top of the post has a coating, whereas its sides are frost free--evidence of the different radiative environments of the top and sides. Radiative cooling was insufficient to lower the temperature of the sides below the frost point. It is scenes such as this post that undoubtedly are responsible for engendering the misconception that dew and frost fall from the sky just like rain and snow."

Well, I'm finished quoting from Dr. Bohren, and I'll finish with just a few comments.

The fact that I do not include a copy of figure 9.2 is inconsequential, since there's a good verbal description of the post.

Replace the sky with the roof of a carport and you can see that the radiative cooling of the car will be less in the carport than it would be if it had a clear view of the sky, so the car simply does not cool as much under the carport. Also, if you have noticed cars that are parked under a clear sky, often the top surfaces get frost (or dew) while the side surfaces do not, just like the post of the (non-existent!) figure 9.2.

I suggest that you buy or borrow a copy of Dr. Bohren's book, as well as his other great book Clouds In a Glass of Beer (John Wiley & Sons, 1987). Dr. Bohren is a master at describing interesting atmospheric phenomena.

John Link, MadSci Physicist

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