Mods, not sure if this one has a factual answer or not, please feel free to move if appropriate.
I have been wondering about the following phenomenon: I go out to my car parked outside in the driveway and find Frost or dew on the wind shield but not on any of the other windows. I have noticed this on other cars also. Any explanation?
This isn’t to say that there aren’t any other cars in the vicinity with windows that are completely covered with dew or frost, but perhaps I just noticed the ones that fit that pattern.
In addition; say my car is as described, but perhaps my neighbours car windows are completely frosted / covered in dew. Maybe I am just parked closer to the house?
Now I reckon that how close I have my car parked to my house would factor into this, but if so why are the door windows on both sides equally free of frost / dew while the wind shield is covered?
For that matter why the front window and not the back one?
I’ve answered this in a previous thread, but I can’t seem to find it. Here’s a short version:
There’s three ways heat is transferred around: conduction, convection, and radiation. For a car window, especially on a windless night, radiation dominates.
Almost all objects with temp greater than absolute zero (and all have that) give off blackbody radiation in proportion to their temperature. But they also receive blackbody radiation from other objects. Or to put it another way, objects are exchanging heat with each other until they all have the same temp.
For a fairly flat obect like a car window, the blackbody radiation mostly goes in a direction perpendicular to the center of the object. The window will exchange heat with whatever that perpendicular line intercepts. A side window is fairly upright, so its perpendicular will usually intercept some terrestrial object such as a building. Thus the side window will end up as the same temp as the outside of that building, which is likely to be above freezing.
The windshield is much more horizontal than side windows so its perpendicular usually goes up into the sky. On a clear night, it’ s going to be exchanging heat with open space, which is much colder than freezing. So it drops below 0 C and frosts up.
So for a really stupid question. Does it have to be below freezing for it to frost? My gut says yes. However, as a gardener, I pay attention to the night time temps and it seems that we have had frosts when the temp was above freezing.
IF it can frost above 32 degrees, how do I predict when it will frost? I watch weather.com when it is going to be chilly for info but I can’t see anywhere on their site where they predict frosts.
Yes, and no. You can get frost with the ambient temperature between 32 and 40 degrees F and the air is calm. Because of the radiation effects mentioned previously, the places where the frost develops (car windows, roofs of unheated buildings) the surface temperature is below 32.
An object radiating off heat can become colder than the ambient air temperature, most likely when it’s facing a clear sky. Hence frost when the air is above freezing.
You have many variables to control for before you can reach any conclusions. We should be telling you about some of these potential variables to look out for:
different types of cars:
>different interior and exterior colors offer different opportunities to heat up from sunlight and cool down after sunset
different locations:
>where does the sun cross during the day? Are some spots in more shade than others (they get a jump on cooling down)
>>are some cars near sewers or other potential geo-thermal spots?
different uses:
>are some cars in use during the day/eve, while some just sit there all day?
For gardeners, the important figure is the “grass minimum” which as the name suggests is the temperature recorded by a thermometer with its bulb just touching the tips of short-mown grass. The temperatures quoted on the weather forecast, though, are air temperatures, which are measured in a screened cabinet with the thermometer 5 feet above the ground.
The grass temperature can easily fall several degrees below the air temperature, but the exact difference depends on lots of factors, including cloud cover, wind and air currents. On a clear night in a frost hollow, you can easily get a grass frost even with an air temperature of 40F or more.
Thanks for answering my question on ground frost. I knew that I saw frost when the air temp was above freezing but it didn’t seem possible. I’m sorry if this is a hijack but I still don’t quite understand. I thought that the ground held the heat. Why would the temperature be lower close to the ground? Also, my real question…How do you predict when you will have frost?
Thank you for that cogent explanation. I teach an apprentice class for plumbers and they need to know the ways that heat gets transferred, so I know a little about radiation, convection and conduction in plumbers terms. I didn’t know that radiation figured so prominently in heat loss for an automobile. I did know that the side of a car parked closest to a building usually gets the least amount of snow.
To the extent that I thought about it, I thought that convection played the larger role, your reply suggests that is not the case.
Thank you for that cogent explanation. I teach an apprentice class for plumbers and they need to know the ways that heat gets transferred, so I know a little about radiation, convection and conduction in plumbers terms. I didn’t know that radiation figured so prominently in heat loss for an automobile. I did know that the side of a car parked closest to a building usually gets the least amount of snow.
To the extent that I thought about it, I thought that convection played the larger role, your reply suggests that is not the case.
I gave the abbreviated version in my last post so it didn’t have details about why radiation dominates heat transfer. In most situations, radiation is the least important heat transfer mechanism.
Conduction is not a big factor. The windows are connected to the rest of the car only along their edges, so there’s not much condiction there. Air is a very poor conductor, so ditto.
You’re only going to get convection if 1) there’s a heat source nearby (e.g. a sewer (as suggested by Philster) or 2) there’s wind to move the air around. So on windless nights, the amount of convection is negligible and radiation is the main factor for cooling the windows.
And that answers zydecat’s question: this effect is most common on windless, cloudless nights. I once heard (I think from a physics prof) that this effect is sometimes used by desert dwellers to make ice without a freezer. Put out a shallow tray of water on an insulated surface on a clear, calm night and it’ll be frozen before morning. It works best in the desert because there’s so little humidity in the air to give a Greenhouse Effect.