A few minutes ago, I went out to get the newspaper. As I walked pass my car to get back in, I noticed that there was ice on my car as my porch light reflected off of it. However, it was only on the roof of my car. The hood, trunk and all the windows were free of ice. Earlier this week, there was some ice on my car, but it was all over the car. Currently, the weather is 43F with 58% humidity in Southern California. Does anyone have an explanation for why this happened?
Is the sun hitting any part of your car yet? Is your car parked in such a way that an eddy current of wind might be catching part of it (like parked up near a garage or wall)?
Sometimes, if the temp and humidity are just right, I’ll find that there’s no frost right over my engine block (even after it hasn’t been run for a 12 hours) or no frost on the uninsulated parts of the hood and roof.
Most likely radiational cooling. The angle on the hood and the trunk is probably slightly different than that of the roof so they are exposed to less sky than the roof is. On a clear night the temperature of the sky can cool things below ambient temperature.
the car is a somewhat isolated and limited thermal reservoir. the roof can loose lots of heat to the sky being flat and has a small amount of heat contained in it or near it. the hood, trunk and windows can be angled to not loose as much heat. the hood and trunk have greater amounts of heat contained near them.
Air temperature isn’t surface temperature.
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The surface can cool below freezing, but the surrounding air can remain above freezing
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The news reports the temp where they measure it: Sometimes downtown, at an airport, et. It might be cooler where you live.
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Different parts/surfaces cool at different rates. Various materials and placement on a car will result in different part heating/cooling differently.
Everything you described is normal.
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As noted above, it was most likely a combination of radiative cooling (I’m guessing that last night was calm and clear) and the fact that the roof is somewhat thermally isolated from the rest of the car. Radiative cooling can actually cause objects to cool well below the ambient air temperature at night; here’s an interesting article on the subject.
The roof is almost always the first part of the car to get frost on it, as you’d expect - it is basically just a thin sheet of metal with a relatively large airspace below it that can cool down quickly, plus it’s almost horizontal for maximum exposure to the sky.
The temperatures noted in weather reports are air temperatures, which are taken at a standard height aboved open ground (usually 5ft IIRC) in a screen (basically a ventilated box). The thermometer is protected by the box so it doesn’t lose heat by radiation into space. On a clear night, the ground temperature is usually several degrees lower than the air temperature. Cars, being metal, are even better at losing their heat into space than the ground is, so ice will form on a car before it forms on the ground. You can get ice on cars when air temperatures are in the mid- to high 40s.