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Name: Joe 
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Question:
I was walking along a country rode in the cold the other day and noticed I could not see my breath. My sister pointed out whenever a car drove by we could see our breath, for a minute or so after the car passed. Why is that? It was raining/snowing on and off with a temperature in the low to mid 30's.


Replies:
Joe

The air temperature was apparently not low enough for saturation of your breath to occur, and so your breath did not become visible. I also am reading "between the lines" in what you have written, assuming that the vehicle exhaust did not produce visible droplets either.

If a vehicle engine is hot after having run for a while, the exhaust that comes out of the tailpipe expands so rapidly and stays so warm as it mixes with the air, that the water content of the mixed exhaust is too low and the temperature is too high to cause saturation of the mixed air; thus visible droplets don't appear.

Second, the particulates from the vehicle exhaust apparently provided nuclei on which the water vapor from your breath (which has a higher water vapor density than the air) could condense and cool sufficiently to result in visible droplets.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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