Seeing Breath, Air Movement
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.
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
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012