I walk the same route at the same time and I have a little thermometer on my bag. At 2C last Tuesday my breath came out in big clouds, and at 1C on Thursday there was basically nothing. Same jacket, same pace, same me. Temperature alone clearly does not explain it, so what is the variable I am missing?
Wind is the other variable people forget. A breeze breaks up the plume before the droplets grow enough to scatter light, so identical conditions look dramatic in a sheltered doorway and invisible in the open. Try exhaling with your back to the wind next time and see if it comes back.
Lighting matters too, embarrassingly. With the sun in front of you it is obvious, with the sun behind you the same fog is nearly invisible. I chased this for a week before realising I walked east on one day and west on the other.
Small thing that might help the pattern make sense: nothing about you changes much here. Exhaled air is at body temperature and saturated whether you are hydrated, out of breath, or freezing cold. So when the fog changes and you did not, it is the atmosphere doing it.
If you want to test it deliberately, breathe out into the shadow of a wall or over a cold window on a dry day. Cooler stiller local air makes fog appear in conditions where the open street gives you nothing, which is a decent confirmation that it is about the mixing and not about you.
@gradschool_gary · 9mo ago · 3 replies
Humidity, or more precisely how close the outside air already is to saturation.
Your breath leaves at roughly 35C and essentially 100 percent relative humidity every single time. What happens next depends entirely on the air it mixes into. The saturation curve is strongly nonlinear, so when a warm wet parcel mixes with a cold one, the mixture can briefly hold more water than it can carry at its new temperature, and the excess condenses into the fog you see.
If Thursday's air was dry, the mixture stayed under saturation the whole way and there was nothing to see. Damp mornings after rain fog beautifully at temperatures where a dry cold snap gives you nothing.
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@slow_thinker_sam · 9mo ago
This is why dew point is the number to watch rather than temperature. Small gap between air temperature and dew point, expect visible breath. A few degrees of gap and it disappears.
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@little_endian_lu · 9mo ago
Worth adding that the nonlinearity is the whole trick. If the saturation curve were a straight line, mixing two unsaturated parcels could never make a saturated one and breath would never fog at all.
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