Originally Posted by stompk
Why is it that one can see your breath at around 20f and the appleman chart says it needs to be -45f for a contrail to form? If human breathing and jet exhaust are just water vapor, please explain the difference.
The difference is in the temperature of the water vapor emitter in each case, and the temperatures in each case of the ambient conditions.
Turbojet exhaust water vapor at 2000 deg F is an IGNITION source. It's arriving into air at -40 deg F, and freezing into microscopically-fine ice crystals which will either remain or evaporate depending whether the ambient air is "wet" or "dry" (the degree to which it is saturated). Contrails have been known to form at warmer temperatures than -45, so the Appleman Chart is somewhat rough and ready.
Water vapor will condense out of one's breath at 20 deg F. But the water droplets which are formed evaporate again because your ambient air at 20 deg F was relatively warm, and much less humid than it was a moment before, when it was in amongst your 100% humid exhalation, and water evaporates more easily in just such conditions. It's different at the North and the South Pole. The temperatures are low enough there to freeze out nearly all the water - permanently.
Evaporation is an equilibrative process. At any point in time, there are roughly as many water molecules evaporating as condensing. The humid column of your exhaled air rises from the water droplet cloud which condensed from it, leaving the droplet cloud at the mercy of the drier ambient air. Pfft.
Leaving your personal aerosols floating as fine particulate matter... ...it's those you need to worry about, believe me.
The unusual properties of water merit years of study.