[Chrataxe]

Holy corvus, I section was here!!!  Anyhoo, an observation I have noticed lately and have been curious about.  If it is a warm day and I breath out, I do not see my breath.  On most cars, on a warm day, I can't see exhaust.  But, on a cold day, both are visible.  So, this brings me to my question....why?  I have to the conclusion that it is a matter of heat.  It seems when gas leaves a "hot" area to a cooler area, the gas is visible.  I don't know if that is right or not, but it seems so.  Which, in my mind, would also account for smoke and steam off of boiling water (and other stuff).  Am I right in this assumption?  And why is the gas visible?  Is it b/c it is coming from a higher temp and then condenses at cooler temps?

Been bugging me for a while now...

[billnotgatez]

Among other things you exhale water vapor at about 37 C
One of the results of internal combustion engines is water vapor at a fairly high temperature
Steam from a boiling pot is more visible on a cold day
 

[jdurg]

Holy corvus, I section was here!!!  Anyhoo, an observation I have noticed lately and have been curious about.  If it is a warm day and I breath out, I do not see my breath.  On most cars, on a warm day, I can't see exhaust.  But, on a cold day, both are visible.  So, this brings me to my question....why?  I have to the conclusion that it is a matter of heat.  It seems when gas leaves a "hot" area to a cooler area, the gas is visible.  I don't know if that is right or not, but it seems so.  Which, in my mind, would also account for smoke and steam off of boiling water (and other stuff).  Am I right in this assumption?  And why is the gas visible?  Is it b/c it is coming from a higher temp and then condenses at cooler temps?

Been bugging me for a while now...

You are not seeing gas.  Gases are invisible unless it is a colored gas like NO2, Cl2, Br2, or I2.  What you are seeing are droplets of water condensing in the air.  Hot air can hold a great deal more water vapor in it than cold air can.  (Hence why it's a LOT more humid during the summer months than the winter months).  When it's cold outside and you breathe out, you are taking warm, moist air from inside your lungs and placing it in a dry, cool environment.  Since your breath cools off rapidly, the moisture that is in your breath can't remain in the now cooler air.  As a result, it condenses out and you see the fine little water droplets as a 'fog'.  The same thing is happening with car exhaust.  So you're not seeing the gas condensing.  You're just seeing the water vapor that was once held in the gas condensing out since the now cooler gas cannot hold the water any longer.

[noname]

If you want to do something cool, use your fingers to hold your lips together, then force air into your mouth, and pressurize it.  Then slowly exhale it from the mouth, and it is a vapor.  You can get the same effect from opening a 2-litre pop bottle.

[Bakegaku]

jdurg is right, it happens because when the moist air from your breath cools down water will condense out of it.  

Something interesting I noticed is after I take a hot shower, the room will be pretty warm (not the 37ºC of my body) but very moist, and I can see my breath.  It's pretty odd, but I believe it's because the air is already saturated with moisture, and when my breath cools down to the room's temperature (say 30ºC) it moisture will still condense out of it.

[Chrataxe]

Good stuff jdurg.  I asked my chemistry teacher after posting and he said the same thing.  But, what about smoke.  My teacher said that smoke isn't so much gas (as jdurg said, you can't see gases) but more of tiny particles of ash and what not else.  But, being a smoker and not smoking indoors, I've noticed that on cold days, the smoke I blow out is much "thicker" than it is on warmer days and it appears to linger longer (assuming no wind, which is virtually impossible in West Texas).

[jdurg]

Smoke is indeed a suspension of particulate matter in air.  When you exhale smoke, you are exhaling fine particles of ash and soot which are so light that they stay suspended in the air.  On a cold day, when you exhale smoke, you see those suspended particles as well as condensed water vapor from your breath.  As a result, the smoke appears thicker.  As for how long it appears to hang around, that could also be a result of air density.  Cold air is much denser than warm air, so on a cold day the smoke has a difficult time moving away through the denser air so it seems as if it is sticking around longer.  On a warm day, the air gives less resistance so the smoke can dissipate easier.

[Chrataxe]

Thats what I was thinking....lol, speaking of densities of gas, since I first started coming here and saw your (jdurg) sig, I have put a lot of thought into the chemistry of farting....what a hobby...thanks!!!

[jdurg]

Lol.