With the cool fall mornings, that we’ve been enjoying lately, why can I see my breath some days and not others? The answer lies in a collision of two factors: the difference in the dew point and air temperature and the amount of water vapor in your breath. When we exhale, in addition to the carbon dioxide and other gases, we also release water vapor. Fog forms when temperature and the dew point of the air are close to identical. This can occur by cooling the air or raising the dew point. If these two are close enough, the water vapor in your breath is just enough to raise the dew point on a very small scale. Thus producing that very small puff of steam fog, that quickly fades as it mixes with the relatively drier air around it.
Mary Knapp, Weather Data Library