Why you can see your breath sometimes - two unsaturated parcels mixing and resulting in a saturated parcel

Take two parcels of equal size - one you breathe out, one from the surrounding air.

	Temperature	Saturation vapor pressure	Dew point	Actual vapor pressure	Relative humidity
Breath air	88°F	45mb	85°F	41mb	91%
Outside air	34°F	6.5mb	20°F	3.7mb	57%

Combine the air parcels - temperature and actual vapor pressure can be averaged linearly

Resulting temperature: 61°F
Saturation vapor pressure for that temperature: 18.5mb (determined from a chart or graph)

Resulting actual vapor pressure: 22.4mb (average of two components)
Relative humidity: 22.4/18.5=121%

Since the resulting parcel has an actual vapor pressure greater than the saturation vapor pressure, excess vapor condenses so that the actual vapor pressure decreases to be the same as the saturation vapor pressure. The condensed vapor makes up the brief "cloud" you see. The condensation does not last very long because the moist air rapidly mixes with more of the drier surrounding air and the liquid water is evaporated into the drier air.

A supersaturated parcel can result from two unsaturated parcels because temperature and water vapor content can be averaged linearly using the values from the two parcels, while saturation vapor pressure depends exponentially on temperature and cannot be determined for the resulting mixed parcel by averaging the SVP values for the two initial parcels.