Your exhaled breath is warm — roughly 34 °C (93 °F) — and nearly 100% saturated with water vapor from the moist lining of your lungs. Cold air, by contrast, can hold far less water vapor: at freezing (0 °C / 32 °F), air holds less than one-fifth as much moisture as it can at body temperature. When the two meet, the warm breath rapidly cools below its dew point, and the water vapor that can no longer stay dissolved as an invisible gas condenses into a cloud of microscopic liquid droplets. That cloud is what you see.
The same physics produces fog on a cold mirror, steam from a hot cup of coffee, and contrails behind airplanes. Temperature and humidity both matter — you’ll typically see your breath when the air is below about 7 °C (45 °F), but on a very dry day you might need it to be closer to freezing. Conversely, on a cold, humid day near a lake or after rain, your breath can be visible at warmer temperatures because the air is already close to saturation.
It’s not the water droplets themselves you’re “losing” — your body loses water through every exhale regardless of temperature. The cold just makes it visible.
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